Spotted knapweed occurs in all 56 counties in Montana. This plant has a root system comprised of a taproot as well as lateral roots. Some research suggests that the root has allelopathic properties in that it produces a chemical which acts like a natural herbicide killing seedlings that grow nearby.
Young spotted knapweed plants form a grayish-green rosette. After the rosette stage, stems and leaves are blue-green but can appear to be silver-gray due to the tiny hairs that cover these surfaces.
Pink to purple flowers bloom from June to September. Flowers are surrounded by oval bracts that have black tips giving them a “spotted” appearance.
A single spotted knapweed plant can produce as many as 300 flower heads which are capable of generating 140,000 seeds. The seeds can remain viable for up to 5 years in undisturbed soil. The seeds of this plant are oval, tan and 1/8” in length; seeds have a short tufted pappus or mature flower sepals which have a feather or hair-like quality like a dandelion seed.
When mowed repetitively this plant can adjust and grow flowers and seeds close to the ground, so it can range from 2” tall to 4’ tall depending on management.
Biological Control of Knapweed
- Knapweed Root Weevil (Cyphocleonus achates)
- Blunt Knapweed Flower Weevil (Larinus obtusus)
- Knapweed Flower Head Weevil (Larinus minutus)
Integrated Weed Management (IWM)
- Learn to recognize knapweeds and report new occurrences to the County Weed Board.
- Eradicate small patches of knapweeds before they have a chance to spread.
- Refrain from driving vehicles through infestations and wash vehicles in a designated area before traveling into non-infested areas.
- Purchase and transport only certified noxious, weed-seed free forage.
- Minimize soil disturbance on range and other non-crop lands.
- Use IWM to contain large knapweed infestations.
- Seed desirable perennial grass species on areas disturbed by construction, mining or other activities.
See Montana Knapweeds, MSU Extension EB0204 2011 for more information