Species specific: Spotted knapweed

Fast facts about spotted knapweed

Highly prolific noxious weed that can significantly reduce desirable forage production.
Black-tipped bracts beneath thistle-like pink flowers help distinguish this weed.
Capable of taking over nearly all available plant space in areas such as fields or pastures if left unmanaged.

What to look for

Spotted knapweed’s (Centaurea stoebeand L.) is a winter-hardy noxious weed that competes with native plants and legume crops that can live for up to five years.

The upright biennial forb averages 2 to 4 feet tall and has rosette leaves that are blueish-green, hairy and covered with shiny specs and translucent dots. Leaves are 4 to 8 inches long and decrease in size on the upper parts of the plant.

Its most distinguishing feature is the black-tipped (spotted) bracts beneath the thistle-like pink flowers, which bloom from June to October, giving the plant it’s common name. The flowers produce fruits between June and February that contain tightly packed seed heads capable of producing more than 1,000 seeds per bud. The seeds can remain viable in soil for years and easily spread such as by wind, on the bodies of passing animals or on the undercarriage of vehicles.

Because of its prolific nature and the ease with which its seeds spread, it is common to see population blooms occur 2 to 3 years after the first signs of the weed.

It has a deep taproot that helps it survive winters and drought and also produces a chemical (catechin) that acts as a natural herbicide to compete with other plants. Catechin is secreted by the root system at the first signs of stress, allowing spotted knapweed to readily displace desirable forages, especially during winter dormancy and in recovering pastures or disturbed fields.

Once an infestation begins, spotted knapweed can take over 95% or more of available plant space without intervention.

Where it is found

What was once a noxious weed isolated to rangelands of the western United States, including Idaho, Montana, Nevada, Oregon, Washington and Wyoming has now spread across much of the Midwest and parts of eastern United States.

How to treat it

During the vegetative plant stage in both spring and fall, apply 16 to 20 fluid ounces per acre of HighNoon® herbicide.

Alternatively, apply 20 to 42 fluid ounces per acre of GrazonPD3^TM herbicide or 24 to 34 fluid ounces GrazonNext® HL herbicide.

GrazonPD3™ is a Restricted Use Pesticide. GrazonPD3 is not registered for sale or use in all states. GrazonPD3 is not for sale, distribution or use in Nassau and Suffolk Counties in New York State. Under normal field conditions HighNoon® is nonvolatile. HighNoon has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with HighNoon and to manure and urine from animals that have consumed treated forage. Consult the label for full details. HighNoon is not registered for sale or use in all states. GrazonNext® HL has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with GrazonNext HL and to manure and urine from animals that have consumed treated forage. Consult the label for full details. GrazonNext HL is not registered for sale or use in all states. GrazonNext HL is not for sale, distribution, or use in New York State and San Luis Valley of Colorado. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.