Over the past few years, various penstemons (from ≈280 species) have become increasingly popular as a garden flower. These plants grow and tolerate a wide range of temperatures and have a variety of flower types and colors. Of particular interest to us is Penstemon digitalis cv. Husker Red, because its tall showy flower spikes have potential as a specialty cut flower. Although cuttings are the preferred method of propagating ‘Husker Red’ penstemon, for locations and regions where cuttings are not readily available, seedlings could be screened and then selected for stock plant production. However, seeds of many Penstemon species possess a dormancy that limits seed germination. As a result, individual germination requirements vary widely both among and within species (Kitchen and Meyer 1991; Meyer et al., 1995). In perennial production classes at the University of Nebraska–Lincoln, fresh penstemon seed sown without any soaking will take 4 to 5 weeks to germinate with only a maximum of 50% success rate (unpublished data).
Gibberellic acid (GA3, GA4, and GA7) has been shown to break dormancy and increase germination in seeds of several genera (Bewley and Black, 1982, 1985), including certain penstemon (Atwater, 1980; Meyer et al., 1995). Using Intermountain region penstemon species, Kitchen and Meyer (1991) working with five concentrations of GA3 (0 to 500 ppm) in combination with a cold treatment (2 °C) found that seeds that were treated with GA3 and stratified (to simulate natural conditions that a seed must endure before germination) had higher germination percentages. Raeber and Lee (1991) soaked seeds of Penstemon parryi in six concentrations of GA3 (0 to 500 ppm) for 24 h and found 87% to 97% seed germination. Neither low temperature stratification nor NaCl soaks were as effective as 500 ppm GA3 for seeds of P. parryi. Mexican species such as P. harwegii and P. gentianoides as well as P. barbatus ‘Coccineus’ germinated without stratification at 21 °C (Nau, 1996; Way and James, 1998).
Given the desire to grow ‘Husker Red’ penstemon as a specialty cut flower in a country where cuttings/plugs are not available and difficult to impossible to send from the United States and because seeds of ‘Husker Red’ are usually sown while fresh (when used for breeding and selection purposes) and often result in uneven seed germination (Lindgren, personal communication) or are reported to take up to 8 weeks for germination (Swayne, 2000), three experiments were conducted to determine if GA3 soaks would promote seed germination to facilitate stock plant establishment.
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