Milkweed for Monarchs

February 2018

I was recently asked about a milkweed article by Sara Williams and so by popular demand here it is.

If you’re worried about the survival of the monarch butterfly (Danaus plexippus) in your part of the world, why not add some prairie or swamp milkweed (Asclepias incarnata) to your flower borders? The butterfly milkweed (Asclepias speciosis) is given much wider publicity in books and garden magazines; however it doesn’t survive in Zone 2b of the prairies whereas the swamp milkweed will!

The monarch butterfly is one of the best known but most threatened of the butterfly species in North America; in Canada, it is a Species of Special Concern ( The monarchs that migrate from Eastern Canada and the Prairies in late summer/early fall travel more than 4000 km to their wintering grounds in Mexico. It is estimated that about 90% of these lovely black, orange and white butterflies have disappeared in the last 10 years. Loss of habitat due to urban sprawl, logging and agriculture in all parts of its migratory route – perhaps most importantly in its overwintering habitat in Mexico – is the major cause of its decline.

In Canada, the female monarch deposits her eggs exclusively on the lower leaf surface of milkweeds. The larvae hatch three to five days later with food in easy reach: i.e. the milkweed leaves on which they hatched. Over a period of a few weeks they shed their skin four times, increasing in size each time. After pupating as a chrysalis for a further 2 weeks, it emerges as an adult butterfly at which stage it can feed on the nectar of a number of different flowers other than milkweed (including annuals such as alyssum, marigold and zinnia). Over a season, up to 2 – 3 generations are produced in Canada. The final generation that emerges at the end of the summer feed on nectar to build up their energy for the big migration. As native milkweed species disappear from once marginal land, so does the food source of the monarch butterfly larvae. And so their numbers decline. Your planting of milkweed will help provide a more continuous source of food along the lengthy migration path of the monarch butterfly.

For a plant with the common name of swamp milkweed, this perennial is exceedingly drought-tolerant and vigorous in the driest of situations. They will spread (by rhizomes) if conditions are to their liking. If that happens and it’s not to your liking, plant them in the back lane. Native from Nova Scotia to southeastern Saskatchewan, its genus name is from the Greek asklepios, the god of medicine, referring to its ancient medicinal properties, while incarnata means flesh pink and describes the flowers.

The showy white or pink flowers are born on 60 to 90- cm (2-3 ft.) stems in early summer above 8-15 cm (3-6 in.) alternate leaves. Several cultivars are available. ‘Carmine Rose’ has rose-pink flowers. ‘Cinderella’ is a dusty rose-pink. ‘Milkmaid’ and ‘Ice Ballet’ both have white flowers. ‘Soulmate’ has white flowers with rose pink bracts.

Start seeds about six weeks prior planting outdoors. Use a well-drained potting media, covering seeds with about 3 mm of the media. Place under timed lights or in a sunny window. Germination should take place within two weeks. Transplant seedlings into biodegradable pots. Give them time to establish themselves before planting outdoors in full sun.

Swamp milkweed is a good border plant or in a bog garden and they make excellent cut flowers.

Sara Williams is the author of the newly expanded and revised Creating the Prairie Xeriscape; Gardening, Naturally: A chemical-free handbook for the Prairies; and the Saskatoon Forestry Farm Park & Zoo: A Photographic History. Sara is also a founding member of the Saskatchewan Perennial Society, and continues to be actively involved in perennial gardening.