Native shrublands and their associated grasses and forbs have been disappearing from the Great Basin as a result of grazing practices, exotic weed invasions, altered fire regimes, climate change and other human impacts. Native forb seed is needed to restore these areas. The irrigation requirements for maximum seed production of four key native forb species (Eriogonum umbellatum, Lomatium dissectum, Penstemon speciosus, and Sphaeralcea grossulariifolia) were studied at the Oregon State University Malheur Experiment Station beginning in 2005. Species plots were supplied with 0, 100, or 200 mm of subsurface drip irrigation per year using a randomized complete block design with four replications. Irrigation in each plot was divided into four equal increments applied between bud and seed set with timing dependent upon the flowering and seed set phenology of each species. Seed was harvested in each year of production through 2011, and the optimal irrigation rate was determined by regression. The four native forb species differed in their responses to irrigation. Lomatium dissectum seed yields were optimized with 140 mm of irrigation. Eriogonum umbellatum seed yields were optimized with 173 to 200 mm of irrigation in dry years and progressively less to no irrigation in the wettest year. Penstemon speciosus seed yields were optimized with 107 mm of irrigation in dry years and were reduced by irrigation in wet years. Sphaeralcea grossulariifolia seed yields did not respond to irrigation. Water requirements of these species are low, and these results can be used by seed growers to produce native forb seed more economically.
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Vol. 35 • No. 1