War on Weeds: Showy milkweed

Richard Old

Richard Old

This weed may invade your land. Be ready to oppose it.

The Enemy: Showy milkweed (Asclepias speciose Torr.)

Strategy: This native variety of milkweed is a rhizomatous-rooted perennial. It generally grows up to 3 feet tall. (The one in my front yard got up to 5 feet tall.) It produces light-green, opposite leaves that are somewhat oval and about 7 inches long covered with small hairs. As the name applies, anytime you break off a part of the plant it seeps a milky latex. Unlike leafy spurge, it is NOT toxic. Mostly pink flowers are found on the top of each branch of the plant and they produce a large, large oval pod. Once the pod dries, it splits open to expose the small seeds that each contain a tuft of hair for seed dispersal.

Attack: Although a native, it can become a nuisance in pastures, range and roadsides. The running roots can penetrate under roadsides and the plant gathers moisture and nutrients in pastures. Colonies of the plant can get quite large when not disturbed. The plant does not appear to be toxic to livestock, even though they will not graze it.

Defense: This plant is on the watch list as it one of the key plants for survival of the monarch butterfly. We are not in the monarchs’ flight pattern, but the plant should be preserved where possible. I read herbicides are reducing monarchs’ numbers, so as a professional weed-control guy, I want to know which herbicides are being used since this is one of the toughest plants to kill. Mechanical control is ineffective due to its root system. In pastures and rangeland, Tordon 22K has been used, which will reduce the stand less that 30 percent. Other herbicides used in pastures, range and roadsides seem to ineffective.


To learn more, call Bonneville County Weed Superintendent Jeffrey Pettingill at 208-529-1397 or email weeds@co.bonneville.id.us.


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