***IMPORTANT NOTE***

According to Idaho Code 22-2407, Idaho property owners carry the primary burden of controlling noxious weeds on their land.  County Weed Superintendents may only treat noxious weeds on private property if the landowner fails to fully mitigate the situation.  In such instances, the County will treat the noxious weeds and bill the property owner for the weed control efforts. 

Control Scotch Thistle

Scotch Thistle plants are on the increase in our county. We need your help if we are to control this noxious weed. This is the largest of the thistles found in this region, and has been mistaken for a tree from a distance. Some of these plants will get fourteen feet tall, with leaves a foot, or more, wide. Scotch Thistle is a very prolific seed producer. A well developed plant will produce dozens of flower heads, with as many as a hundred seeds per head. It shades out other vegetation near it and soon takes over the growing area.

Scotch Thistle is a biennial plant. It grows from a seed to form a rosette during its first year of life, concentrating its energy into forming a large tap root that will support it's huge top the following season. These rosettes have a blue-green color and are covered with very fine leaf hairs. Scotch Thistle rosettes can become two to three feet across, and have spiny, winged stems. It over winters in this rosette stage, and in the spring, draws nutrients from its thick tap-root, to form one of the largest weeds we have to contend with.

Herbicides can be used on the mature plants to try to stop growth and seed production. It is preferable and more effective, to treat Scotch Thistle when it is young, and still in the rosette stage. Early spring and late summer are excellent times to use herbicides on this noxious weed.

The seeds are dispersed by the wind and can remain viable in the soil for several years. If you must treat an area for Scotch Thistle, you will need to monitor the sight for new sprouts for several years.

Scotch Thistle is particularly invasive, and few management practices will select against it. There are several herbicides that are effective in controlling this weed. Herbicide selection is done case by case, and if you need more information regarding the use of herbicides on Scotch Thistle, feel free to call us at Ada County Noxious Weed Control, at (208) 577-4646. We are doing everything we can to control this weed, and will be happy to help you select a herbicide to control these plants.