Weed Wisdom Aug. 2017
by Dr. Bob Blackshaw
Showy Milkweed (Asclepias speciosa) and common milkweed (Asclepias syriaca) are perennial weeds native to North America and are best known as a primary food source for the monarch butterfly. Showy Milkweed is the species commonly occurring in western Canada and it is typically found in undisturbed areas such as pastures and roadside ditches. However, with widespread adoption of conservation tillage, Showy Milkweed presence in field crops has increased in recent years.
Identify Showy Milkweed by looking for large leaves (7-12 cm wide, 15-25 cm long) covered in soft downy hairs and a thick, milky sap throughout the plant. Flowers are pinkish to purplish in colour and form distinctive large ball-like clusters at the top of 75-125 cm tall plants. Grayish seed pods are 8-12 cm long and contain dozens of flat, reddish-brown seed with tufts of hair that facilitate long distance movement in wind and water. Reproduction occurs not only by seed but also by creeping rhizomes that allow formation of dense patches.
While Showy Milkweed is a desirable plant in many instances, control may be warranted in some cases. A large, fibrous root system enables Showy Milkweed to compete strongly with crops for soil water and nutrients. There are documented cases of reduced cereal yields by 25-45% because of Showy Milkweed. It is toxic to livestock, so avoid grazing and haying of infested areas.
Showy Milkweed control can be difficult. Mowing is generally ineffective and tillage is only partially effective as it can break and spread rhizomes that lead to new infestations within a field or between fields if equipment is not properly cleaned. As with most perennial weeds, herbicides capable of translocating to the root system are required to control showy milkweed. The preferable herbicide timing is late summer or early fall as the greatest herbicide movement to roots occurs when plants are storing food reserves for winter survival.