Phlox Research Report

Gardening on a higher level

Research Report

Phlox

For the Mid-Atlantic Region George Coombs | Manager, Horticultural Research

INTRODUCTION

Eastern Tiger Swallowtail on Phlox paniculata 'Jeana'

P

hlox represents a horticultural love affair that dates back to the earliest period of exploration and discovery in America.

The showy and often fragrant flowers are now a staple of European and American gardens. Since their discovery centuries ago, horticulturists the world over have been selecting cultivars for improved horticultural traits. Now, with hundreds of phlox to choose from, today’s gardener can be overwhelmed with choices. While many of these selections claim to be disease resistant, their performance in the mid-Atlantic has never been truly examined.

From 2015-2017, Mt. Cuba Center tested the performance of several different species of native phlox along with their related cultivars. The evaluation of 94 selections of eight sun-loving species is discussed first. This includes the ever-popular garden phlox (P. paniculata) beginning on page 3, followed by other sun-loving species like Carolina phlox (P. carolina) and smooth phlox (P. glaberrima) on page 9. Page 18 begins the results of our evaluation of 43 selections of two shade-loving species, woodland phlox (P. divaricata) and creeping phlox (P. stolonifera). While these evaluations are extensive, they do not include other native species like annual phlox (P. drummondii) or mat-forming species like moss phlox (P. subulata).

Phlox is a large genus that includes more than 60 different species native to a variety of habitats throughout North America. Of those, 17 species can be found in the eastern United States. Phlox were initially described by European naturalists in colonial Virginia. The first such record comes from British naturalist John Banister who made detailed drawings of downy phlox (Phlox pilosa) and moss phlox (Phlox subulata) in 1680. The name Phlox, however, was not officially designated until 1737 by the father of modern taxonomy, Carolus Linnaeus. He coined the term Phlox, which is derived from the Greek word for “flame,” referring to the intense pink flower color of many species.

Throughout the colonial era, new species of phlox were discovered and sent back to British gardeners.

Philadelphia’s famed colonial botanist, John Bartram, is credited with sending the first specimens of Phlox paniculata to Europe in 1744. This species eventually became wildly popular in European and American horticulture. In fact, by 1917, a survey of American nurseries counted 584 named selections of Phlox paniculata.

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Breeding and selecting of new cultivars of Phlox paniculata is still ongoing today. Much of this work is happening in the Netherlands where breeders are focused primarily on compact, disease resistant selections. However, the climate and disease pressures of Europe do not translate equally to the United States.

In fact, Mt. Cuba Center found that many of the best selections for the mid-Atlantic region are plants that were selected in America rather than Europe. This includes most of the top performers whose origin stories often tell a narrative of chance discovery and humble beginnings.

While the beauty of phlox has been admired for centuries, today’s gardeners are also falling in love with its ability to attract butterflies. As natural areas are lost to development, it is increasingly important to make our personal landscapes more productive for wildlife.

Providing nectar sources for butterflies is one crucial step gardeners can take, and there are few plants better to use than phlox. For this reason Mt. Cuba Center evaluated butterfly preference as well as horticultural performance of Phlox paniculata (see pg. 16).

SUN PHLOX Phlox paniculata Garden phlox (Phlox paniculata) is the most popular species of phlox used in today’s landscapes. In fact, of the 94 different types of sun-loving phlox in our trial, 66 are selections and hybrids of P. paniculata. Throughout the trial, each selection was evaluated for floral display, foliage quality, habit, and powdery mildew resistance. Garden phlox did prove challenging to grow in our trial garden due to powdery mildew infections that were exacerbated by having so many plants in one location.

However, several cultivars excelled despite this challenge. The following are the 10 best selections of Phlox paniculata for the mid-Atlantic garden.

Phlox paniculata 'Jeana' ««««« Phlox paniculata 'Jeana' is, without a doubt, the best-performing phlox from the trial. This cultivar was discovered growing along the Harpeth River near Nashville, Tennessee and named after its discoverer, Jeana Prewitt. Although there were many plants of Phlox paniculata in the area, 'Jeana' in particular stood out for its exceptionally mildewfree foliage. This trait carries through to the garden and is one of the main reasons 'Jeana' performed so well in the trial. This 5' tall beauty also produces an impressive floral display from mid-July through early September. Interestingly, the individual flowers, or pips, are much smaller than any other garden phlox. However, that does not deter the butterflies that feed on its nectar. In fact, we found 'Jeana' attracted more butWterflies than any other garden phlox in the entire trial. With a top rank in both horticultural and ecological evaluations, Phlox paniculata 'Jeana' is hard to beat.

Phlox paniculata 'Glamour Girl' ««««« Phlox paniculata 'Glamour Girl' is a medium height cultivar (3’ tall) with stunning coral-pink flowers in midsummer. The large blooms begin in early July and last for nearly six weeks. In addition to its gorgeous flowers, 'Glamour Girl' also stands out for its vigorous and lush habit. Some powdery mildew was observed throughout the trial; however, it was never severe and did not cause much actual damage to the leaves. In a garden without hundreds of other phlox plants 'Glamour Girl' would likely be exceptionally clean.

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TOP PERFORMERS

Phlox paniculata 'Delta Snow' «««« Phlox paniculata 'Delta Snow' hails from Mississippi where it originated as an old pass-along plant, a term used for plants that were loved so much that neighbors freely shared them with one another. Part of the longstanding appeal of this cultivar is its exceptional resistance to powdery mildew. In fact 'Delta Snow' has been recognized as one of the most disease resistant cultivars in several trial programs throughout the United States. Although its claim to fame is disease resistance, 'Delta Snow' also produces an incredible number of large inflorescences with white flowers accentuated by a bright lavender center.

Phlox × arendsii 'Babyface' «««« Phlox × arendsii 'Babyface' is thought to be a hybrid of Phlox paniculata and Phlox divaricata, which is typically denoted as P. × arendsii. Although this cultivar may have been derived from such a hybrid, it looks and behaves like Phlox paniculata. 'Babyface' does develop some powdery mildew, and frequently drops its lower leaves as a result. However, all of that can easily be forgiven thanks to its first-class floral display. The broadly pyramidal inflorescences are made up of small pink flowers with a darker pink center.

The buds are also extremely attractive thanks to the dark calyx found at the base of each bud. This provides as much as two additional weeks of interest prior to the flowers opening. 'Babyface' also has a relatively late floral display which peaks in early August, a few weeks later than most cultivars.

