We often meet visitors in the gardens who would like us to point out the milkweed plants we have growing here at Marin Art and Garden Center. With the critical situation of the Monarch butterfly population on our minds, many of us would like to help these long-distance travelers find a reliable food supply for growing caterpillars to support their species’ survival.
Monarch butterflies are famous for their annual migration; here in the West, butterflies mainly from Oregon and Nevada spend the winter along the coast from Mendocino to Baja California in Mexico. Their winter requirements are quite specific: sites must feature dappled shade, high humidity, a source of fresh water, and temperatures above freezing for the season. They also need a food supply, and while adult butterflies feed on late-blooming flowers, Monarch caterpillars only eat milkweed; in fact, the monarch butterfly is also known as the “milkweed butterfly.” But these plants are rapidly disappearing, due to the loss of habitat stemming from land development and the widespread spraying of weed killer on the fields where they live.
Planting milkweed in your home garden is easy and rewarding, but be prepared: Monarch caterpillars are eating machines and each one will mow through about 20 leaves. So make sure you have enough milkweed plants or the caterpillars will starve!
If you’re planning to grow milkweed in your garden, there are a few considerations to take into account. Milkweed plants are food for caterpillars but poisonous to humans. Do not get milkweed sap on your skin or in your eyes. Milkweed is also toxic if eaten, so keep plants away from young children and pets.
There are many varieties, some which thrive in full sun, some in humid conditions, and some even in very dry conditions. To find out more about the various species of milkweed and which are best for your area, check out this factsheet courtesy of Monarch Joint Venture. Milkweed seeds can be very slow to germinate and do take some time and patience. For best results and to speed up the germination process, place the seeds in wet paper towels in a plastic bag in the refrigerator for 30 days prior to planting. Then plant the seeds in a sunny place about ½” deep and water them for at least two weeks.
After about two months, your milkweed plant will be big enough for caterpillars to eat. As a perennial, they will come back every year, but in the autumn, you should cut back the plants. It’s also important to cut back the milkweed to encourage migration; although there are overwintering sites in Marin, it’s best for the butterflies to be in a frost free location. Also, the more butterflies that overwinter in a single location, the greater the genetic diversity, so it’s better for them to congregate in larger numbers.
In this video, Garden Manager Steven Schwager explains how to cut back milkweed, for the best health of our Monarch visitors.