Propagating penstemons from seed requires some knowledge about the general background of the seed and also about germination procedures. For example, some penstemon species and varieties require no pre-germination seed treatment. This is especially true of species from mild climates. However, for most of the penstemon species, some type of seed treatment is beneficial to enhance germination and reduce seed dormancy. The method to use is often dictated by the facilities available. Not everyone has a greenhouse in which to start plants.
The first step in propagating seed is to acquire the seed. Seed may be collected in the wild or from a garden. Make sure federal and state regulations are followed when collecting seed from the wild. Through the American Penstemon Society and many commercial seed collectors, you can obtain seeds and grow species and varieties for a wide variety of locations and growing conditions.
It is a good idea to sow a quantity of seed that is several times the number of plants you desire. Very seldom will you get 100% germination of seed. The American Penstemon Society’s Book on penstemon (Growing Penstemons: Species, Cultivars and Hybrids) can be used as a starting point in determining seed germination conditions. Conditions may include but are not limited to stratification, scarification, alternating temperatures, chemical treatments, light versus dark, and seed aging. These techniques will also vary with the quality of the seed. You may also find out that in one year a technique works very well and the next year, it does not. Penstemon seeds often germinate best after being stored dry for 6 months to a year, so do not discard seed that is not fresh. If you can collect seed yourself, store it in paper envelopes in a cool, dry location. You will usually get reasonable germination for about five years after collection, and possibly even longer for some species. For example, the following table shows the results of a study comparing the germination of penstemon seed from one to ten years old. Germination of Penstemon strictus was good for seed up to 6 years old.
Many penstemon seeds require a long cool, moist stratification time before they will germinate. Cold stratification consists of growing the seed in a moist medium at temperatures just above freezing. This is especially true for northern species. Southern species and many large flowering hybrids require no stratification. Almost every grower has his or her own method of accomplishing this. Probably the most common method is to prepare a mix of new perlite and vermiculite and thoroughly dampen it, pack it into new or washed pots, sow the seed on top, cover lightly and place the pots outside in the winter. The pots need to be protected from birds, rodents, insects and heavy, washing rains, but allowed to undergo variations in temperature. Clean sand may also be used with the perlite and vermiculite, and the proportions of the ingredients should be adjusted so that the seeds of plants that grow in dry conditions are sown on a mix that contains less vermiculite. The seeds can be sown anytime from November through early March in cold climates and germination will begin when the weather warms up. Germination of wild seed often takes place over several months or years as nature’s way of insuring that some plants will survive the vagaries of weather. Some species respond with higher germination if temperatures of the growing are allowed to fluctuate.
Another method of providing stratification is to place seeds on damp sand or perlite in a plastic fold-over sandwich bag with a plastic tag on which is written in pencil the name, date and seed source. Several of these can be folded, bundled and placed in the refrigerator (not the freezer). They should be checked regularly. Add a couple of drops of water if needed to keep the packet moist. Remove from the refrigerator for planting at the first sign of germination. Once germination has initiated, the seeds should immediately be placed on top of a growing medium mix in containers and moved to a location where they get very bright light and gradually warming temperatures. A warm sun room will serve for a few days to get them growing, but they should be moved to a cool shed, basement, garage or porch where a bank of fluorescent lights can be hung 3 – 5 inches above them if they are not in daylight. A greenhouse is an ideal location to do this. The young seedlings can be fed with dilute solutions of fertilizer once they have true leaves. Growing at a temperature between 40 and 60 degrees F will keep them from drying out rapidly and prevent damping-off disease. It is convenient to have the lights on an automatic timer and set them to be on 14 – 16 hours a day. The plants should be separated into individual pots or containers in soil that is a mix of garden soil with sand, pumice or whatever lightener you prefer as soon as they have two pairs of true leaves. A soluble fertilizer, highly diluted, can be added. Move the trays or pots outside when the weather is good and back under cover or under the lights if the temperature drops rapidly. They need gradual exposure to outdoor conditions of all types, including strong sun, wind and dryness. In temperate climates, when the seedlings have two or three leaves, they can be planted out in prepared beds. If you have a cold frame that does not get too hot, they can be placed in it, but it is easy to lose small plants if the temperature inside rises too rapidly. When transplanted from pots into the garden, some protection should be used for a week. This method gets plants off to a rapid start.
If you have a generous amount of seed, the simplest and easiest way to start it is right in the ground. Choose a location that will be in the shade during the part of the winter when the sun is very low, but that rain and snow can reach. In the late fall, loosen the soil and scatter the seed thinly on top so it is spaced with enough room to move the plants after they have developed. Put a very thin layer of coarse sand or potting soil over the seeds. You can then forget them until they begin sprouting in the spring. They will become sturdy plants and easy to transplant to permanent locations by mid-spring.
Some seed will respond to scarification, the process of nicking the seed coat, to break dormancy. For example, studies have shown that Penstemon haydenii responds to the nicking of the seed coat. It also responds to letting water run over the seed for about 24 hours. (Penstemon haydenii is an endangered species so seed of it is not available. However, we can learn from the studies that have been conducted on this species.)
It has also been reported that some species of penstemon seed require absolute light for germination. This has not been documented in other studies. In addition, growth regulators have been shown to increase germination.
These suggestions are only to help you get started. You will probably develop your own methods that work best for you in your climate. Don’t get discouraged if the level of germination is not what you had hoped for. Just try again. A list of references in the next paragraph can be reviewed for those interested in more information on germinating penstemon seed.
A link to Jim Swayne's Penstemon Seed Germination spreadsheet and notes to the terminology used on the spreadsheet. Mr Swayne also would like to acknowledge Alplains and Peter James book 'Gardening with Penstemons' for their contribution to his page.
Click here to open or save a great article by Bob McFarlane on germination and growing plants on in a hoop house.