Milkweed & Monarch Butterflies

Written by: Brenda Van Ryswyck, Natural Heritage Ecologist

Photo Credit: Brenda Van Ryswyck

Photo Credit: Brenda Van Ryswyck

A lot of people are concerned with the Monarchs and their decline but recent studies show that (not counting weather weather) it’s the loss of habitat (milkweed) here in North America that is having the biggest impact on their population, currently. They are losing milkweed plants to lay their eggs on and for their caterpillars to eat. One of the best things you can do to help the Monarchs is to let milkweeds grow on your land, hedgerows or ditch or intentionally plant it in your garden. Common Milkweed seems to be preferred by Monarchs but it is not always “well behaved” in the garden and can spread by runners. If you want a better behaved one then try Butterfly Milkweed – Asclepias tuberosa (AKA Butterfly Weed) for a dry site (it is super drought tolerant and is often available at traditional garden centers); or, plant Swamp Milkweed – Asclepias incarnata for a wet site or rain garden (we have some in the bioswale).

We routinely receive questions about collecting eggs or caterpillars and raising them, but I don’t recommend this. Mistakes can be made and you could kill them.  So I recommend people plant milkweed for Monarchs (but do NOT recommend people collect eggs or caterpillars unless they are first trained by someone who has done it and have the proper permit. Yes, you need a permit to have Monarchs!)

If you purchase milkweed plants, then make sure you purchase only plants grown without pesticides and that you yourself do not use pesticides in your yard or garden. A big concern right now is that many garden flowers are grown using large amounts of pesticides: especially concerning are the prevalence of neonicotinoids as they are incorporated into the plants themselves, and, if a caterpillar were to feed on these plants they would die. You can also buy many pesticides in garden centers and hardware stores, which will also be toxic to the Monarchs (and other) caterpillars. Ask your greenhouse if the plants have had any pesticide treatments and only purchase plants that are guaranteed not to, or buy organically grown plants. For more information on the prevalence of pesticides in garden plants see “Gardeners Beware”.

Monarch caterpillars can feed on any of the milkweed species – Asclepias sp but there are three that are common in Halton and we can recommend planting. Common Milkweed – Asclepias syriaca is the most common and seems to be the one preferred by Monarchs. It has broad leaves and balls of purple flowers that have a wonderful fragrance. Until recently this species was on the Provincial Noxious Weed list as it was an agricultural pest and could be toxic to cattle. Recent removed in part because of the monarch decline but also because more management options have been developed for farmers to deal with Common Milkweed. But still, please be considerate and do not plant Common Milkweed next to agricultural fields. Common Milkweed does have a habit to spread through runners and thus is not often the first choice in a formal garden setting but the next two species are “well behaved” and do make great additions to any formal (or informal) garden. Butterfly Milkweed – Asclepias tuberosa (or just Butterfly Weed) is a very drought tolerant species that loves full sun and has brilliant bright orange flowers. Swamp Milkweed — Asclepias incarnate is a species that likes soil a little more moist, so if you have a spot that stays moist or sometimes holds a bit of water after a rain this is a good species for that spot. It gets light pink flat-topped flowers. I have it in my rain (downspout) garden and it has done well there.



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