THE ISSUE:  Purple Starthistle 

Purple Starthistle (Centaurea calcitrapa L.) is a biennial or short-lived perennial that is native to the Mediterranean region. It invades pastures, rangelands, forests, disturbed areas, fields, and bottom land. It can form dense monocultures that out compete native species. It has sharp spines that make it unpalatable to livestock, and it is toxic to horses if consumed. 

Purple Starthistle grows from rosettes that have spines in the center, after bolting it can grow 2-3 feet tall. When young it is covered in fine hairs that fade with maturity. Lower leaves (4-8 in long) are deeply lobed while upper leaves are not. Each plant has many pink-purple flower heads. Each flower head contains 25-40 individual flowers. Flowers have spined bracts (1” or longer) just under the flower. It reproduces solely by seed. Seeds are about 1/8” long and are not plumed, which distinguishes it from its relative Iberian Starthistle.

Integrated Pest Management (IPM) Options: 

• Prevention— Learn to identify this plant. Never transport unknown plant material. Always plant clean seed! Report any sightings. 

• Mechanical— Hand digging can be effective on small infestations. Be sure to remove plant at least 2” below the soil surface before flowering occurs. Mowing is not an effective option; rosettes grow low to the ground allowing them to avoid damage. 

• Cultural— Establish a healthy stand of beneficial plants that will compete for essential resources.

• Chemical— Apply herbicides in the spring while plants are in the seedling or rosette stage. 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 and Bear Lake Counties. 208-547-3205
[email protected]