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SUN PHLOX

Phlox paniculata 'Lavelle' «««« Phlox paniculata 'Lavelle' originated in the garden of Jeana Prewitt and is thought to be a seedling of the cultivar 'Jeana'.

Although 'Lavelle' is not as disease free as 'Jeana', it still boasts above-average resistance to powdery mildew. ‘Lavelle’ has white flowers with pale pink floral tubes that are held on 4' tall stems. The floral display is one of the longest in the trial, lasting from early July through late August. 'Lavelle' did suffer from being placed next to the sidewalk which reflected a lot of heat and greatly contributed to spider mite infestations.

Despite these challenges, 'Lavelle' proved vigorous and resilient and would likely perform even better in a less stressful environment.

Phlox paniculata 'Robert Poore' «««« Phlox paniculata 'Robert Poore' is named after the landscape architect who discovered it at an abandoned homestead in Mississippi decades ago. He shared it with Barbara Boys of Southern Perennials and Herbs who recognized its superior performance and introduced it to the nursery trade. Phlox paniculata 'Robert Poore' grows as tall as 5' and bears large, broadly pointed, magenta colored inflorescences. This cultivar is frequently cited as one of the most disease resistant in our area. However, it did develop some powdery mildew in the second and third year of the trial, demonstrating just how severely the trial garden was affected.

Despite the mild powdery mildew, 'Robert Poore' proved to be a sturdy and robust selection with an excellent display of intensely fragrant flowers.

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TOP PERFORMERS Phlox paniculata 'Dick Weaver' «««« Phlox paniculata 'Dick Weaver' is a vigorous cultivar with an abundant floral display throughout July and August. The flowers are strongly fragrant and have a magenta color similar to 'Robert Poore'. However, 'Dick Weaver' differs from 'Robert Poore' with a slightly shorter habit (4' tall) and smaller, more dome-shaped inflorescences. Although the inflorescences are not as large as other cultivars, the great quantity provides an impressive display nonetheless. 'Dick Weaver' originated in Michigan but was passed around several times before arriving at Plant Delights Nursery. They decided it was so remarkable that it needed recognition and named it after the co-founder of We-Du Nursery who shared the plant with them.

Phlox paniculata 'David' «««« Phlox paniculata 'David' is probably the most well-known cultivar of garden phlox and was even named Perennial Plant of the Year in 2002. Its popularity stems from its reputation for excellent resistance to powdery mildew, which was confirmed in Mt. Cuba Center’s trial. Fragrant, pure white flowers bloom in midsummer atop 3' tall stems. This cultivar was originally discovered as a seedling at the Brandywine Conservancy in southeastern Pennsylvania from locally sourced wild populations. It was named after the husband of F.M. Mooberry who oversaw horticulture at the organization.

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SUN PHLOX Phlox paniculata 'Ditomdre' (Coral Crème Drop) «««« Phlox paniculata 'Ditomdre' belongs to the Candy Store™ series and is sold under the trade name Coral Crème Drop. It is a compact cultivar, reaching approximately 3' tall. Coral Crème Drop did develop some mildew during the trial, but the disease did not affect the health of the leaves as much as other cultivars. Coral Crème Drop is particularly eye-catching thanks to its salmon-pink flowers highlighted by dark stems and buds. Although dark stems and foliage can be seen on other cultivars, the coloring usually fades before the plants bloom. Coral Crème Drop maintains the dark coloration longer than any other cultivar, resulting in a more dramatic floral display.

Phlox paniculata 'Shortwood' «««« Phlox paniculata 'Shortwood' is actually not short at all. Growing to approximately 4.5' tall, it represents one of the taller cultivars of garden phlox in the trial. It was named for the garden of author and perennial plant expert Stephanie Cohen who calls her own garden Shortwood (since "Longwood was already taken"). However, the cultivar 'Shortwood' was actually developed by Sinclair Adams as a cross between Phlox paniculata 'David' and Phlox paniculata 'Eva Cullum'. 'Shortwood' has been cited by other trial gardens as one of the most mildew resistant cultivars available, but that was not the case in our trial. 'Shortwood' developed a fair amount of mildew, but excelled instead due to its incredibly sturdy habit and long-lasting display of pure pink flowers from late July through early September.

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ADDITIONAL RECOMMENDED SELECTIONS Several other cultivars of Phlox paniculata scored slightly lower than the top performers but could still be recommended.

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Phlox paniculata 'Thai Pink Jade'

Phlox paniculata 'Barthirtytwo' (Volcano White)

Phlox paniculata 'Barfourteen' (Purple Flame)

Phlox paniculata 'Blushing Shortwood'

Phlox paniculata 'Party Punch'

Phlox paniculata 'Blue Paradise'

Phlox paniculata 'Speed Limit 45'

Phlox paniculata 'Dasfive' (Ice Cap)

Phlox paniculata 'Pink Lady'

Phlox paniculata 'Barsixtytwo' (Coral Flame)

Phlox paniculata 'LSS Vincent'

Phlox paniculata 'Laura'

SUN PHLOX Other Phlox Species Although much of the trial was devoted to garden phlox (Phlox paniculata), selections from several other less common species were also evaluated, including Phlox amplifolia, P. carolina, P. glaberrima, P. maculata, P. ovata, P. pilosa, and P. pulchra. Many of these differ from Phlox paniculata with an earlier bloom time (late spring) and improved resistance to powdery mildew. Although not as readily available as garden phlox, the following selections are highly recommended for their outstanding garden performance.

Phlox carolina ssp. carolina 'Kim' ««««« Phlox carolina ssp. carolina 'Kim' is a fantastic selection of the species that was discovered by Jan Midgley in Alabama. It performs much better than any other cultivar of Carolina phlox in our trial. What sets 'Kim' apart from other members of the species is its lush and vigorous habit (2’ tall) that remains sturdy and disease free all season long. The leaves are also a lighter shade of green, almost lime colored, which can prolong its horticultural interest in the garden. However, the most impressive feature of 'Kim' is its show-stopping light pink flowers which blanket the plant from late May through early June.

