In reality, there was only one species of milkweed that was really any problem, and that species was Common Milkweed (Asclepias syriaca). It could get quite aggressive especially on the farming landscape, as it thrives on disturbed sites. But the list of Noxious Weeds was compiled years before there were chemical controls and other methods of tilling that helped keep it from being too problematic in the agricultural setting.
The Monarch butterfly is highly dependent on milkweeds, especially Common Milkweed, as the adults lay eggs on the milkweed's leaves and the caterpillars consume the leaves. The milky sap that the caterpillars ingest as they eat the leaves is what makes eating the caterpillars so distasteful to predators. The very distinctive colours of the caterpillar serve as ample warning to caterpillar-eating birds to beware.....they are not a tasty meal!
In recent years, as the Monarch butterfly declined and became legally listed provincially and nationally as a Species At Risk (SAR), there was a conflict with retaining milkweeds on the Noxious Weed list and requiring landowners to destroy the critical plant of a SAR. So it was a major, yet long overdue, event earlier in 2014 when milkweeds were removed from that list.
With that as background, let me introduce you to eight of the other nine milkweeds of Ontario (The 10th species, Four-leaved Milkweed [Asclepias quadrifolia], is extremely rare and in Ontario is currently found only in one or two places in the east. I have yet to see it).
Swamp Milkweed (Asclepias incarnata) is a tall species that is commonly found in wet areas, as its name suggests. Wetlands, roadside ditches and the like are great places to see this one. Its reddish purple flowers and relatively long narrow leaves, as well as the habitat, are useful characteristics.
Poke Milkweed (Asclepias exaltata) is an uncommon species most often found along woodland edges. It has wide leaves, and pinkish white downward hanging clusters of flowers.
Butterfly Milkweed (Asclepias tuberosa) is probably the most distinctive milkweed of all! It is found in grassy woodland openings, in prairies and on grassy sandy beaches.
Green Milkweed (Asclepias viridiflora) is not a plant that many people would recognize as a milkweed. In fact most people would walk by it and not give it much thought at all. This next photo is of a fairly robust plant along a sandy trail through the southeast beach of Rondeau, with closer views following.
Whorled Milkweed (Asclepias verticillata) is generally not common anywhere in Ontario, but where it does occur, can be fairly abundant. The leaves are quite narrow which together with its white flowers, separate it from all other Ontario milkweeds.
All of the preceding species occur or formerly occurred at Rondeau. Whorled Milkweed has not been seen for at least two decades.
Prairie or Sullivant's Milkweed (Asclepias sullivantii) is restricted in Ontario to the extreme southwest. It grows in some of the better quality tallgrass prairies, and also may be found along some highway road allowances where the vegetation may have some structural characteristics of a prairie. It looks a lot like Common Milkweed, but the underside of the leaves are not hairy, the leaves are more upright and the main leaf veins are more purplish.
Purple Milkweed (Asclepias purpurascens) has vivid, deep purple flowers. It is a plant of some of the best quality tallgrass prairie and savanna in southwestern Ontario.
One of the rarest milkweeds in Ontario is Tall Green Milkweed (Asclepias hirtella). To date in Ontario it has only been found in the tallgrass prairies of the Ojibway Prairie area in Windsor, and probably totals less than two dozen plants. It is a relatively common species in the mid-western tallgrass prairies, however. The next photo is one I took on a wonderful Missouri prairie in 2007, whereas the second one was taken on film in the 1990s from a small population in the Ojibway area.
So get out and enjoy some of our native milkweeds....the butterflies sure do!
Here is a bit of additional info: currently there are two stamps for sale via Canadian Postal outlets.....one of a Monarch adult for 22 cents, and a Monarch larva for 2 cents