Weeds commonly impair the growth of forest crop trees. Control measures rely increasingly on herbicides, large-scale use of which dates from the development of chlorophenoxyacetic herbicides in the 1940's. Weed control is not to kill weeds but to divert growth resources into crop trees. The herbicide tool becomes increasingly powerful and versatile because of new herbicides and new ways of using them. Of the organic herbicides, only phenols have high mammalian toxicity. It is highly improbable that problems could arise from use of herbicides in normal forestry operations. Herbicides make available to crop trees nutrients that would otherwise be taken up by weeds and those that are released by killed weeds. Control measures are especially important during plantation establishment. Grasses and herbs, and woody weeds are the main types of weed growth. Effective control measures may be devised for almost any situation.