Phlox glaberrima 'N³ Tasache rvfo hakof' (N³ Springfall) ««««« Phlox glaberrima 'N³ Tasache rvfo hakof' is a fantastic new selection of smooth phlox from Nearly Native Nursery in Fayetteville, GA. It has a Muskogee language cultivar name in honor of the Native Americans that once inhabited the region where it was discovered. In English, Tasache rvfo hakof roughly translates to "spring and/or through fall." What makes N³ Springfall such a great garden plant is that it forms tidy mounds of foliage that are completely immune to powdery mildew. The floral display is also gorgeous, with copious numbers of vivid, lavender-pink flowers in June.

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TOP PERFORMERS P. carolina 'Bill Baker', P. glaberrima 'Morris Berd', P. 'Forever Pink' ««««« There is a great deal of taxonomic confusion surrounding Phlox glaberrima and Phlox carolina, such that these three cultivars may all be more closely related than their names imply. In fact, they all can be used very similarly in the garden, which is why they are grouped together here. Each produces medium-green foliage approximately 2' tall and blooms with pink flowers in late May/early June. Heavy rains can cause the plants to flop while in full flower, but 'Forever Pink' is usually the sturdiest.

After blooming, all three form an attractive mound of foliage that can persist to varying degrees through winter. This is especially prominent with 'Morris Berd' which maintains its basal foliage throughout the entire year. Such overwintering basal foliage can be useful in reducing soil erosion caused by harsh winter rains.

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Phlox carolina 'Bill Baker' foliage before flowering

Phlox 'Forever Pink'

Phlox glaberrima 'Morris Berd'

Phlox glaberrima 'Morris Berd' foliage after removing flowering stems

SUN PHLOX Phlox amplifolia «««« Largeleaf phlox (Phlox amplifolia) is an exciting species that has never been used in horticulture prior to our evaluation.

The foliage grows approximately 2' tall and is relatively immune to powdery mildew. In June, it produces large, airy inflorescences of pink flowers that rise 1-2' above the mass of foliage and continue to bloom for about six weeks. However, the most interesting thing about this species is its rhizomatous habit. Largeleaf phlox produces underground runners that slowly spread outward to form a large mass, similar to the way bee balm (Monarda spp.) grows, though not as aggressive.

Such a habit could be very beneficial in meadows and naturalistic plantings where largeleaf phlox can be allowed to meander.

Much of the outstanding garden performance of this species likely comes from its ability to grow in drier soils, especially compared to Phlox paniculata which requires consistently moist conditions. In the wild, Phlox amplifolia can be found growing in dry-mesic upland sites from Indiana south to Alabama, and from Arkansas east to Virginia. Further selection and perhaps hybridization of Phlox amplifolia could lead to more adaptable summer-blooming phlox for the garden.

Phlox 'Minnie Pearl' «««« Phlox 'Minnie Pearl' was discovered along a Mississippi roadside by Karen Partlow and was originally thought to be a naturally occurring hybrid between Phlox maculata and Phlox glaberrima. However, more recent research suggests it is a selection of P. carolina ssp. carolina. This cultivar was introduced by Plant Delights Nursery who named it after famed country comedienne, Minnie Pearl.

It grows approximately 2' tall and is covered with snow white flowers in late spring. The mildew-free foliage is also a glossy, dark green which adds to its appeal even after the flowers are finished.

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GARDEN CULTURE Where to Plant The most important consideration for healthy phlox is to plant it in the proper site. Most species of phlox, such as P. paniculata, P. carolina, and P. maculata, prefer consistently moist, fertile soil. This closely mimics the conditions found along river courses and floodplain edges where these species naturally occur. In a garden setting, when the soil dries out these species are more likely to develop disease. This is especially true for powdery mildew on P. paniculata. Other species like P. amplifolia and P. pulchra can tolerate slightly drier conditions, and while they still prefer moist soil, they don’t require as much consistency as P. paniculata. Phlox pilosa prefers relatively dry, well-drained soil and can rot if grown in soil that stays wet for long periods of time. In terms of sun exposure, all of the above mentioned species do best in full sun to light shade.

Powdery Mildew Powdery mildew is a disease common to many garden plants and is the main concern gardeners have when growing garden phlox. It is ubiquitous in the environment and only requires proper weather conditions to develop on the leaves and stems of susceptible hosts. Powdery mildew usually first appears when days are warm and nights are cool. Cool nights provide the high humidity that enables spore germination (initiation of new infections) and warm days provide the low humidity for spore dispersal (spreading of existing infections).

The disease itself is typically only cosmetic. Lower leaves are the first ones affected and frequently fall off by midsummer. If an infection becomes more severe, the defoliation will extend further up the plant. In extreme cases, plants can lose so many leaves that they are unable to survive the winter. Instances like this were observed in Mt. Cuba Center’s trial; however, this level of severity is unlikely to occur in most home gardens where there are not hundreds of phlox growing in close proximity.

Site selection, as mentioned above, is critical in preventing disease development. However, the easiest way to avoid powdery mildew on Phlox paniculata is to select disease resistant cultivars like 'Jeana', 'Delta Snow', and 'David'. The powdery mildew resistance ratings listed on the chart on pages 14-15 are based solely on observations from our trial. Because of the added disease pressure caused by growing so many phlox in one location, the disease ratings may be more severe than what most gardeners will likely experience.

Spider Mites Spider mites are small arachnids that feed on plant juices inside the leaves. Phlox paniculata can be prone to spider mite outbreaks, especially when the weather is hot and dry. Natural populations of beneficial mites typically help to control spider mites. However, periods of hot, dry weather can cause spider mite populations to increase much faster than the beneficial mites. Active infestations cause yellowing of the foliage and often produce webbing on the undersides of the leaves.

Most home gardeners will not see significant outbreaks like what was observed in Mt. Cuba Center’s trial simply because we had uncommonly high numbers of phlox in one garden. If damage does become significant, spider mites can be controlled without chemicals by spraying a hard, steady stream of water upwards at the undersides of the leaves. This knocks down enough mites to keep damage at tolerable levels.

Design When deciding where to plant phlox in your garden it’s important to think about how tall the plants will be when they bloom and more importantly, how tall the neighboring plants will be. This is especially valuable since neighboring plants can be used to obscure the loss of any lower leaves due to powdery mildew or drought stress.

There are cultivars available in almost any height, so phlox can easily be incorporated into any garden.

Fragrance is also an important consideration when designing with phlox. Strongly fragrant selections, like those listed on pages 14-15, have a scent that is noticeable several feet away. This can greatly add to the enjoyment you receive while spending time on a patio or sitting area.

