Both red (Trifolium pratense L.) and white (Trifolium repens L.) clover seed yields can be highly variable and low in western Oregon. The objectives of this study were to: i) determine crop water requirements and supplemental irrigation timing, and ii) quantify the effects of soil and water status on inflorescence production, seed yield and seed yield components for red and white clover seed crops. In each species, five supplemental irrigation treatments were applied in 1990 and 1991 to first and second year seed crops grown on a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. Non-irrigated controls were also maintained. In red clover, increased plant water stress reduced the duration of the season-long bud and flower production, stem length, potential floral capacity (PFC), and seed yield (SY). Root rot index (RRI) increased with increasing levels of plant water stress, indicating that supplemental water applications reduced second-year root rot severity. The reduction in SY from increasing plant water stress was primarily caused by a decrease in floral fertility, and less conclusively by reductions in inflorescence number per unit area. One irrigation to fill the soil active profile during peak flowering provided adequate water to maintain efficient seed production. In white clover, SY was maximum in 1990 when water application was delayed until 68% of the available soil-water was used by the crop which maintained an even flush of flowers and restricted vegetative growth. In 1991, all irrigation treatments yielded the same or less than the non-watered control. This was due to the excessive vegetative growth from stolons that had grown between the planted rows the previous and present crop year. In both years excessive amounts of irrigation water favored profuse vegetative growth and reduced SY. Inflorescence density was increased by constraining soil-water in 1990 and was the yield component that most affected SY both years. Crop water stress index (CWSI) was a useful indicator of plant stress status and can be used to schedule irrigations in red and white clovers grown for seed under typical climatic conditions of western Oregon. / Graduation date: 1993