BTA's Plant Collections

Revealing the Hidden Bounty of Boyce Thompson Arboretum’s Plant Collections

Text by: Becky Stephenson, Plant Collections Manager Photos by Becky Stephenson & Joseph Pacheco

Serving as only the second full-time dedicated plant records employee in history at a large arboretum founded in the early 1920’s is no small task! Since I started last summer, I have been working tirelessly to bring our plant records current and assess the diversity of our collection. With a total size of 343 acres (135 acres of which are cultivated), we are considered a very large operation as botanical gardens go. Over the course of a century, our horticulture staff has diligently tended to our many plants, removing the old and propagating the new as needed. It is now my job to determine what we currently have in our collection, evaluate its ecological importance, ensure it is being appropriately managed and displayed through new policies and protocols, and begin laying the groundwork for where we hope to go in the future.

Top photo: Astrophytum asterias blooming in the Cactus Display House. Above: Tammy Knight and Becky Stephenson auditing the aloe collection.

Photo by Joseph Pacheco

A plant collection is not a static landscape. It is essentially a living library, with each plant having its own unique accession number and qualifier—basically its social security number—which is tied to all of the information and history associated with the life of that plant: where it came from, how it was grown, its conservation status, its current health, and its activity throughout the years. Every individual plant in the collection has its own embossed metal accession tag, which is placed on or near it. This tag is the plant’s social security card and includes its species name, accession number, and qualifier. It has a very important job, withstanding both weather and time to serve as the physical link between the plant and its corresponding data.

Historically, this data was housed in logbooks and a system akin to a library’s card catalog. In our modern era of technology, it is now retained in a computer database specific to gardens, essentially a botanical social security database. Routine field checks are performed to assess each individual plant’s health and activity over time. Bloom dates, flower color, fruit output, growth rate, illness, relocations, deaths, resurrections, and countless other events—including the truly unusual such as being stolen by a packrat and replanted elsewhere (true story)—are all recorded and tracked year to year.

Logging information in this way allows trends to be identified and can prove invaluable to researchers. It tells the story of a plant’s life, and this story can then be compared with those of other members of its species, both at botanical gardens and in the wild. Many plants, like our large eucalyptus trees and saguaros, are exceptionally long-lived. Keeping accurate and detailed data on what these plants are doing today, even if it seems insignificant, is part of a much bigger picture. Unlike backyard gardens and landscapes where plants are grown mainly for their aesthetic value, a botanical garden keeps accurate and detailed records of every single plant in its assemblage for the purpose of fostering genetic diversity, promoting awareness and education, and sharing this knowledge with other institutions to assist with research and conservation efforts.

But not all accessions are created equal. Generally speaking, plants of wild provenance, that is, plants that were collected as adults or seed directly from their native ranges in the wild, are of greater value for both research and conservation efforts than cultivated plants because they have high genetic diversity. Cultivated plants, that is, plants that have been “domesticated” by humans, pull from a smaller gene pool and are also selectively bred for certain traits resulting in reduced genetic diversity. And genetic diversity is crucial for the long-term survival of a species. We are losing plant diversity at an accelerated rate as threats like urbanization, disruptive agricultural activity, poaching, and climate change increase. It is now estimated that one third of the world’s vascular plant species face extinction in the wild. Botanical gardens, therefore, have a very important role to play, ensuring that plant genetic diversity is stored either as seeds or living plants. At present, 3,595 plants represented by 907 taxa in the Arboretum’s collection are of wild provenance. A large portion of our Australian plants were collected on an overseas expedition during the 1990’s, our desert legumes were grown from seed collected in arid regions all over the world, many of our aloes were mail ordered from Africa, and several of our cacti were salvaged from the diminishing wilds of the desert southwest prior to their destruction.