Maintenance Phlox are relatively easy to maintain throughout the year, especially when planted in a cool and moist, yet sunny location. In periods of drought, additional water may be necessary to keep plants healthy and disease free. Aside from that, phlox can be cut back to the ground in the fall or late winter each year. In the proper location, they are reliably hardy throughout the mid-Atlantic region.

Powdery mildew

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Spider mite damage is visible on the top of the leaf as an interveinal yellowing, while webbing can often be observed on the underside.

SUN PHLOX Visitor Favorites Mt. Cuba Center visitors were asked to participate in our phlox research by picking three of their favorite plants in the trial.

The goal of this program is to provide direct feedback from the consumer to the nursery industry about which plants the gardening public finds most exciting. Over 400 votes were collected, and the five most favored cultivars, in order, were: 'Jeana', 'Ditomdre' (Coral Crème Drop), 'Blushing Shortwood', 'Dasfive' (Ice Cap), and 'Babyface'. All five of these cultivars were either top perfomers or additional recommended selections.

A dark-form female Eastern Tiger Swallowtail butterfly feeds on Phlox paniculata 'Blushing Shortwood'. Some females have evolved this darker coloring to mimic their poisonous relative, the Pipevine Swallowtail, and avoid predation.

About the Sun Phlox Trial This evaluation took place at Mt. Cuba Center, located near Wilmington, DE (USDA Hardiness Zone 7a/6b). Ninety-four taxa representing eight species were trialed for a three-year period (2015-2017). Plants were evaluated to assess their habit, vigor, floral display, and disease resistance. Five plants of each taxon were spaced linearly on 2' centers. They were grown in full sun in a soil best described as clay-loam with a pH near 6.5. Each species or cultivar was measured weekly and assigned three different ratings, each on a scale of 1-5 (1 being very poor and 5 being excellent). The floral display rating was based on flower coverage and overall appeal and then adjusted for bloom periods longer or shorter than average. The rating for plant/foliage quality included attributes like habit, vigor, and foliage retention. The rating for powdery mildew resistance was based on the amount of foliage affected by the disease. The plant and adjusted floral ratings were then averaged, after which points were added or deducted for powdery mildew resistance. Points were also deducted for deaths of two or more plants. Throughout the sun phlox trial, plants were given minimal care. No fungicides were used, and supplemental water was provided only during the first year to encourage establishment and during any extremely dry periods. This strategy is designed to test the plants in a manner similar to how most landscapes are maintained. However, many of these plants would have performed better with more frequent water. mtcubacenter.org

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PLANT CHARACTERISTICS & PERFORMANCE SUMMARY RATINGS Phlox

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Rating

Avg. HxW

Flower Color

Bloom Time

Powdery Mildew Resistance

Fragrance

P. amplifolia

4.1

««««

44” x 36”

pink

early June–mid-July

excellent

P. 'Aurora'

3.7

««««

24” x 30”

white w/pink eye

early June–late July

excellent

P. carolina

3.3

««««

30” x 30”

pink

late July–late Aug

excellent

P. carolina 'Bill Baker'

4.3

«««««

24” x 36”

pink

late May–early June

excellent

P. carolina 'Gypsy Love'

3.2

«««

28” x 20”

pink

mid-June–early July

excellent

P. carolina 'Lil’ Cahaba'

3.4

««««

24” x 40”

pink

mid-June–late June

excellent

P. carolina ssp. alta

3.4

««««

30” x 20”

pink

late June–early July

excellent

P. carolina ssp. carolina

3.4

««««

24” x 16”

pink

early June–early July

excellent

P. carolina ssp. carolina 'Kim'

4.5

«««««

24” x 48”

pink

late May–mid-June

excellent

P. 'Forever Pink'

4.3

«««««

24” x 32”

pink

late May–mid-June

excellent

P. glaberrima 'Morris Berd'

4.3

«««««

24” x 36”

pink

late May–early June

excellent

P. glaberrima 'N³ Tasahce rvfo hakof' (N³ Springfall)

4.3

«««««

28” x 36”

pink

mid-June–early July

excellent

P. glaberrima ssp. triflora

3.5

««««

24” x 36”

pink

late May–early June

excellent

P. glaberrima ssp. triflora 'Triple Play'

3.3

««««

20” x 36”

pink

late May–early June

excellent

P. maculata 'Alpha'

3.2

«««

36” x 24”

pink

mid-June–early July

fair

P. maculata 'Flower Power'

3.1

«««

40” x 30”

white

late June–early July

fair

mild

P. maculata 'Natascha'

3.4

««««

40” x 24”

pink & white

late June–early July

excellent

mild

P. 'Minnie Pearl'

4.2

««««

24” x 36”

white

late May–mid-June

excellent

mild

P. 'Miss Lingard'

2.8

«««

30” x 30”

white

mid-June

very poor

P. ovata

2.8

«««

20” x 24”

pink

early May–late May

excellent

P. paniculata 'Alexandra'

2.1

««

36” x 24”

pinkish red

late June–mid-July

poor

strong

P. paniculata 'Barfourteen' (Purple Flame)

3.8

««««

30” x 24”

pinkish purple

late June–late July

good

mild

P. paniculata 'Barsixtyone' (Violet Flame)

3.5

««««

24” x 24”

dark lavender

late June–mid-July

fair

strong

P. paniculata 'Barsixtytwo' (Coral Flame)

3.4

««««

18” x 18”

coral

late June–late July

good

mild

P. paniculata 'Barthirtyone' (Volcano Ruby)

2.8

«««

30” x 28”

pinkish purple

late June–late July

excellent

mild

P. paniculata 'Barthirtysix' (Volcano Red)

3.1

«««

40” x 28”

red

early July–late July

good

strong

P. paniculata 'Barthirtytwo' (Volcano White)

3.8

««««

24” x 20”

white

late June–mid-July

fair

mild

P. paniculata 'Bartwelve' (Pink Flame)

3.3

««««

30” x 24”

pink w/dark pink eye

early July–late July

good

mild

P. paniculata 'Bartwentynine' (White Flame)

3.2

«««

20” x 16”

white

early July–late July

fair

mild

P. paniculata 'Blue Paradise'

3.5

««««

36” x 30”

violet blue

late June–mid-July

good

strong

P. paniculata 'Blushing Shortwood'

3.6

««««

44” x 30”

