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THE ISSUE: Iberian Starthistle

Iberian Starthistle (Centaurea iberica) is a native to northern Africa, southern Europe, and the Mediterranean region. It can act as an annual or biennial depending on moisture availability. While it has “thistle” in its name it is more closely related to “knapweed” species. Iberian Starthistle displaces precious forage on rangelands and successfully invades roadsides. It has sharp spines which discourage grazing animals and reduces recreational opportunities.

Iberian Starthistle seeds germinate in the fall or early spring forming a rosette. The plant bolts in late spring to early summer and it can grow up to 6 feet tall. Leaves are narrow, linear, and divided with fine hairs. Flowers are purple-white and are armed with 1 inch long tan colored spine-like bracts. Seeds can be spread by water, wind, animals, and humans.

Look-a-likes: Unlike Purple Starthistle (Centaurea calcitrapa L.), Iberian Starthisle has plumed seed (tufts of hair protruding from one end of the seed). Mature seed is needed to distinguish between the two species.

Integrated Pest Management (IPM) Options:

n Prevention— Learn to identify and do not transport seed (remove seed from clothing, pets, and equipment).

n Mechanical— Hand digging can be effective if the stem is cut 2” below the soil surface, but plants often resprout, so make sure to revisit infestations. Mowing is not an effective option, because plants will resprout. If flowers are present mowing will only help distribute seed.

n Biological—None known.

n Chemical— Apply herbicides in the spring while plants are in the seedling or rosette stage. It will likely take more than one application to kill this plant. Refer to Pacific Northwest Weed Management Handbook (https://pnwhandbooks.org) for herbicide recommendations (Search “starthistle”). Always read and follow herbicide label directions.

Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou County. 208-547-3205 JLHatch@uidaho.edu

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