Perennial weeds are an increasingly important problem in California. This may be due in part to the increased use of preemergence herbicides which reduce the competition to perennial weeds from the normally more prevalent, faster growing, annual weed species. In the process we have increased the relative influence of perennial weeds. Perennial weeds are usually deep rooted; and most often regenerate themselves by growing from underground rootstocks (rhizomes and stolons). Preemergence weed killers are usually ineffective against such large underground storage organs because they are designed to kill only newly germinating weed seedlings close to the surface of the soil.