Thistle – Weed ID

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Thistle is hard to miss with it’s large growth pattern and aggressive spines. It will bloom in it’s second year of growth and is easily spotted from a distance. Thistle may be beneficial to pollinators, but it is best when it is not found in pastures or hayfields. Livestock do not usually eat thistle at all, and if they do, they are not able to consume enough to provide any sort of benefit. Thistle populations grow rapidly as they produce as many as 4,000 seeds from one single plant. If thistle is not controlled, it will reduce the productivity of your forages and ultimately impact your bottom line. This is why it is most beneficial to start scouting for thistle early and continue to do so in years to come.

While thistle comes in a variety of shapes, sizes, names, species, etc., control methods remain similar. For optimum control, it is important to understand that thistle is a biennial plant. This means that it is actively growing in it’s first year of life and produces seed in the second year. During it’s first year of growth, the thistle plant is in the rosette stage, as pictured below in Fig. 1. In the second year, thistle is considering to be “bolting” and then “flowering”, as pictured below in Fig. 2 & 3.

Thistle in Rosette Stage

Fig. 1 Thistle in Rosette Stage

Thistle in bolting stage

Fig. 2 Thistle in bolting stage

Pre-bloom thistle

Fig. 3 Pre-bloom thistle

Timing of control and stage of growth have everything to do with what your best option may be.

-Manual

  • Digging, pulling up, removing the root from below the soil’s surface.
  • Only effective if before bloom.
  • Only practical and feasible if small quantity of thistle.
  • If you find a plant with flowers about to bloom, clipping those blooms off and disposing of them (do not leave laying in the pasture) will at least reduce the number of seeds produced that growing season but additional measures may need to be taken.

-Mowing

  • Does not effectively control thistle in the rosette stage as it does not kill the root.
  • It is best to mow when plants are in the bolting stage (growing upright). This prevents them from being able to bloom.
  • The limitation is that not all thistles bolt and bloom at similar times. You will also be mowing in April-June for thistle control. This may interfere with hay production or grazing plans.
  • Mowing will have to be done multiple times and a few years in a row to see effective control.

-Chemical

  • Most effective when in the rosette stage. Rosettes respond well to chemical control and are more easily controlled using an herbicide than in comparison to mowing or manual control.
  • Timing is everything because if plants are allowed to grow into the bolting or flowering stage, the effectiveness of herbicide treatments reduces greatly.

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