Spotted knapweed is a biennial or short lived perennial non-native noxious weed which originated in Eurasia. It grows from 1-4 feet tall, with the fine gray-green foliage and lavender flowers that bloom from June to September. Spotted knapweed grows on disturbed areas, overgrazed rangeland, roadsides, meadows and sandy soils in Nebraska.
Spotted knapweed reproduces only by seed, but must be closely monitored if allowed to go to seed. One spotted knapweed plant can produce 40,000 seeds. One patch in Sheridan County on the highway right of way was sprayed for eleven years straight, and when skipped for one year, the spotted knapweed showed up again, proving how prolific the seed bank can be.
Once established, spotted knapweed reduces livestock and wildlife forage by out-competing native species. As a versatile invader, it can tolerate both dry conditions and high moisture areas.
The EC-130 Guide for Weed Management in Nebraska lists several herbicides that can be used to control spotted knapweed. One of the challenges of spraying spotted knapweed is the lack of leaf surface, because of the fine foliage. Spraying at the rosette stage is recommended, and will give better results, as there is more leaf surface.
Several counties in eastern Nebraska have used biological control insects to help control spotted knapweed. Since we only have a few acres infested in Sheridan County, and we want to keep it that way, using bio-control is not recommended at this time.