Spotted knapweed (Centaurea stoebe L.), a perennial invasive forb that reproduces largely by seed, often forms new flowers after prescribed sheep grazing or mowing is applied during the bolting or flowering stage. It is unknown if these new flowers produce viable seeds by the end of the growing season. The purpose of this 2-yr study was to determine the appropriate timing (or timings) or combination (or combinations) of timings of defoliation on spotted knapweed to reduce its viable seed production. Spotted knapweed plants on foothill rangeland in west-central Montana were hand-clipped at seven different timings and frequencies of defoliation: June (bolting stage); July (late-bud-early flowering stage); August (full-flowering stage); June + July; June + August; July + August; or June + July + August. Unclipped plants were controls. Plants clipped in the bolting stage were defoliated at 35-40% relative utilization. Plants clipped at all other timings had 100% of their buds and flowers removed, plus 3 cm of each bud or flower stem. Plant response was evaluated from mid-August through September, whenever the seed heads of each treatment's plants reached maturity but while their seed-head bracts remained tightly closed. Clipping at any timing or combination of timings reduced the number of buds and flower heads per plant (P , 0.01), number of seeds per plant (P , 0.01), percentage of viability of seeds (P , 0.01), and number of viable seeds per plant (P , 0.01) compared with no clipping. Clipping during the bolting stage reduced the number of viable seeds by nearly 90% compared with no clipping. Clipping during the late-bud-early-flower or full-flower stage reduced the number of viable seeds by nearly 100% compared with no clipping. Spotted knapweed defoliation via prescribed sheep grazing or mowing in summer should suppress viable seed production of spotted knapweed. Resumen Spotted knapweed (Centaurea stoebe L.), es una herbácea invasiva perenne que se reproduce principalmente por medio de semilla, algunas veces produce nuevas estructuras florales despue´s de pastoreo prescrito con ovejas o de cortes aplicados durante la época previa a la floración o durante la etapa de floración. Se desconoce si estas nuevas estructuras florales producen semillas viables durante la etapa final de la época de crecimiento. El objetivo de estos dos añ os de estudio fue determinar el calendario(os) apropiado o la combinación (es) de calendarios de defoliación sobre Spotted Knapweed para reducir la producción de semillas viables. Plantas de Spotted knapweed en pastizales con lomeríos en la parte central-oeste de Montana USA, se cortaron a mano en 7 diferentes épocas y frecuencias de defoliación: en Junio (etapa previa a la floración); Julio (finales de la brotación/ principios de la época de floración); Agosto (etapa de floración completa); Junio + Julio; Junio + Agosto; Julio + Agosto; o Junio + Julio + agosto. Las plantas que no fueron cortadas se utilizaron como control. Las plantas cortadas en la época previa a la floración fueron ...
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