Growing milkweed in your garden is either extremely easy (it just materializes out of nowhere!) or difficult (it's not in the spot you'd really like it to be...). There seems to be no in between.
Perennial Milkweed:
Most milkweed (there are approximately 110 varieties in North America that produce the milky sap known as latex) are tropical and can be grown as annuals in Canada. There are some varieties that are perennials in Canada. Some of these include:
- Asclepias syriaca (common milkweed) - Zone 3
- Asclepias tuberosa (butterfly weed) - Zone 3
- Asclepias incarnata (swamp milkweed) - Zone 3
- Asclepias verticillata (whorled milkweed) - Zone 4
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The best way to try to start these plants is by transplanting them very early in the spring. Milkweed do not transplant very well, so it is important to dig them up as soon as you see them sprouting in the spring, water their roots extremely well, and plant again immediatly. They have a long horizontal root (runner) that must be dug up with the plant in order to make the transplant successful. They may or may not bloom that first year.
Another way is to start the plants from seed. Most milkweed need to be vernalized, which means that they need to go through a period of cold. Keep seeds (harvested in the fall when the pods are about to open and seeds are brown) outdoors for the winter, or else put them in your freezer for 8 weeks. About 7-14 days before you wish to plant them, put them between damp paper towels and in a bag in your crisper drawer of your fridge. It is recommended that you start the seeds indoors in March in thoroughly soaked soil-less mix (less than 1 cm down). Keep mix damp - germination is 7-10 days if maintained at least at 24°C. If they have not sprouted in that time, the seeds may need scarification - physical abrasion to break down the outside later. Monarch Watch recommends putting the seeds in a container with sand and shaking the container for 30 seconds or so. Be aware that germination still may not occur. Any seedlings that have germinated and reach a height of 10-15 cm should be hardened off (gradually exposed to more and more sunlight) after all danger of frost has passed. Transplant into your garden in a sunny location.
Annual milkweed:
Tropical Milkweed
Asclepias curassavica (tropical milkweed) - zone 8 - is a great plant to grow from seed as an annual, and it germinates much more easily. The seeds do not need to be vernalized, and can be started indoors in March as well. They tolerate a much higher temperature and can easily germinate in temperatures above 30°C.
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For more information about butterfly nectar plants, click here.
For more information about essential larval host plants, click here.