white w/pink blush

mid-July–late Aug

fair

mild

P. paniculata 'Bright Eyes'

3.0

«««

44” x 30”

lt. pink

late July–mid-Aug

good

mild

P. paniculata 'Dasfive' (Ice Cap)

3.4

««««

40” x 24”

white

mid-July–mid-Aug

good

mild

P. paniculata 'David'

3.8

««««

34” x 24”

white

early July–early Aug

excellent

strong

P. paniculata 'David’s Lavender'

3.1

«««

52” x 36”

pinkish lavender

late July–late Aug

good

mild

P. paniculata 'Delilah'

2.8

«««

24” x 24”

purple

early July–late July

fair

mild

P. paniculata 'Delta Snow'

4.2

««««

48” x 40”

white w/ purple eye

early July–late Aug

excellent

mild

P. paniculata 'Dick Weaver'

3.9

««««

50” x 36”

magenta

early July–late Aug

good

strong

P. paniculata 'Ditomdre' (Coral Crème Drop)

3.8

««««

36” x 28”

coral

mid-July–late Aug

fair

mild

P. paniculata 'Ditomfav' (Cotton Candy)

3.3

««««

30” x 24”

lt. pink w/dark pink eye

early July–mid-Aug

poor

P. paniculata 'Ditomfra' (Bubblegum Pink)

3.3

««««

30” x 24”

pink

early July–early Aug

excellent

strong

P. paniculata 'Ditomsur' (Grape Lollipop)

2.6

«««

24” x 14”

reddish purple

early July–late July

excellent

mild

P. paniculata 'Düsterlohe' (Nicky)

2.9

«««

34” x 24”

purple

late June–late July

fair

moderate

P. paniculata 'Eva Cullum'

3.0

«««

48” x 34”

pink w/dark pink eye

mid-July–mid-Aug

fair

P. paniculata 'Franz Schubert'

2.4

«««

24” x 24”

not observed

did not flower

good

P. paniculata 'Glamour Girl'

4.3

«««««

36” x 30”

coral pink

early July–late Aug

good

mild

P. paniculata 'Grenadine Dream'

3.2

«««

36” x 24”

pinkish red

late June–late July

good

mild

P. paniculata 'Jeana'

4.8

«««««

60” x 48”

pink

mid-July–early Sept

excellent

strong

mild

SUN PHLOX Phlox

Rating

Avg. HxW

Flower Color

Bloom Time

Powdery Mildew Resistance

Fragrance

P. paniculata 'John Fanick'

3.2

«««

54” x 40”

lt. pink w/pink center

mid-July–mid-Aug

fair

P. paniculata 'Juliglut' (July Glow)

2.0

««

34” x 24”

pinkish red

late July–mid-Aug

good

P. paniculata 'Junior Dance'

3.1

«««

26” x 20”

coral pink

late June–mid-July

poor

P. paniculata 'Katherine'

3.0

«««

30” x 28”

pale lavender

late June–late July

good

mild

P. paniculata 'Laura'

3.4

««««

30” x 20”

purple w/ white eye

early July–early Aug

good

mild

P. paniculata 'Lavelle'

4.1

««««

48” x 36”

white w/ pink tube

early July–early Sept

good

mild

P. paniculata 'Light Blue Flame'

3.0

«««

28” x 28”

pale blue

mid-June–mid-July

good

moderate

P. paniculata 'LSS Vincent'

3.7

««««

44” x 30”

pink

early July–late Aug

good

mild

P. paniculata 'Orange Perfection'

3.1

«««

28” x 20”

orange

late July–mid-Aug

excellent

P. paniculata 'Party Punch'

3.5

««««

36” x 32”

pink

late June–late July

fair

moderate

P. paniculata 'Peacock Cherry Red'

2.8

«««

36” x 28”

pinkish red

early July–late July

poor

mild

P. paniculata 'Pina Colada'

3.0

«««

28” x 16”

white

late June–mid-July

poor

moderate

P. paniculata 'Pink Lady'

3.4

««««

36” x 30”

pink w/white eye

early July–mid-Aug

good

moderate

P. paniculata 'Pixie Twinkle'

3.0

«««

16” x 16”

pink w/dark pink eye

early July–late July

good

mild

P. paniculata 'Purple Kiss'

3.3

««««

32” x 24”

pinkish purple

early July–late July

good

mild

P. paniculata 'Purple Rain'

3.3

««««

32” x 24”

pinkish purple

late June–late July

good

moderate

P. paniculata 'Red Caribbean'

2.4

«««

32” x 24”

pink w/red eye

late June–mid-July

poor

mild

P. paniculata 'Red Riding Hood'

2.3

«««

32” x 28”

red

early July–late July

poor

mild

P. paniculata 'Robert Poore'

3.9

««««

60” x 48”

magenta

early July–mid-Aug

good

strong

P. paniculata 'Shortwood'

3.8

««««

54” x 36”

pink

late July–early Sept

fair

mild

P. paniculata 'Speed Limit 45'

3.5

««««

44” x 30”

lt. pink

late July–mid-Aug

good

mild

P. paniculata 'Tequila Sunrise'

3.1

«««

32” x 32”

coral

mid-July–early Aug

fair

moderate

P. paniculata 'Thai Pink Jade'

3.8

««««

42” x 28”

lt. pink w/ pink eye

mid-July–late Aug

fair

mild

P. paniculata 'The King'

2.7

«««

32” x 24”

purple

early July–late July

fair

mild

P. paniculata 'Tracy’s Treasure'

3.5

««««

44” x 30”

pink blush w/ pink eye

early Aug–mid-Sept

poor

P. paniculata 'Twister'

2.8

«««

36” x 30”

white & pink

early July–late July

good

P. paniculata 'Younique Bicolor'

2.3

«««

28” x 20”

pink blush

late July–early Aug

good

P. pilosa

2.8

«««

12” x 18”

pinkish lavender

mid-May–early June

excellent

P. pilosa ssp. ozarkana

2.7

«««

20” x 36”

pink

late May–mid-June

fair

P. pulchra 'Bibb Pink'

3.1

«««

20” x 20”

pink

early June–mid-June

excellent

P. 'Sherbet Cocktail'

2.5

«««

36” x 20”

pink, white, green

early July–late July

good

P. 'Solar Flare'

4.1

««««

24” x 32”

white

mid-June–late June

excellent

P. × arendsii 'Babyface'

4.2

««««

32” x 32”

pink w/ dark pink eye

late July–late Aug

good

P. × arendsii 'Susanne'

1.1

«

32” x 30”

not observed

did not flower

poor

Rating Key: 5=excellent, 4=good, 3=fair, 2=poor, 1=very poor Plants in bold are the top-performing selections.

mild

mild

mild

mild

Fragrance rated as strong, moderate, or mild. No desgination means no fragrance observed.

