Milkweed

A Toxic Relationship

Milkweed is a fascinating group of plants that have developed a set of defense mechanisms to keep predators away. The main source of this is also where the plant gets its common name—from the milky sap. The milky sap and all parts of the plant contain high levels of a compound toxic to all vertebrates. Monarch butterfly larvae have evolved with this plant and developed the ability to store this lethal compound in their body, making them toxic to vertebrates as well. This is what gives monarchs their characteristic bad taste.

The second defense these plants have is the characteristics of the milky sap itself. Milkweeds actually don’t like being preyed upon by monarchs. These plants are taken advantage of for their toxic sap by fellow prey looking for defense. To protect themselves, milkweed plants keep their lethal latex under high pressure, so that when they are damaged or bit into, toxic soup quickly gushes out. In some cases, such as when the predator may be a very small caterpillar, this sticky liquid can trap them and even drown them. It also acts as a sticky gum to chewing mouthparts of predators, preventing them from eating more.

More Orange Flowers for Your Garden

Milkweed Care Must-Knows

As the name might imply, milkweed plants are tough and need very little care. These are plants that are typically native to tough terrains full of competitive plants. Many species of milkweeds are native to grassland prairies, where plants compete for scarce resources. This has made milkweed plants tough competitors—they can thrive in poor soils, low water conditions, and harsh sun. With the exception of a few species of milkweeds that are adapted to swampy conditions, this plant is extremely drought-tolerant as long as it has well-drained soil. Milkweeds are generally not tolerant of shade, so give them as much sun as you can. It is also important to note that milkweeds are very slow to come up in the spring, so note where plants are to prevent damaging the tender new growth. See more native plants of the Mountain West.

The main milkweed pest is the oleander aphid. This aphid is a bright acid yellow with black appendages, and it likes to feed on the tender top growth of milkweeds. These pesky bugs quickly form colonies and will cover the whole stem in just a few days. Luckily, they don’t tend to cause severe or lasting damage to milkweed plants—damage is usually cosmetic. The large amounts of honeydew they secrete can cause black sooty mold that is also unsightly. To take care of these pests, spray with a harsh stream of water or insecticidal soap. If left alone, they will generally be preyed on by a variety of local beneficial insects, at no cost to you!

Fiber Facts

A fun fact about milkweeds: The fluffy fiber produced on the seeds that acts as a dispersal method is also sometimes called the Silk of America. This plant “silk” is very unique as the fibers are actually hollow tubes and are extremely water resistant. During World War II, stuffing for life jackets was in short supply, so they substituted milkweed silk. The fibers are also hypoallergenic and can be used to stuff pillows.

More Varieties of Milkweed

‘Hello Yellow’ Milkweed

Asclepias tuberosa ‘Hello Yellow’ has the same butterfly-attracting qualities as the species but produces clear yellow flower clusters instead of orange ones. Grows to 3 feet tall. Zones 4-9

Common Milkweed

Common milkweed has large ball-shaped clusters of deep pink blooms with a light fragrance. Zones 3-9

Butterfly Weed

Asclepias tuberosa offers fiery orange flowers that harmonize well with red, yellow, and other hot hues in the perennial border. Because it has a long taproot, it grows best in well-drained soil and is difficult to transplant once established. Grows to 3 feet tall. Zones 3-9

‘Soulmate’ Swamp Milkweed

Asclepias incarnata ‘Soulmate’ is a long-blooming selection with delightful rosy pink blooms. Zones 3-8

Purple Milkweed

Purple milkweed has deep purple-red blooms from May to July. Zones 3-8

Prairie Milkweed

Asclepias sullivantii is similar to common milkweed except that it has smooth stems and leaves and larger flowers. The leaves have a distinctive upward sweep. The plant grows 3 feet tall. Zones 3-7

Whorled Milkweed

Whorled milkweed has thin, needle-like leaves and is smaller than many other milkweeds, at only 1-2 feet tall. Flowers are held at the tops of the plant and are a greenish white color. Zones 4-8

Swamp Milkweed

Asclepias incarnata ‘Soulmate’ is a long-blooming selection with delightful rosy pink blooms. Zones 3-8

Plant Milkweed With:

Catmint is one of the toughest perennials you can grow. It’s a proven performer during hot, dry weather, and the silvery foliage and blue flowers look great most of the season. Deadhead or cut back hard after first flush of bloom to encourage more flowers. Average, well-drained soil is usually sufficient. Tall types may need gentle staking; it sometimes seeds freely. As you might guess from the common name, catmint is a favorite of cats. They’ll often roll around in the plants in delight.

One of the longest bloomers in the garden, coreopsis produces (usually) sunny yellow daisy like flowers that attract butterflies. Coreopsis, depending on the variety, also bears golden-yellow, pale yellow, pink, or bicolor flowers. It will bloom from early to midsummer or longer as long as it’s deadheaded.

Like so many grasses, fountaingrass is spectacular when backlit by the rising or setting sun. Named for its especially graceful spray of foliage, fountaingrass also sends out beautiful, fuzzy flower plumes in late summer. The white, pink, or red plumes (depending on variety) continue into fall and bring a loose, informal look to plantings. This plant self-seeds freely, sometimes to the point of becoming invasive.