Lessons Learned From My Milkweed

Barbara Mrgich
Adams County Master Gardener

(7/18) I have a nice little patch of common milkweed (Asclepias syriaca) that has established itself right at the end of my driveway. Milkweed, of course, is the only host plant for Monarch butterflies. I like to watch it for eggs and caterpillars because even though the common variety is not the most attractive milkweed, the Monarchs seem to like it the best.

It is true that common milkweed can be a thug in the garden. It propagates itself by both stolons and seeds. (Stolons are like underground roots that snake through the soil, then surface to become a new plant.) I have let it establish itself at the end of my driveway where it is bound by the street, the asphalt driveway, and a patch of mown grass in the hopes that it will not invade my gardens.

Other forms of the plant such as Butterfly weed, Asclepias tuberosa, and Swamp milkweed, Asclepias incarnata, are both less aggressive and more attractive plants. I grow both in my gardens, and have never had a problem with aggressiveness from either.

It is actually very easy to control the seed spread of milkweed by simply cutting off the large seedpods that they all develop after their bloom is finished. The seed won’t spread until the pod bursts open, giving you plenty of time to get out there and snip!

Last year, aphids were all over the common milkweed. Aphids are tiny insects that suck the life out of a plant. By August, sooty mold covered it, and it was so unsightly I really didn't think it could possibly support a caterpillar, so I cut it down. As I was cutting, I found a caterpillar! I took it inside and raised it on Butterfly weed. It went quickly into chrysalis, and emerged as a beautiful butterfly which I released into my gardens.

If you break off a stem or leaf of Asclepias, it bleeds a white sticky liquid that looks like milk. Hence, its common name. The white sap is highly toxic if eaten by insects, animals or humans. It’s best not to get that milky sap on your hands either because it can cause skin irritation, and possible blindness if it gets in your eyes. It contains chemicals called cardiac glycosides and is especially lethal to grazing animals such as cattle and horses.

So, knowing this, I stand and examine my milkweed, and see that it is just crawling with bees and lots of different pollinators. I wonder, "If it is so toxic, how are they not being poisoned?"

Well, I learned that the poison is only in the leaves and stems, not in the nectar and pollen.

Monarchs have developed a tolerance for the poison over the eons of their interactions with the plant, and by now they can’t live without it. It’s the only thing their caterpillars can eat.

They eat the milkweed leaves which make both the caterpillar and the butterfly poisonous to birds and anything else that would like to eat them. Both the caterpillar and the butterfly carry the bright colors that serve as a warning to would-be predators.

Earlier, I mentioned sooty mold. Sooty Mold is an ugly black fungus that sometimes grows on the leaves of garden plants. The interesting thing is that it does not grow INSIDE the plant like other molds; it simply grows on the outside of the leaf. If you scrape it or wash it, it will come off. Why does it so often end up on milkweed? This is because it grows on a sweet substance called Honeydew. And the honeydew comes from the aphids that secrete (poop) it.

There are different colored aphids, but the ones on my milkweed look like tiny black dots which you may see clustered on the stems and leaves.

In an effort to keep my milkweed healthy this year, I would stand and inspect it when I walked down for the mail. If I found a small patch of aphids, I used my clippers to cut the stem off and throw the whole thing in the trash. I should add here, that in a normal garden situation, you can easily just wash them off the plant with a strong stream of water from the hose, but these are farther from the house than the hose can reach. So for me it was easier to simply clip off the infected part. It seemed to work, as this year, I had very little aphid problem. The plants stayed clean all summer and into fall.

Please don’t let all this discourage you from growing milkweed in your garden. Monarchs need the plant for the species to survive. Don’t forget the other varieties which are readily available. Asclepias tuberosa (Butterfly weed), and Asclepias incarnata (Swamp milkweed) are both beautiful in the garden, and, in my experience, have not attracted the aphids. If you happen to have an out-of-way sunny location where you can grow common milkweed, that’s even better.

Milkweeds not only support Monarch butterflies, they are excellent pollinator plants for a wide variety of bees and other beneficial insects. Once you have established a nice sized patch, you will enjoy seeing Monarchs flit around your garden all summer.

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