Visit mtcubacenter.org/research/trial-garden for detailed information about each plant.

Plants That Did Not Complete Trial Phlox P. glaberrima 'N³ Hvt-ke Mes-ke'

Reason Only able to evaluate one plant.

(N³ White Summer)

Phlox

Reason

P. paniculata 'Lord Clayton'

Poor establishment and died.

P. paniculata 'Red Super'

Poor establishment and died.

P. paniculata

Not true to type.

P. paniculata 'Tiara'

All plants died.

P. paniculata 'Bareightysix' (Blue Flame)

All plants died.

P. paniculata 'White Admiral'

Abnormal growth habit, possibly virused.

P. paniculata 'Barsixty'

All plants died.

P. pilosa 'Bungalow Blue'

All plants died.

P. pilosa 'Eco Happy Traveler'

All plants died.

P. pilosa 'Slim Jim'

All plants died.

(White Eye Flame) P. paniculata 'Barthirtythree' (Purple Eye Flame)

Only one out of five plants was true to type.

mtcubacenter.org

15

POLLINATOR ATTRACTION

P

hlox is frequently listed as a great plant for attracting butterflies to the garden. Its funnel-shaped flower tubes are filled with the sugary nectar that butterflies feed on. However, Phlox paniculata, the species most utilized by butterflies, has been changed from its wild form to accommodate varied preferences in flower color and habit size.

Throughout the process of developing those selections, little attention was paid to whether butterflies were still able to utilize the plants. In order to assess the ecological value of Phlox paniculata cultivars, Mt. Cuba Center enlisted both the help of citizen scientists as well as a graduate student at the University of Delaware.

Mt. Cuba Center’s Pollinator Watch is a group of volunteers that monitor pollinator visitation in the trial garden in order to determine if any phlox species or selections are more preferred than others. With the ability to quantify which selections are most frequently visited by butterflies, Mt. Cuba Center can recommend to gardeners those plants that will attract and support the greatest number of pollinators in their own backyards.

Best Phlox for Butterflies (539)

This project was carried out over the second and third years of the trial in order to ensure consistent patterns of preference. Dedicated citizen scientists monitored the phlox trial in hour-long shifts and recorded the number of butterfly visits for each selection. Numerous shifts were performed weekly while the phlox were in bloom. The final counts were then averaged per plant so that cultivars with fewer than five plants were not disadvantaged.

The results of this work show that butterflies in our area overwhelming prefer Phlox paniculata 'Jeana' to every other cultivar in the trial. In fact, when averaged per plant, 'Jeana' received 539 visits over the two-year period, with the next highest cultivar being 'Lavelle' at 117 visits per plant. The chart on the right details the best cultivars for attracting butterflies. Interestingly, several of the topperforming selections from a horticultural perspective are also among the most preferred by butterflies. The relationship between horticultural performance and pollinator attraction is, in some ways, not surprising.

Traits such as flower production and increased bloom time provide more opportunities for pollinator visits, and heathier plants are likely better able to produce floral rewards such as the nectar.

Pollinator Watch volunteer recording butterfly visitation

16

1. P. paniculata 'Jeana' * (117)

2. P. paniculata 'Lavelle' *

(116)

3. P. paniculata 'John Fanick'

(105)

4. P. × arrendsii 'Babyface' *

(71)

5. P. paniculata 'Speed Limit 45'

(61)

6. P. paniculata 'Dick Weaver' *

(60)

7. P. paniculata 'Blushing Shortwood'

(58)

8. P. paniculata 'Tracy’s Treasure'

(56)

9. P. paniculata 'Thai Pink Jade'

(55)

10. P. paniculata 'David’s Lavender'

(50)

11. P. paniculata 'Robert Poore' *

(48)

12. P. paniculata 'LSS Vincent'

(45)

13. P. paniculata 'Shortwood' *

(43)

14. P. paniculata 'Party Punch'

(43)

15. P. paniculata 'Delta Snow' *

This chart shows the 15 cultivars most frequently visited by butterflies in Mt. Cuba Center’s trial from 2016-2017. The number on the left indicates the total number of butterfly visits per plant observed over that two-year period.

Top-performers from a horticultural perspective are denoted with a asterisk (*).

When selecting plants for butterfly attraction, bloom time is another important attribute to consider. At Mt. Cuba Center, most butterflies first appear in early to mid-July.

This coincides with the bloom time of many selections of Phlox paniculata, which is why they are great choices for butterfly gardens. However, some cultivars of P. paniculata, as well as several other species of phlox, bloom prior to July. These early blooming phlox are often finished, or at least past peak, by the time butterflies arrive and are thus not the best choices for attracting butterflies.

American Lady butterfly

SUN PHLOX Swallowtail Host Plants Eastern Tiger Swallowtail American hornbeam (Carpinus caroliniana) Ash (Fraxinus spp.)

Birch (Betula spp.)

Elm (Ulmus spp.)

Hawthorn (Crataegus spp.)

Maple (Acer spp.)

Serviceberry (Amelanchier spp.)

Sweet bay (Magnolia virginiana) Tulip tree (Liriodendron tulipifera) Black cherry (Prunus serotina) Willow (Salix spp.)

Spicebush Swallowtail Eastern Tiger Swallowtail nectaring on Phlox paniculata 'Jeana'

It’s relatively simple to determine which selections are most preferred by butterflies. However, it is much more difficult to determine why 'Jeana' is preferred to such a greater extent than other similarly healthy and floriferous cultivars. In fact, the incredibly small flower size of 'Jeana' might make you suspect it would be the least loved, since the flowers are so atypical for Phlox paniculata.

