Many new plants come to our program through the extensive collections at Denver Botanic Gardens, but surprisingly enough, these new-to-horticulture species never seem to come with propagation instructions! That detective work often falls to the staff there, lead by the creative and determined Mike Bone, Senior Horticulturist.
The following was written by Brien Darby, a production/greenhouse assistant at DBG. One of his primary tasks in the coming years will be to organize and manage the production of plants for Plant Select®. We are very fortunate to be associated with such wonderfully talented and passionate horticulturists!
In 2009, I was given the exciting task of overseeing the Plant Select® germination trials. These trials have been conducted at Denver Botanic Gardens for several years, but were often overseen by volunteers and/or staff with fragmented involvement. This year, I had the opportunity to witness the trials from beginning to end; what this really boils down to, however, is meticulous note-taking on each of our trial species. I would like to share some of my findings.
The species selected for these trials are chosen for two reasons; there was either sufficient evidence that the plant was already hardy and drought-tolerant enough to survive the weather extremes of Colorado, or there was sufficient interest to determine whether the species could survive. In either case, these trials are conducted to determine a propagation protocol that would provide quick germination without sacrificing strong plant growth. The three variables enlisted were scarification, light, and stratification treatments.
As an example, one species explored was Sorbus scopulina. In one of the trials, the seeds were subjected to a 30 day cold stratification (being placed in a freezer). Another trial explored a 60 day cold stratification. The final trial utilized a scarification method that involved leaching the seeds prior to sowing for five days in the tank of a toilet. In this example, the 30 day and leaching trials resulted in zero germination. However, the 60 day cold stratification trial resulted in germination rates above 50 percent. As of right now, 27 of these seedlings are thriving.
In this case, based on the above information, we can easily make the recommendation that Sorbus scopulina undergo a 60 day stratification in order to germinate the most seed. The possibility exists to explore a different set of trials (starting with a 60 day strat.) with this species for the 2010 trials.
Lest there be any confusion regarding the propagating terms used in this entry, it should be noted that subjecting seed to freezing conditions is not a stratification treatment. It is a freezing treatment. This treatment has not been reliably shown to be beneficial to germination for any seed. Stratification, on the other hand, refers to the subjection of seed to both moist and cool conditions for some prescribed period. The use of the term “cold stratification” is redundant since the temperature conditions are embodied in the simple term “stratification”. The earliest known use of the term harkens back to the 17th century in a German text about growing trees. It should also be noted that leaching seed is not a scarification treatment. It is a leaching treatment.
Reports of most germination treatments are not complete unless a “control” treatment of “no treatment” are included.