Swamp Milkweed

Swamp milkweed (Asclepias incarnata) is known for its ability to thrive in swampy areas and wet meadows. It is a monarch butterfly host plant and is essential to their life. Despite its love of wet soils, due to its long taproot, it can tolerate average well-drained soils with ease. Showy, fragrant pink and white duo-tone flowers displayed throughout the summer months make lovely flower arrangements. The colourful blooms give way to attractive seed pods in the fall, which persist into winter. 

This is a tall plant, though height can vary considerably depending on the amount of sunlight. Swamp Milkweed can often be found growing in full sun or partial shade in moist or damp soils, such as along lakes, rivers, swamps, or drainage ditches. As would be expected, it does not grow in areas that are typically hot and dry. Unlike common milkweed, the central stem may branch. Leaves can be up to 6” (15cm) in length (but are often shorter), are much narrower (1-4 cm width) than common milkweed, and taper to a sharper point.

Swamp Milkweed and Monarch Butterflies

All varieties of the milkweed are essential to the life cycle of the monarch as it is the sole source of food for the caterpillars. However, the Swamp Milkweed is one of the monarch’s favourites. The flowers of Swamp Milkweed are so generous with nectar that they attract pollinators of all kinds, including a wide variety of bees, wasps, flies, butterflies, and skippers. It will occasionally be visited by the Ruby-Throated Hummingbird.

Milkweed earned its name from the toxic milky white sap that emerges when the plant stock or leaf is injured. Skin and eye irritation can occur so it’s best to avoid bare-hand contact. The plant is toxic to cats, dogs and horses but repels deer and rabbits.

Caring for Swamp Milkweed

Easily grown in medium to wet soils in full sun. Surprisingly tolerant of average well-drained soils in cultivation even though the species is native to swamps and wet meadows. Plants have deep taproots and are best left undisturbed once established. Foliage is slow to emerge in spring.

The plant grows in clumps about 2-3’ wide. It is a tidy plant unlike its counterpart, the invasive common milkweed. However, the conspicuous seed pods do need to be removed before they dry and crack to prevent more plants from developing. This low maintenance beauty is generally pest and disease free. No fertilizing is needed. The stalks remain upright when given full sun exposure. Should this not be the case, you’ll find it leans towards the sun. The stalks turn a delightful red and purple in the autumn. Keeping the stocks in place throughout the winter benefits existing insect larvae. Once spring comes, the stocks can then be cut back to ground level before new growth begins.

Companion Plants for Sap Milkweed – Asclepias incarnata

Companion plant suggestions include those that are shorter than the Swamp Milkweed and mingle easily with other sun and moisture lovers.

• Cardinal Flower (Lobelia cardinalis)
• White Turtlehead (Chelone glabra)
• Great Blue Lobelia (Lobelia siphilitica)
• Monkey Flower (Mimulus ringens)
• Blue Flag (Iris versicolor)

Propagating Asclepias incarnata

Swamp Milkweed is easy to grow by harvesting seed pods before they crack and dry which generally occurs about two months after blooming. The mature, brown coloured seeds contained within the pods are covered in a silky fluff which needs to be removed before planting. You can store the seeds in a cool place until ready to directly sow outdoors late fall. Select a sunny moist site and sow the seeds by covering them with a light scattering of soil. Germination will occur in the spring. For spring planting, mix seeds with moist sand and refrigerate for 30 days before direct sowing.

Alternatively, indoor germination is possible. However, seedlings have very sensitive roots and often do not easily tolerate transplanting. The seeds must cold stratify for three months in the refrigerator before being planted. Using peat pots that will be eventually planted directly in the garden bed is a recommended method to avoid transplant shock. About 20 days after seeds are planted and exposed to 20C the plants can be hardened off. They are ready to plant outdoors in early Spring when the plant is about three inches tall.

Where to Plant

Although this plant is home to the Wetlands it can adapt and will thrive in various other locations by being kept well watered until established. It can tolerate heavy clay and wet ground as well as medium to moist well drained soil. Full sun to partial shade is acceptable. An open sunny area is the preferred spot to encourage maximum height and blossoms. Your new plant will bloom the first summer it is planted. The plant grows well in Zones 3-8.