Field Guide Weed Management Bull Thistle

Bull Thistle (Cirsium vulgare)

Crop Impacts: Pastures


About Bull Thistle:
The Bull Thistle is a biennial weed that reproduces by the wind blowing their seeds. Bull Thistle has a two year life span. They are in rosette stage in the first year and flower and set seeds in the second year. The seeds of this weed are short-lived on the soils surface but can lay dormant in the soil for many years. The root of this plant is edible and is best eaten when mixed with other vegetables.

Family: Composite or Aster Family (Compositae)

Bull Thistle 2 Bull Thistle 3

Bull Thistle Scouting and Prevention:

Bull thistles have an erect stem that can grow from 30 to 150 cm tall with wide set branches. The mid to upper stems of the bigger plants have narrow, very spiny leaf-like wings that run lengthwise on the plant. On smaller plants, the whole stem and branches are spiny-winged. As the stem gets thicker, the wings start drying and disappearing. The leaves of a bull thistle are large, woody and are flatted and basal rosette during their first year of growth. All of the leaves are deeply lobed with a 1 cm hard and sharp spike coming out the tip of each lobe with the margins lined with smaller spikes. The bottom part of the leaf has wooly-hair and is light green and soft to touch. The top of the leaves are dark green with many sharp spikes. The most distinguishing aspect of this weed is the flowers that grow at the tips of large branches and are about 2.5 to 7.5 cm wide. These purple to pink flowers are egg-shaped and grow out of bloomed shaped green brackets covered in yellowish spines. The seeds of this weed are shiny, grayish-brown with darker lines and hairs at the tip of the seed, flowering from July to August.

Common locations

- Pastures
- Lawns
- Gardens
- Cultivated land

Prevention

Prevention of Bull Thistle is less expensive and less time-consuming then trying to control it. Contaminated hay is the main source of the spread of bull thistle. Make sure when you seed a new area that you do so with certified weed-free seeds. If there is an infested area on your property, be sure to drive around instead of through it. Make sure to give all equipment that has been in infested fields a good clean so that no seeds are transferred.

Bull Thistle Control:

Cultural Control

If you are interested in mowing Bull Thistle, the best time to do so is when the flower is at full bloom, but before the seeds release in mid-summer. Mowing can work if it is done twice in a season, which will reduce the population over time and prevent seed production. Take note that mowing will not kill first year rosettes of bull thistle. Cultivation or shoveling is another form of cultural control that works well if it cuts the weed below the crown or the rosette of the plant. It is important that the roots are dug up and the flower stems should be properly disposed of to prevent them from forming viable seeds.

Chemical Control

It is important to know that Bull Thistle is susceptible to glyphosate and dicamba herbicides. That being said, 2,4-D provides good control for top growth when the plant is still young and has yet to flower. Bull thistles in the rosette stage are very sensitive to the supplication of herbicides when applied early in the growing season. GrazonNextHL herbicide provides decent control after the Bull Thistle has flowered. However, it will only provide short term control due to the fact that the seeds have already set. Scouting pastures and previously infested areas in January to March will reveal Bull Thistle in its rosette stage to allow for cheap herbicide application.