Breaking primary seed dormancy in Gibbens’ beardtongue (Penstemon gibbensii) and blowout penstemon (Penstemon haydenii)

This study established that chilling removes primary seed dormancy in 2 rare penstemons of the western US, Gibbens’ beardtongue (Penstemon gibbensii Dorn [Scrophulariaceae]) and blowout penstemon (Penstemon haydenii S. Watson). Wild-harvested seeds were subjected either to moist chilling at 2 to 4 °C (36–39 °F) for 0, 4, 8, 12, and 16 wk or to approximately 2 y of dry storage. Seeds of both species were dormant at harvest and responded positively to chilling. Penstemon gibbensii germination increased linearly with length of chilling, and collections from sites with longer winters required a longer chilling period to break dormancy. With longer chilling durations, most seeds germinated during the chilling treatment. Penstemon haydenii germination increased to nearly 100% after 4 or more wk of chilling followed by incubation under a cool, diurnally alternating temperature regime (10–20 °C [50-68 °F]) but did not germinate during chilling treatments regardless of duration. Under constant (15, 20, 25 °C [59, 68, 77 °F]) or warmer (15–25 °C, 20–30 °C [59–77 °F, 68–86 °F]) alternating post-chilling temperature regimes, germination was consistently < 15%. Without chilling, dry storage increased germination (from 0–15%) in P. haydenii. By contrast, P. gibbensii seeds showed no increase in germination following dry storage, where germination in both recently harvested and 2-y-stored seeds averaged 16% without chilling. These insights will assist propagation and reintroduction strategies for restoring populations of these rare species.