Seed priming is a simple way to jumpstart garden plants. It is a treatment that hydrates seeds so that the germination process begins, but radical emergence does not occur. Priming can be as simple as soaking seeds in water overnight or a more complicated procedure using various treatment conditions to enhance germination of agronomic, vegetable, or ornamental crops. Primed seed usually has faster seedling emergence with greater uniformity than non-primed seed, especially under adverse soil conditions. For example, in some parts of the United States, producers use primed lettuce seed during the hottest part of the growing season, when lettuce does not typically germinate well due to the extreme heat. When temperatures are cooler, growers switch back to non-primed seed. Primed pansy seed is also used in summer plug production in greenhouses to overcome thermodormancy.
While not a new technique, priming can be beneficial. As early as the 4th century B.C., Theophrastus observed that cucumber seeds soaked in water before planting hastened seedling emergence. Later in 1600, Oliver de Serres discovered that soaking wheat, rye, or barley grains for two days in manure water and then drying them before planting prevented grains from "being eaten away by soil pests". Charles Darwin also experimented with a form of osmotic priming when he submerged cress and lettuce seeds in salty sea water in 1855. He not only demonstrated that seeds survived the cold, salty water for several weeks, but they also germinated more quickly after reaching land. The significance of these early observations was not realized until 1963 when J.E. Ells treated tomato seed with a nutrient solution to enhance germination and early seedling growth. In 1973, polyethylene glycol was used as a priming treatment, resulting in a commercial practice used today by the seed industry.
Currently, various osmotic seed priming treatments are used by seed companies. Such compounds as polyethylene glycol, mannitol, potassium nitrate, potassium chloride, gibberellic acid, or hydrogen peroxide can be used for osmopriming triploid watermelon, cucurbit seeds, etc. However, the concentration of the osmotic solution, temperature, and treatment duration varies among the type of seed. After osmopriming, seeds are washed and dried using forced air or fluidized beds. Similarly, these solutions can be applied to vermiculite, diatomaceous silica, or calcined clay to hydrate large seeds by the method of matrix priming. While primed seed is more expensive, it is advantageous when planting new seed varieties that are available in limited quantities. Because triploid (seedless) watermelon seed is expensive to produce, it is often purchased as primed seed to enable producers to plant earlier in the growing season when soils are cool.
Drum priming or hydropriming is also used by commercial seed companies. In this method, a limited amount of water is sprayed onto seed as they slowly rotate in a drum. Alternatively, humid air is added to seed in the drum under computer control to achieve the desired level of hydration. Hydropriming is often used for field crops of grains and beans.
While any of these primed treatments can be used, these types of commercial seed treatments can reduce the storage life of the seed. Also, seed stored at high temperature and humidity will lose viability more rapidly than that stored under optimal conditions. Thus, primed seed obtained from commercial sources may have low germination when stored for more than one season. When using stored seed for home use, it is helpful to soak seed in water for a limited time and plant it in a small flat for germination. Later, when seedlings are small they can be transplanted into larger containers to avoid wasted space in the garden.
REVISED: June 13, 2012