Currently, there are 18,322 living plants with active records in the Boyce Thompson Arboretum (BTA) collection. Some of these plants, including many of our eucalyptus trees, were planted when the Arboretum was founded while others were acquired as recently as this year. As of today, 3,918 different taxa (that is, species, subspecies, varieties, and cultivars) are represented, including 381 species of known conservation concern. These numbers, like a library’s holdings, are dynamic as plants are lost and added. While many talented people have (luckily!) kept various versions of records on our plants over the years, we have only recently devoted a fulltime staff member to managing the collection’s data. As a result, there is a lot of information to sort through. I am in the process of conducting “detective work” – interviewing long-standing staff members who possess vital and irreplaceable knowledge of our plants, reviewing old notebooks and documents, performing plant inventories, poring over old maps, and conducting process-ofelimination analysis. Many plants that have been growing in the BTA is a member of Botanic Gardens Conservation International (BGCI), which is the world’s largest plant conservation network. This joins our collections to those of 600 other botanical institutions in 100 different countries—a combined force of 60,000 plant scientists and horticulturists (and even more plants) from all over the globe. In addition, we currently hold three collections nationally accredited through the American Public Garden Association’s Plant Collections Network (PCN): our eucalyptus collection, our southwestern oak collection, and our desert legume collection. Accredited collections actively promote collaboration and allow for both knowledge and plant material to be shared through specialized networks. We have been able to trade plants and work with other gardens on research projects and conservation efforts on a national level with these affiliations. This offers a glimpse of the many opportunities a well-maintained plant collection affords.

Aloe Flowers

Our collections are already sought after by the scientific community. The Ephedra collection, acquired from H.B. Wallace, has been used in a taxonomic study at the University of Alaska and plant material from our collections has been exchanged with researchers at gardens as far as Germany. Data on our plants is currently being utilized by the Desert Lab in Tucson, Arizona as part of a project aiming to conserve the wild plants that gave rise to crops in the Sonoran Desert. In addition, over 350 species in our collection are threatened with extinction in the wild. Caring for and propagating these plants and working with a network of other gardens will help conserve these species in a variety of ways, from potentially reintroducing genetic diversity into wild populations to increasing their numbers in cultivation to diminish the incentive for poaching.

We are in the process of identifying and quantifying additional collection opportunities. An audit of our sizable aloe collection, which could potentially be one of the largest outside of Africa, where most aloes are native, is currently in progress. Several aloe experts have worked at BTA over the years, leading to a surprisingly significant accumulation of these succulent plants. Our current resident aloe expert is Horticultural Specialist Tammy Knight, whose family donated hundreds of plants to BTA–many of which have wild provenance. At present, there are just over 300 taxa in the aloe collection, including 34 species of conservation concern. I expect this number to be even larger when the audit is complete. Next, we will conduct an audit of our diverse cycad collection. This will allow us to work with other institutions to conserve these very imperiled plants which are considered the most threatened group of plants in the world. We hope to get both the aloe and cycad collections accredited through the PCN in the coming years.

Astrophytum ornatum blooming in the Cactus Display House

Photo by Becky Stephenson

Acanthocalycium spiniflorum blooming in the Cactus Display House

Photo by Becky Stephenson

Opuntia macrocentra blooming in the Wallace Desert Garden

Photo by Becky Stephenson

The future for BTA’s plant collections is bright. Based on my current projections, about one third of our existing plants are not yet accessioned, so a full inventory of the entire collection is underway. This will add many plants to the records, and will allow us to accurately manage and report our complete diversity of taxa. Plants will then be mapped and updated records will be shared with other institutions and eventually the public, allowing for more collaborative conservation and research opportunities which will further increase educational capabilities. Thus far, the rare and endangered plant collection, the Wallace Desert Garden, the eucalyptus collection, the southwestern oak collection, and the two display greenhouses containing specialty cactus and succulents from around the world have been fully assessed. In addition, a plan is in the works to survey the entire property (all 343 acres) to inventory our native flora for the first time in BTA history. Many of these Sonoran Desert plants are threatened with extinction, and identifying them will allow us to conserve native plants in their natural habitat in addition to the many other species we house and protect outside of their native ranges.

Bringing our records current will open countless doors as we strive to become a leading botanical institution. We will soon be able to add our comprehensive collection data to plant conservation networks like BGCI, allowing us to play a more instrumental role in saving plants from extinction, and actively participate in more research projects encompassing everything from cultivating new agricultural crops to identifying novel medicinal compounds. We have known for years that our large, diverse collection is special—and it is time the world knew it too!

Acanthocalycium spiniflorum blooming in the Cactus Display House

Photo by Becky Stephenson

Aloe helenae in the Succulent Display House

Photo by Joseph Pacheco

Obregonia denegrii blooming in the Cactus Display House

Photo by Becky Stephenson