Asclepias syriaca

Note: this plant is legally considered a noxious weed in Ontario (but grow it anyway).

Asclepias syriaca, known in English as "common milkweed", "butterfly flower", "silkweed", "silky swallow-wort", or "Virginia silkweed", and in French as asclépiade commune, bébé lala de lait, herbe à ouate, or herbe aux perruches, is a perennial native to eastern and central North America. The "milk" in milkweed is its thick white sap, which is poisonous to many animals. Some insects, most famously the monarch butterfly larva, are able to feed on milkweed. Monarchs store the toxins in their bodies as a deterrent to predators.

The field behind my mom's apartment buidling in Almonte (eastern Ontario) is dominated by various non-native flowers, but the milkweed is one native that is holding its own. When I checked the undersides of the leaves on this vigorous clump, I found many eggs, probably monarchs. You can see how a big group of milkweed like this would be a butterfly magnet!

The clusters of muted pink flowers are attractive to many different pollinators.

Now, technically, this milkweed is considered a noxious weed under the Ontario Weed Control Act. (Asclepias viridiflora, Asclepias incarnata, and Asclepias verticillata are also listed as noxious.) Milkweed is toxic to livestock and can also invade fields of crops, reducing their yield, so if you are farming or live near a farm you'll want to control or remove milkweed. But I don't think that applies to most of us here in Toronto.

The asclepias are a vital genus of plants for monarch butterflies; they are the only plants that their larvae can eat. If milkweeds are eliminated everywhere, then so to will be the monarch butterflies. So I think that those of us who aren't near farms should make an effort to include some of the asclepias in our gardens if growing conditions are suitable.

The Ontario Weeds Act FAQ states:

As long as the population of milkweed planted doesn't negatively affect agricultural or horticultual land by spreading seed and new vegetative plant material (i.e. root stock) into fields, nurseries or greenhouses then it is acceptable to plant milkweed in your garden. It is recommended that you consult with your local weed inspector and/or neighbours so that all parties involved are comfortable that the impact to agriculture or horticulture is negligible.