The enemy: Purple aster (Machaeranthera canescens) or Hoary aster

Strategy: This perennial plant is often mistaken for the invasive knapweeds that are listed as noxious. It generally has lavender petals with numerous yellow flowers in the center inflorescence. Its leaves have small spiny edges on them. It has a larger seed head with bracts that are usually curved out and down. This plant can stand about 3 feet tall and grows in disturbed sites, grazed areas and ornamental settings, as well as on ditch banks and pastures.

Purple aster

Purple aster

Attack: As each plant produces hundreds of seeds, new invasions are prevalent and establishment is quick. The weed germinates later on in the spring, thus becoming stronger as other plants are drying out, hence this weed establishes where others do not exist and is tolerant to droughty sites. The plant has very little forage value and will force out other valuable plants by robbing the plants of sunlight and soaking up nutrients.

Defense: This is one perennial plant that can be controlled with a shovel. Any plant with a tap-root can be controlled as long as you dig down and get 3-4 inches of the root, thus removing the crown and stabilization of the plant. Early in the summer, one can use 2.4-D at a medium rate (1.5 pints per acre). Once the plant starts to bolt, one must switch to stronger products such as Milestone, Curtail, Opensight or Tordon 22K. These products will all control this broadleaf weed without harming the desirable grasses that are needed for forage. Very little can be done to culturally control the weed other than to keep your land healthy, thus fighting any invader that is trying to get established. Remember, this plant resembles other knapweeds, so consult with your weed professional for help.

To learn more, call Bonneville County Weed Superintendent Jeffrey Pettingill at 208-529-1397 or email