Fields for seed production can be established by transplanting greenhouse grown containerized stock or from direct seeding. Direct seeding should take place in the fall to allow for natural stratification of the seed. Greenhouse materials can be established by seeding into cones or flats in winter for natural stratification or by stratifying the seed for 8 to 12 weeks in cold/moist conditions. Pre-chilling requirements are reduced with a liquid smoke treatment (Shaw et al. 2003). Germination can also be enhanced by watering with a weak solution of gibrillic acid (250 ppm) though treated seedlings appear to be less vigorous than non-treated. Royal penstemon plants have also been successfully propagated from herbaceous stem cuttings (Dave’s Garden 2008). Seed should be sown to a depth of 0 to 6 mm (0 to 0.25 in).
In herbicide tolerance screening trials royal penstemon has shown sensitivity to pre-emergence applied pendimethalin and benefin. Limited damage was observed to plants with post-emergent herbicide treatments. Plants showed good tolerance to post-emergent applied clethodim, dimethanamid-P and pendimethalin and had seed yields comparable to untreated plants, but untreated plots had highest yields overall. Penstemon plants showed high sensitivity to prometryn and linuron (Shock et al. 2008).
Royal penstemon seedlings grown in weed barrier fabric for seed production.
Derek Tilley, USDA NRCS Idaho PMC
Because of the limited number of available herbicides that can be used on broadleaf forbs, planting into weed-barrier fabric (pictured above) is a viable alternative to control weeds in forb seed production fields. Holes should be three to four inches in diameter at 9 to 16 inch spacing.
Pollinators: Successful pollination is essential for commercial seed production of royal penstemon. In pollinator studies manual pollination more than doubled the weight and count of seeds/capsule when compared with auto-pollination. Outcrossing also yielded five times more seed than self-pollinating and bumblebee pollinations enhanced seed yield an additional 25% (Cane 2005).
Royal penstemon is visited by selective pollinators. In one study hived honey bees favored distant Dalea ornata, while wild bumblebees and ground nesting species were attracted to royal penstemon from surrounding rangelands. (Cane 2005). Other identified pollinators include Pseudomasaris vespoides and Osmia spp.
Seed harvest can be accomplished by hand or by direct combining. Harvest should occur when the stems and capsules begin to dry and open. Seed can be cleaned with a small clipper or air-screen cleaner. Seed yields range from 20 to 50 lb/ac.
