GARDENING WITH ANDREW STEENS - OF SWANS AND BUTTERFLIES

Well, I was going to do an article on the effect of this wet summer on our crops, but to be fair, I’m sick of even thinking about rain! So, instead I’ll write about something entirely more uplifting and ethereal: Butterflies and swans.

I think most people love seeing the majestic Monarch butterflies flitting through their garden, and for kids young and old the almost magical process of changing from a fat stripey caterpillar to a gold studded green chrysalis and then a beautiful new butterfly never fails to entrance.

As most people will know, Monarch caterpillars feed exclusively on swan plants. The swan plant gets its common name from the swan shaped seed pods that form after flowering (kids can have swan races with these, as they float effortlessly on water, blown by the wind or a well-directed puff).

What most people don’t know though is that the common swan plant, Gomphocarpus fruticosa and the giant swan plant, Gomphocarpus physocarpus, are not the only members of the familyand there are in fact approximately 140 species worldwide with most of these native to North America.

In New Zealand, we can easily obtain seed or plants of two other species, Asclepias curassavica and A. incarnata, both of which are more attractive and wellbehaved plants in the garden compared to the somewhat rangy and plain swan plants.

Asclepias curassavica is commonly known as Tropical Milkweed, as it originates from the West Indies where its vibrant orange and red flowers are a common sight on roadsides. This species is sometimes grown as a cut flower so there are a wider range of colours that have been developed including dark pink, white and gold. Asclepias incarnata, with pink and white flowers, is known as the swamp milkweed and is more tolerant of wet soils. It develops a rhizome over time and is a useful early food source, however it loses its leaves later in the season so is not so useful for late caterpillars.

These are tough perennials that will last several years, unless slaughtered by too many caterpillars. They prefer rich organic soil, slightlyacidic, with frequent watering but not too wet and warm summers with temperate winters, so our conditions are normallyperfect. To avoid suffering from root rot due to excess rain, planting in raised beds is recommended, except for swamp milkweed.

Propagation of Asclepias is easy by seed and over time you will hopefully find seedlings pop up by themselves. It’s easy to pull them out where they are not wanted and just leave the others to grow as they will. Root and stem cuttings taken in spring are also possible if you need to bulk up a particular variety. Do remember though that the plants contain a toxic milky sap that can cause irritation to skin and eyes.

It is this chemical that makes the butterfly and its caterpillars toxic to predators. Except for paper wasps unfortunately. These voracious predators will clean out any small caterpillars so it is a good idea once you see some eggs or baby caterpillars to enclose the plant in a mesh bag. These are now available at most garden centers, with very handy drawstrings to tie around the stem. This is also a good technique for preventing the adult butterflies from laying too many eggs and stripping the plants of all their leaves.

Once the caterpillars are near full size, the mesh can come off, as paper wasps struggle to deal with large caterpillars. As the wasps get ready for winter, they switch from predating on caterpillars to other food sources and this is also when many caterpillars can reach full term, hopefully just before the winter cold causes any remaining chrysalis to abort.

In the last stage of the caterpillars’ life, cut pumpkin can be used as an emergency food source if the swan plants have been stripped. However, this is not recommended for small caterpillars as they will often develop deformities. Also, remember to replace with fresh pumpkin regularly, so they don’t feed on mouldy food.

Aside from the food source provided by the leaves, the flowers of Asclepias and common swan plants produce a large quantity of nectar which attracts Monarchs and other butterflies throughout the growing season. To keep the butterflies well fed, a good selection of other flowering plants in the garden is useful, particularly many of the compositae (daisy) family such Monarda, Rudbeckia, Echinacea, Gaillardia, Coreopsis and Tithonia.

As a bonus, a garden full of Asclepias and Compositae will not only attract and keep many butterflies, but also provide a palette of gorgeous summery colour in the garden for at least three quarters of the year.

At least that will cheer us up during this incessant rain!

Andrew Steens