A:   Tar weed, also known as curlycup gumweed, is a biennial or occasionally a short-live perennial that reproduces from seeds. Plowing or other tillage practices are very effective at killing existing plants. New plants are likely to begin developing from seed following tillage, but they are unlikely to develop rapidly enough to affect oat hay as long as the oats are planted into a recently tilled, weed-free seedbed. Young plants, prior to forming flower buds, can be killed using 1 quart of Grazon P+D per acre. Mowing also is effective at limiting their development.