Common Milkweed (Asclepias syriaca) and Butterflyweed (Asclepias tuberosus) keeping company in an old field.
A monarch butterfly nectaring on common milkweed flowers (A. syriaca).
It’s all about chemistry when it comes to the relationship between a caterpillar and the leaves it eats. Through the process of coevolution, a caterpillar is able to overcome the defensive chemicals of a certain plant; this host plant responds with more chemicals, and is able to avoid complete defoliation. The chemistry between Monarch and Milkweed is one of nature’s most famous and amazing relationships: not only has the monarch caterpillar adapted to consuming the milkweed’s toxic leaves, but it has evolved to literally incorporate the toxins. The caterpillar assumes the plant’s chemistry, becoming as poisonous to predators as the plant is to most fauna (including livestock and people).
By adapting to the chemical make-up of milkweed, the monarch has become an extreme specialist, feeding solely on the genus Asclepias. Consequently, as milkweed has diminished in North America as a result of mowing, herbicide-spraying and loss of habitat, so have the monarch butterflies. With the decrease in monarchs, we may lose one of the greatest wonders of the natural world, the monarch migration.
(Left) A tiny monarch caterpillar eating swamp milkweed flowers. (Below) An older and larger caterpillar on butterflyweed.
We can help counteract the threat to the monarch population in our own yards by planting milkweeds--showy summer wildflowers that are easy to grow and care for, are deer-resistant, include species to suit almost any light and soil situation, and produce copious nectar for all kinds of pollinators, including bees, a variety of butterflies, and the odd hummingbird.
There are fifteen milkweed species indigenous to Indiana, ranging from Sand Milkweed to Swamp Milkweed, and to woodland species like the dainty Four-leaved Milkweed that blooms in our southern Indiana forests. Ten of these native milkweeds are available commercially, thanks to local sources such as Stranger’s Hill Organics and Ecologic’s seasonal sales, as well as online native plant nurseries, which usually list species for each geographic area.
Be aware that although conventional garden centers may carry the orange Butterflyweed, the plants may not be pesticide-free (deadly for leaf-eating caterpillars). Also avoid planting the exotic Balloon or Swan milkweeds. Yes, monarch caterpillars will eat them, but this may be to the butterflies’ detriment. Monarch authority Lincoln Brower posits that the butterflies leave a chemical fingerprint on their Mexican overwintering trees that helps guide the returning generation back to the same location. Feeding on exotic milkweeds alters this chemical fingerprint so much that it could disrupt the monarch migration. In California and the southern U.S., planting nonnative tropical milkweed north of its natural range has already created havoc among monarch populations; the year-round bloom has altered the monarchs’ breeding and migration patterns, and these changes have made the butterflies more susceptible to parasites.
There are plenty of milkweeds to choose from among our Indiana species, anyway. If you don’t have the right habitat or room for the classic Common Milkweed, a robust spreader with large clusters of fragrant, light purple flowers, try one of the following species:
Bumblebees pollinating butterflyweed (A. tuberosus)
Butterflyweed: Like Common Milkweed, the drought-tolerant orange Butterflyweed is a lover of full sun and well-drained soil. It can be started from seeds (found at most garden centers) or transplanted when young, but its taproot makes moving a mature plant problematic. Butterflyweed does not have milky sap, and this makes it more prone to being nibbled on by deer than other species.
Swamp milkweed (A. incarnata): A gorgeous plant that offers pollen for bees and butterflies.
Swamp or Marsh Milkweed: Carnation-pink flowers make this a beautiful milkweed for the rain garden or any low wet area, although it can adapt to medium soils. Grow in full sun to part shade. Flowers are sometimes white. In my yard, this species is more susceptible than others to the bright orange oleander aphid, itself a remarkable example of insects incorporating plant toxins to protect themselves from predators. Be careful using insecticidal soaps or oils on the aphids, as even organic pesticides can harm soft-bodied larvae of other species (such as the monarch). I've developed the sophisticated control method of squashing them digitally.
Stunning purple milkweed (A. purpurascens) growing in open woods.
Purple Milkweed: A sturdy, vigorous milkweed of open woods and thickets, with magenta flowers closely packed on domed umbels. Grow in part shade, in sandy to medium soil. It may spread to create small colonies.
Poke Milkweed (A. exaltata), a subtle, shade-loving species.
Poke milkweed: A tall, lanky milkweed of woodlands, this milkweed can be grown in full shade but prefers dappled sunlight and soil rich in organic matter. Its pinkish-white flowers are held in loose, drooping clusters.
Sullivant's Milkweed (A. sullivanti).
Sullivant’s Milkweed: This handsome milkweed is similar to Common Milkweed in appearance but doesn’t spread as rambunctiously. Grow in full sun, in moist to medium soil. Clay-tolerant.
Whorled Milkweed (A. verticillata).
Whorled milkweed: A shorter milkweed that spreads to form attractive drifts of white to greenish-white flowers. Grows in full sun to part shade, in medium to dry soil, but can adapt to a variety of soil types, including clay.
Spider Milkweed (A. viridis).
The green-flowered milkweeds: Short Green, Tall Green, and Spider Milkweed (aka Green Antelope-horn). These are milkweeds of prairie and savannah, adapted to full sun and tolerant of dry, sandy soils.
By incorporating any of these native milkweeds into our plantings, we can contribute to the restoration of the monarch butterflies that depend on them for food, protection, and perhaps even chemical mapping—all while enjoying fragrant, attractive flowers and the plethora of pollinators they bring into the garden. Who knew chemistry could be so beautiful?
Common Milkweed.