Propagation by seeds is practical if the seed is able to produce plants with predictable and desirable features. Seeds are produced and collected by industry specialists who market them through catalogues that illustrate the mature plant to potential buyers. Seed suppliers usually provide the buyer with information about the best time to plant, any pretreatment that may be necessary, and follow-up culture information. A grower who propagates from seeds should select a reputable supplier who will provide seed that is true to species and free of pests, weed seeds, and inert material; it should also have a high germination percentage (as determined by a seed test).
Preconditioning of Seeds
Certain species of plants, particularly those likely to be grown in a nursery, may produce seeds that require preconditioning before germination. As explained in Chapter 5, some seeds possess seed coats that are so hard and impervious to water that they must be altered before water can be absorbed and germination initiated. In their natural habitat, such seeds may require exposure to weathering elements or passage through the digestive tract of animals to break down the seed coat. A commercial grower can overcome such seed coat dormancy by mechanical scarification of the seed. Several techniques are commonly used, including:
• abrasion (such as rubbing with sandpaper or tumbling in drums with coarse sand or gravel)
• soaking the seed in hot water and allowing it to cool over a twelve – to twenty-four-hour period
• soaking the seed in concentrated sulfuric acid until the seed coat is paper thin but the embryo is not yet affected. This may require from several minutes to hours depending on the species.
Other species of plants produce seeds that remain dormant until certain physiological changes (termed after ripening) are initiated and completed within the seed. The initiation of the changes is usually temperature-related, beginning when the temperature drops below 50° F for a required period of time. The chilling is termed stratification, and it can be duplicated by growers when the needs of a particular species are known. To stratify seeds, they are first soaked in water for a half day or longer, then mixed with sphagnum moss or another moisture-retaining medium, sealed in polyethylene bags (for oxygen permeability), and stored in a cooler (not a freezer) for the required period of time. If the moisture – retaining medium is not sphagnum moss (which has a natural fungicidal quality), then a seed fungicide should be added for protection.
Some seeds may possess double dormancy and require both scarification and stratification before they germinate.
A great amount of research continues to be directed to the use of growth regulators as substitutes for the after-ripening period. Most of the results have yet to prove of commercial value, although gibberellic acid and kinetin are of some use to commercial growers.