Herbicides with residual activity more effectively control infestations of yellow starthistle, a facultative winter annual, because seed banks quickly furnish replacement plants after nonresidual herbicide treatments. Picloram has been applied to rosettes in fall or spring, but new infestations of yellow starthistle are often discovered when plants are more noticeable in bud or flower stages. Eradication, containment, and revegetation are facilitated if weed seed rain can be stopped. This study evaluated whether registered rates (0.14, 0.28, and 0.42 kg ae/ha) of picloram, alone and with 2,4-D at 1.12 kg ae/ha, can prevent seed production when applied to yellow starthistle at bud or flower stage. Picloram applied at bud stage curtailed both seed production and germination, reducing seed production by 42 to 86% and viability by 80 to 99%. Neither the picloram rate nor the addition of 2,4-D to the spray solution affected the percentage of nonviable seeds. The addition of 2,4-D further decreased germination of developed seeds only at the lowest picloram rate. At flower stage, picloram and 2,4-D neither killed mature plants nor consistently reduced the quantity and quality (viability) of seeds. Bud stage was the phenological limit for effective reduction of viable seed by picloram, which caused both bud abortion and lower seed germination.
Nomenclature: 2,4-D; picloram; yellow starthistle, Centaurea solstitialis L. #3 CENSO.
Additional index words: Herbicide timing, reproductive-stage application, seed germination, weed seed yield.