Pulling Perennial Weeds Is Hard
Perennial weeds, if they are a problem at all, require a different approach from that for annual weeds. To control perennial weeds by pulling them requires a very diligent effort. Every plant must be dealt with soon after it emerges from the soil. Use a weeding tool for this task, because it makes it possible to cut weed roots down 4 to 5 inches below the soil surface. This completely cuts off the weed's access to sunlight. Cut back this way several times, most perennial weeds will eventually die. However, just a few portions of a perennial weed remain near the soil surface, it will come back to haunt. Only the most persistent efforts will yield success in eliminating perennial weeds by pulling.
Herbicide is Best For Perennial Weeds
Fortunately for those with less persistence, there are two herbicides on the market that are effective against virtually all perennial weeds. They are very specific, working only against those plants that they contact. They are essentially harmless to other plants, the soil, people, and the environment in general.
One common herbicide is made of glyphosate salts; see Herbicides for Ornamentals in Yardener’s Tool Shed. These salts are absorbed through the plant's leaves and then move to the plant's root system, where they block the production of proteins. The plant essentially starves to death in 2 to 4 weeks. A glyphosphate herbicide will kill virtually any herbaceous (nonwoody) plant it touches, so it must be used with great care. Do not spray this herbicide on a windy day. To avoid touching adjacent desirable plants, apply a glyphosateto weed foliage with a small paint brush rather than with a spraying device. A glyphosate herbicide is water-soluble, biodegradable, and is not toxic in any way to the rest of the environment.
The other group of herbicides are made from natural ingredients. These products can be effective in killing the top of the weed, but they are not so effective in killing the roots. You can find a number of these products in Natural Herbicides in Yardener’s Tool Shed.
The best time to use any herbicide on perennial weeds, such as Canada thistle, is when the weather is warm and when the plant is approaching full size, a time when its food production system is working at full swing. Then the herbicide is most likely to be lethal.