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The World of Milkweeds

Unfortunately, we only associate milkweeds with Monarch butterflies. This is a good starting point but, definitely, not the full story of milkweeds. Even the name is interesting. The Genus name honors the Greek god, Asklepios, the god of medicine. There are 100 species throughout the world and Canada has 13 native to her soils. Ontario boasts 11 native milkweeds which we will visit later.

There have been planting programs to bring back milkweeds to the landscape. Why? The obvious answer is that this plant group is the food source for the magnificent Monarch and its caterpillars. The loss of milkweeds in Monarchs spring and summer breeding areas is believed to be a significant factor for their decline in populations. Agricultural intensification, development of rural lands and the use of mowing and herbicides to control roadside vegetation have all lead to the drastic decline of milkweeds.

But beyond the Monarch many insects and hummingbirds drink nectar from its flowers. In fact, because milkweeds produce vast amounts of nectar over a sustained period, it is rated by pollination experts as a plant of special value to native bees. Orioles will use the plant fibers from its stems to construct nests. Other nesting birds use the silks from seed pods to line their nests. The Queen butterfly, dogbane tiger moth and milkweed tussock moth also use milkweed as their larval plant.

In Nature, every act uses biological energy. To produce flowers, leaves and nectar are all carefully planned since energy must be diverted for each process. The production of high quality nectar by milkweeds is no accident and is totally dependent on the process of pollination. The pollination path of milkweeds is quite complicated and intricate.

All milkweeds are incompatible. They can’t pollinate themselves but must depend on insects to transfer pollen between unrelated plants. Compared to other insect visited flowers, milkweeds are very unique. Their pollen is contained in small, waxy sacs (pollinia) that are located in vertical grooves (stigmatic slits) within the flower. Each pollinia contains several hundred pollen grains which are inaccessible to pollinators visiting, milkweed must offer large amounts of nectar as a reward. Not only does milkweed offer large amounts of nectar, the nectar is of very high quality.

When insects come for their nectar reward, their legs or mouth parts may slip into the stigmatic slits and come into contact with the pollinia sacs containing the pollen grains. The sacs are barbed in nature and is removed from the flower as the insect tugs its body part out of the stigmatic slit. The unknowing insect then transfers pollen to the next milkweed. Although this process is complicated it does not require ‘specialist’ insects. Insects need only be large enough to remove and carry the pollinia to the next flower.

Interestingly, not all larger insects visiting milkweeds are equal in their pollinator abilities. A huge array of insects visit milkweeds from bees, wasps, butterflies, moths, flies and beetles. It is the large bees and wasps and butterflies that are the most efficient pollinators of milkweed. And even though Monarch butterflies visit milkweed flowers often, they are not efficient at transferring pollinia. Believe it or not, it is the swallowtail butterfly family that are important pollinators of this plant. From my research, bumble bees, honeybees, eastern carpenter bees are the most significant insect pollinators.

For the rest of this article I would like to zero in on the native milkweeds that call Ontario home.

Green Milkweed (Asclepias viridiflora)

The Latin description of ‘viridiflora’ means ‘. The status of Green Milkweed in Ontario is listed as

Green Milkweed will reach heights of 1 – 2 feet. It will tolerate a variety of growth conditions ranging from rocky to sandy areas, fields, grasslands, meadows as well as damp marsh like areas. Very versatile.

This milkweed will produce pale green flowers in the summer.

Green Milkweed flower

Poke Milkweed (Asclepias exaltata)

The Latin description of ‘exaltata’ means ‘tall’. The status of Poke Milkweed in Ontario is listed as common

Poke Milkweed will attain heights of 2 – 5 feet tall. Its natural habitats are open woodlands, forest edges, and roadsides. It prefers sites with partial to little light.

From June to September it will bloom with flowers ranging in color from white to pale pink or even purple.

Poke Milkweed flower

Prairie Milkweed (Asclepias hirtella)

The Latin description of ‘hirtella’ means ‘. The status of Prairie Milkweed in Ontario is listed as Rare.

This an uncommon milkweed native to abandoned fields, roadsides, wetland margins and prairies. It prefers full sun and reaches heights of 2 – 3.5 feet and widths of 2 – 3 feet. Tolerant of a wide range of soil conditions from sandy to gravelly soils to dry and clay soils. Drought tolerant.

From June till August Prairie Milkweed will produce flowers ranging in color from greenish to white.

Purple Milkweed (Asclepias purpurascens)

The Latin description of ‘purpurascens’ means ‘. The status of Purple Milkweed in Ontario is listed as Rare.

Purple Milkweed is found in woodland edges, roadsides and dry fields. Attaining heights of 1.5 – 2.5 feet it will grow in full to partial sun. Not drought tolerant.

Purple Milkweed will bloom from June till August. It produces beautiful purple flowers.

Sullivants Milkweed (Asclepias sullivanti)

The Latin description of Sullivanti is the name of the botanist who discovered this milkweed. The status of Sullivants Milkweed in Ontario is listed as

This milkweed is linked to tall grass prairie habitat and has declined due to loss of habitat. It will grow in moist to medium conditions in clay or loam soils. Grows in full sunlight up to 2 – 3 feet tall and 1 – 2 feet wide.

In June till August Sullivants Milkweed will produce flowers ranging in colour from light green to pinkish purple.

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Four (4) Leaved Milkweed (Asclepias quadrifolia)

The Latin description of ‘quadrifolia’ means ‘. The status of 4 Leaved Milkweed in Ontario is listed as extirpated

Dwarf Milkweed

Dwarf Milkweed (Asclepias ovalifolia)

This is quite a shorter milkweed growing to heights of .5 – 2 feet. It grows in well drained or sandy areas such as open woods, woodland edges, thickets and roadsides.

Dwarf Milkweed will produce greenish white to greenish purple flowers from June till August.

Dwarf Milkweed in bloom

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Common Milkweed (Asclepias syriaca)

The Latin description of ‘syriaca’ means of ‘syria’. The status of Common Milkweed in Ontario is listed as common

Swamp Milkweed Flower

Swamp Milkweed (Asclepias incarnata)

The Latin description of ‘incarnata’ means ‘flesh’. The status of Swamp Milkweed in Ontario is listed as common

Butterfly Weed

Butterfly Weed (Ascelepias tuberosa)

The Latin description of ‘tuberosa’ means ‘ tuberous’. The status of Butterfly Weed in Ontario is listed as uncommon

This plant will reach heights of 1 – 2 feet and spread 1 – 1.5 feet. Blooming a vivid orange in July and August, it is of special value to native bees and recognized by pollination ecologists for attracting large numbers of native bees. It grows in well drained soils and tolerates drought.

Whorled Milkweed (Asclepias verticillata)

The Latin description of ‘verticillata’ means ‘whorled’. The status of Whorled Milkweed in Ontario is listed as uncommon

A pioneer plant that thrives on open disturbed areas tolerating drought and dry soils. Thrives in full sun to partial shade conditions. It attains heights of 1 – 2.5 feet and widths of 1 – 2 feet.

This milkweed blooms a soft greenish white color from July to September which is later in the year than most milkweeds. 

whorled milkweed

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