A graduate student at the University of Delaware, Keith Nevison, attempted to answer this question by examining the nectar produced by 'Jeana' and several other cultivars. In theory, a greater quantity of nectar or a higher concentration of sugar would be a more desirable food source for butterflies, thus attracting more to the plant. After measuring these qualities, Nevison found no correlation between the increased visitation of 'Jeana' and its nectar volume or sugar content relative to other cultivars. Further sampling by Mt. Cuba Center intern, Caitlynd Krosch, supported Nevison’s assessment that these nectar characteristics are not good predictors of butterfly preference.

Spicebush (Lindera benzoin) Common sassafras (Sassafras albidum) Sweet bay (Magnolia virginiana) Tulip tree (Liriodendron tulipifera)

Host Plant Spotlight A fantastic small tree that can be added to almost any home landscape to support swallowtail butterflies is sweet bay (Magnolia virginiana).

A mature tree grows approximately 20’ tall and bears large (2-3” wide), white flowers in summer.

Although still unproven, the theory put forth by Nevison is that perhaps the smaller flower size, especially the narrowness and shallowness of the flower tube, is actually preferable because it allows butterflies to quickly access the nectar of many flowers without moving as frequently.

Of course, supporting butterflies is not as simple as planting a garden full of Phlox paniculata 'Jeana'. While this cultivar is great at feeding adult butterflies, it is important to remember that all butterflies start out as caterpillars—many of which require specific host plants in order to become winged adults. Swallowtails were some of the most frequent visitors to our phlox trial and especially 'Jeana'. The caterpillar stage of these butterflies feed on several species of native trees (see sidebar).

It’s easy to support the entire lifecycle of these pollinators simply by including one of their host plants in your landscape. mtcubacenter.org

17

TOP PERFORMERS

I

n addition to sun-loving phlox, Mt. Cuba Center also tested selections of woodland phlox (Phlox divaricata) and creeping phlox (Phlox stolonifera). Both shade-loving species are prized for their beautiful spring flowers that bloom in April and May. Many selections have been made over the years for different flower colors; however, only a few are commonly offered at garden centers. From 2015-2017, Mt. Cuba Center trialed 21 selections of P. divaricata, 12 selections of P. stolonifera, as well as 10 hybrid cultivars for their overall garden performance.

In general, we found most of the woodland phlox (P. divaricata) cultivars to be somewhat challenging to grow.

Powdery mildew was a significant problem with woodland phlox, such that they would severely defoliate right after blooming. The first time this happened, we thought the plants had died, but they did return the following spring. The yearly stress caused by the disease seemed to prevent many selections from increasing in size from year to year. However, two selections that were very successful were the straight species Phlox divaricata and the cultivar Phlox divaricata 'Blue Moon'. It should be noted that the straight species may not perform similarly for everyone since each plant will be different depending on its source.

One possible explanation for the difficulties experienced with woodland phlox is that the initial plants were sometimes not in the healthiest condition when they were planted. This weak start may have been enough to hamper their performance for the rest of the trial.

Therefore, it is recommended to only plant vigorous, actively growing plants of woodland phlox in order to ensure better establishment in the garden.

Creeping phlox (Phlox stolonifera) proved much easier to grow than woodland phlox. Of the 12 Phlox stolonifera selections we evaluated, 'Fran’s Purple', 'Sherwood Purple', 'Pink Ridge', and 'Home Fires' were the best-performing cultivars. Each produces a dense mat of ground-hugging foliage as well as an impressive display of large, brightly colored flowers. The rhizomatous, ground-covering habit is also a useful feature that can be used in the garden to suppress weeds. At Mt. Cuba Center, we have been particularly successful in pairing Phlox stolonifera with plants like Pennsylvania sedge (Carex pensylvanica), foamflower (Tiarella cordifolia), and maidenhair fern (Adiantum spp.) to create dynamic interwoven plantings that remain attractive across multiple seasons.

Tiarella cordifolia (foreground) paired with Phlox stolonifera (background)

P. stolonifera 'Pink Ridge'

P. stolonifera 'Fran’s Purple'

Phlox divaricata 'Blue Moon'

18

P. stolonifera 'Home Fires'

P. stolonifera 'Sherwood Purple'

SHADE PHLOX About the Shade Phlox Trial This evaluation took place at Mt. Cuba Center, located near Wilmington, DE (USDA Hardiness Zone 7a/6b). Forty-three taxa representing Phlox divaricata, Phlox stolonifera, and several related hybrids were trialed over a three-year period (2015-2017).

Plants were evaluated to assess their habit, vigor, and floral display. Five plants of each taxon were spaced linearly on 2' centers. They were grown under a shade structure that provided uniform shading of 60%. In a garden setting this would be most similar to dappled shade or an area that receives only a couple of hours of direct sunlight each day. The plants were grown in a soil best described as clay-loam with a pH near 6.5. Each species or cultivar was measured weekly during the spring and monthly during the rest of the growing season. Ratings were assigned on a scale of 1-5 (1 being very poor and 5 being excellent). The floral display rating was based on the percentage of the plant covered in flowers while the plant/foliage rating accounted for attributes like habit, vigor, and foliage retention. The plant and floral ratings were then averaged, after which points were deducted for deaths of two or more plants. Throughout the shade-loving phlox trial, plants were given minimal care. No fungicides were used, and supplemental water was provided only during the first year to encourage establishment. This strategy is designed to test the plants in a manner similar to how most landscapes are maintained.

Plant Characteristics & Performance Summary Ratings Phlox

Rating

Avg. Height

Avg. Width

Flower Color

Bloom Time

P. 'PPPHL0604' (Britney)

2.8

«««

6”

24”

pink

April–May

P. 'PPPHL0617' (Miley)

2.5

«««

8”

24”

pinkish purple

April–May

P. 'PPPHL07101' (Lindsay)

2.9

«««

8”

30”

pink

April–May

P. 'Spring Delight'

2.6

«««

20”

30”

pink

late May–early June

P. 'Violet Pinwheels'

3.1

«««

4”

18”

violet

April–May

P. divaricata

3.5

««««

18”

40”

lavender

April–May

P. divaricata 'Blue Dreams'

2.2

««

12”

18”

bluish lavender

April–May

P. divaricata 'Blue Elf'

2.2

««

6”

16”

light purple

April–May

P. divaricata 'Blue Moon'

3.5

««««

16”

32”

lavender

April–May

P. divaricata 'Blue Perfume'

2.2

««

14”

20”

light lavender

April–May

P. divaricata 'Charleston Pink'

3.2

«««

12”

