Environment and Timing—Right Place at the Right Time
The flower structures of most herbaceous plants that produce viable seed have evolved to allow efficient pollination to occur, whether by wind or insect movement. But it isn’t enough for a plant to simply produce seed. The seed must be viable and remain viable long enough to germinate and develop into a healthy seedling. The presence of pathogens or pests during seed formation, or immediately after ripening, can significantly reduce the ability of a particular plant to reproduce naturally through seed production. For example, the ripe seed of narrow-leaved puccoon, Lithospermum incisum, can be eaten by mammals. In another example, a weevil frequents the seedpods of blue false indigo, Baptisia australis. To compensate for such challenges, some plants produce thousands of seeds, thereby ensuring that at least a few will survive distribution and germinate. These seeds may be so tiny that they have no value as a food source, and can germinate near the base of the parent plant. For example, Penstemon grandiflorus, (Fig. 5a & 5b) shell-leaf penstemon, is most easily propagated by simply scattering the ripe seed on the ground. In other adaptation strategies, seeds may be attached to wings or 'parachutes' that allow them to be distributed over a wide area, increasing the odds of landing in a location with favorable growing conditions.
Germination may be erratic in any herbaceous species, occurring over several months or even years. This in itself increases the probability of successful germination and reproduction in the next generation. Think about what might happen if every seed were to germinate from a plant species at the same time, say during a very dry season. Compare that species’ odds of survival with one in which a few seeds germinate throughout the season, or during the same season over many years, perhaps increasing the chances of at least a few germinating in a favorable environment. Even though the quantity of healthy seedlings will be smaller, at least some will live to ensure the survival of the plant species over time. Consider next some other survival germination mechanisms plants have developed in nature.
The time of seed harvest is critical for germination. The seed of some species only germinates reliably when it is very fresh. This has implications for immediate harvest and sowing. Others have an extended period of dormancy, or even double dormancy. Double dormancy may require not one, but two periods of cold followed by warm to break the sound sleep of the seed. Seed of penstemons and many other herbaceous plants may require stratification, which is the process of placing the seed in a substrate, like perlite, vermiculite, or sand, and exposing them to cold temperatures. Less vermiculite is used in the mix if the species is one that requires very dry conditions. Usually the stratified seeds of penstemon are placed outside and allowed to overwinter, exposed to the elements from November through March, unless the controlled environment of a cold facility is desired. A second method of stratification, used for small numbers of seeds, or in situations where access to an outside location needs protection from marauding creatures like mice, is to use a mix of damp sand and perlite (soilless mixes are the key to good stratification), and place the flats in the refrigerator. Stratification serves as a survival mechanism in that the germination mechanism in the seed is not triggered until conditions are favorable for its survival. For example, seeds will not germinate in the fall when new seedlings can easily be killed by cold winter temperatures. Instead, they germinate in the spring under favorable conditions. Seeds usually require a moist stratification treatment.
Other herbaceous plants have evolved seed coats that are nearly impenetrable, requiring scarification. Sometimes scarification is described as a means of overcoming seed coat dormancy. Stratification, on the other hand, usually is more associated with embryo dormancy and usually permits physiological changes within the embryo to occur. Scarification involves creating an opening in the hard seed coat, either through acid or nicking the seed with a razor blade or other sharp instrument to make moisture and air available to the embryo. Stratification and scarification processes both function to break seed dormancy. Understanding these seed germination strategies is important to time the collection and planting of seed and determine the method of handling and seed treatment that will produce the best results.