Propagation of the genus Penstemon for use in landscape horticulture has been handicapped by a lack of understanding of seed dormancy and a practical method for breaking dormancy for numerous species. The extent of dormancy in seeds of 27 wild populations of Penstemon representing 16 Intermountain species was investigated by subjecting seeds to stratification (moist prechilling) of 2 to 16 weeks at 2°C (36°F) and varying concentrations of gibberellic acid (GA3). Germination varied from 0 to 88% for non-treated seeds and from 13 to 100% for seeds treated with 250 ppm GA3. Collections from 10 species required 12 or more weeks of stratification for complete germination. Three species exhibited reduced germination after stratification when compared to the non-chilled control. Gibberellic acid significantly reduced the stratification requirement of seeds for the more dormant species. Concentrations of 150, 250, and 500 ppm GA3 were equally effective in breaking dormancy for most species. The considerable variability in seed dormancy suggests that the most practical solution to penstemon propagation from seed may be the selection of species and ecotypes with minimal dormancy.
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Kitchen et al. (1991) studied this question.