Yellow and purple nutsedge are difficult to control worldwide. In turfgrass, the availability of herbicides that provide selective control of these weeds is limited. To address this problem, a sulfonylurea herbicide, halosulfuron, is being developed for the control of both yellow and purple nutsedge. To confirm preliminary results, evaluations of this herbicide were performed in both field and greenhouse studies during 1993 and 1994. The objectives of the field studies were to evaluate halosulfuron for turfgrass tolerance (safety to turfgrass) and efficacy for yellow and purple nutsedge control. Greenhouse studies were performed to determine the extent of translocation of halosulfuron in yellow and purple nutsedge. Four species of turfgrass were evaluated for halosulfuron tolerance: Kentucky bluegrass (Poa pratensis L. 'Plush’), tall fescue (Festuca arundinaceae Schreb. 'Confederate'), bermudagrass (Cynodon dactylon (L.) Pers. '419' and 'Vamont') and zoysiagrass (Zoysia japonica Steud. 'Meyers'). Over a two-year period, injury to these turfgrass species did not exceed 10% and in most cases was non-existent. In these studies, yellow nutsedge control with halosulfuron at 0.14 kg ai/ha averaged 90% after six weeks in the four turfgrasses. However, after six weeks, yellow nutsedge regrowth did occur. Purple nutsedge control was evaluated only in Kentucky bluegrass and was uniformly transplanted into the study area. Purple nutsedge control averaged 96% in Kentucky bluegrass at 6 weeks after treatment. Yellow and purple nutsedge contains a well-developed rhizome/tuber system, and as was seen in several of the studies, have the ability to regrow after herbicide treatment. Two greenhouse studies were designed to determine halosulfuron translocation into the tuber and connecting shoot and through a rhizome into another shoot. In the first study, a tuber was only allowed to develop two shoots from separate buds on the tuber. After a month, one of the shoots was treated with halosulfuron, and control ratings were taken on both shoots. In the second study, a plant was placed in one of two connected pots and allowed to grow. A rhizome from this plant (mother plant) was guided into the connecting pot where a new plant developed. After a month, the mother plant was treated with halosulfuron, and control ratings were taken on both mother and new plant. From both of these tests, there is statistically significant evidence that translocation was occurring through both the tuber and the rhizome. This translocation occurred not only at rates used for nutsedge control but at rates well above and below 0.14 kg ai/ha. More work, however, needs to be performed using radiolabeled tracers or immunological techniques to confirm the movement of halosulfuron in yellow and purple nutsedge. Although regrowth of yellow and purple nutsedge was seen in both field and greenhouse studies, halosulfuron does provide good initial control of both species. Sequential applications of halosulfuron are desirable in poorly established turfgrass. / Master of Science