28”

pink

April–May

P. divaricata 'Clouds of Perfume'

2.6

«««

12”

20”

lavender

May

P. divaricata 'Dirigo Ice'

2.8

«««

10”

18”

pale blue

April–May

P. divaricata 'Fuller’s White'

2.2

««

10”

14”

white

April–May

P. divaricata 'London Grove Blue'

3.1

«««

14”

28”

bluish lavender

April–May

P. divaricata 'Louisiana Blue'

2.3

«««

10”

16”

purple

April–May

P. divaricata 'Manita'

2.0

««

12”

16”

pale bluish lavender

April–May

P. divaricata 'Mary Helen'

2.6

«««

12”

20”

purple

April–May

P. divaricata 'May Breeze'

3.0

«««

16”

24”

white

April–May

P. divaricata 'Plum Perfect'

1.2

«

6”

16”

purple

April – May

P. divaricata 'Tika'

2.4

«««

12”

22”

dark lavender

April–May

P. divaricata 'Tinian Sprite'

3.1

«««

10”

18”

pink

April–May

P. divaricata 'White Perfume'

2.5

«««

12”

16”

white

April–May

P. divaricata "white"

3.0

«««

12”

30”

white

April – May

P. divaricata ssp. laphamii

2.8

«««

20”

36”

lavender

April–May

P. divaricata ssp. laphamii 'Chattahoochee'

2.8

«««

6”

20”

lavender w/ purple eye

April–May

P. stolonifera

2.8

«««

10”

increased 1-2' per year

pink

April–May

P. stolonifera 'Bruce’s White'

2.9

«««

8”

increased 1-2' per year

white

April

P. stolonifera 'Fran’s Purple'

4.2

««««

8”

increased 1-2' per year

purple

April

P. stolonifera 'Home Fires'

3.4

««««

12”

increased 1-2' per year

pink

April

P. stolonifera 'Irridescens'

2.8

«««

8”

increased 1-2' per year

purple

April–May

P. stolonifera 'Margie'

3.2

«««

8”

increased 1-2' per year

lavender

April

P. stolonifera 'Mary Belle Frey'

3.1

«««

10”

increased 1-2' per year

pink

April

P. stolonifera 'Pink Ridge'

3.5

««««

10”

increased 1-2' per year

pink

April

P. stolonifera 'Sherwood Purple'

4.0

««««

12”

increased 1-2' per year

purple

April

P. stolonifera 'Variegata'

1.0

«

4”

increased 1-2' per year

pink

April

P. × procumbens 'Pink Profusion'

3.3

««««

8”

24”

pink

April

Visit mtcubacenter.org/research/trial-garden for more detailed information about each plant.

The following cultivars died before completing all three years of the trial: Phlox 'Angelina', P. 'Green Spring', P. 'PPPHL07201' (Paris), P. 'PPPHL07301' (Gaga), P. stolonifera 'Blue Ridge', and P. stolonifera 'Weesie Smith'.

Rating Key: 5=excellent, 4=good, 3=fair, 2=poor, 1=very poor Plants in bold are the top-performing selections.

mtcubacenter.org

19

About Mt. Cuba Center Mt. Cuba Center is a botanical garden that inspires an appreciation for the beauty and value of native plants and a commitment to protect the habitats that sustain them.

Over the past 80 years the landscape at Mt. Cuba Center has been transformed from fallow cornfields into thriving, ecologically functional gardens, thanks to the initiative of the late Mr. and Mrs. Lammot du Pont Copeland.

The gardens at Mt. Cuba Center now represent a variety of habitats, from upland forests and meadows to lowland ponds. With its support of biodiverse communities, Mt. Cuba Center serves as a model for environmentally beneficial gardening. Mt. Cuba Center also conducts original research on native plants in the Trial Garden and manages over 500 acres of natural lands. Mt. Cuba Center is open for visitation April-November and classes are offered year-round.

About Trial Garden Research

3120 Barley Mill Road Hockessin, DE 19707 302.239.4244 mtcubacenter.org

Mt. Cuba Center’s Trial Garden, managed by George Coombs, evaluates native plants and their related cultivars for their horticultural and ecological value. The goal of this research is to provide gardeners and the horticulture industry with information about superior plants for the mid-Atlantic region as well as highlight the important ecosystem services native plants provide. Mt. Cuba Center has conducted trial garden research since 2002, including previously completed evaluations of bee balm, false indigo, coreopsis, heuchera, coneflowers, and asters.

References Armitage, Allan M. Herbaceous Perennial Plants: A Treatise on Their Identification, Culture, and Garden Attributes. 3rd ed. Stipes, 2008.

Cech, R., Tudor, G. Butterflies of the East Coast: An Observer’s Guide.

Princeton University Press, 2005.

Mader, E., Shepherd, M., Vaughn, M., Black, S., LeBuhn, G. Attracting Native Pollinators. Storey Publishing, 2011.

Hawke, R. (1998). "Plant Evaluation Notes: A Comparative Study of Phlox paniculata Cultivars," Chicago Botanic Garden, www.chicagobotanic.org/ downloads/planteval_notes/no35_phloxpaniculata.pdf.

Holme, Heather. Pollinators of Native Plants: Attract, Observe and Identify Pollinators and Beneficial Insects with Native Plants. Pollination Press, 2014.

Locklear, J. Phlox. Timber Press, 2011.

Nevison, Keith. "Considering a Role for Native Plant Cultivars in Ecological Landscaping: An Experiment Evaluating Insect Preferences and Nectar Forage Values of Phlox Species Vs. Its Cultivars." University of Delaware, 2016.

"Phlox 'Minnie Pearl'." Plant Delights Nursery, www.plantdelights.com "Phlox paniculata 'David'." New Moon Nursery, www.newmoonnursery.com "Phlox paniculata 'Dick Weaver'." Plant Delights Nursery, www.plantdelights.com "Phlox paniculata 'Delta Snow'." Niche Gardens, www.nichegardens.com "Phlox paniculata 'Jeana'." New Moon Nursery, www.newmoonnursery.com "Phlox paniculata 'Lavelle'." Lazy S’s Farm Nursery, www.lazyssfarm.com "Phlox paniculata 'Robert Poore'." North Creek Nurseries, www.northcreeknurseries.com

Front Cover: Phlox × arendsii 'Babyface' ©Mt. Cuba Center, 2017. All Rights Reserved.