Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Seed Priming
Seed priming refers to several techniques used to initiate vegetable seed germination prior to planting. This is useful for seeds that have natural dormancy, hard seed coats, or other physiological conditions that can slow germination. It is also useful where rapid, uniform germination is required such as in greenhouse transplant production or direct field seeding of small seeded vegetables. Priming initiates the germination process but does not go to the point that the radicle (seedling root) has emerged.
One common vegetable seed that is often primed is lettuce. Lettuce is prone to heat dormancy in high temperatures and seed priming can reduce or eliminate this problem. Other vegetable seeds that are often primed include onions, leeks, carrots, tomatoes, peppers, and seedless watermelons.
Heat dormancy resulted in uneven emergence timing in these lettuce seedlings. (Empty cells are from large plants that emerged first and were transplanted.)
There are several types of seed priming:
Water priming – this is the oldest and most common form of seed priming where seeds are soaked in water for a period of time prior to planting. This is often used where hard seed coats are an issue. However, water priming provides the least uniformity of all the priming methods.
Osmopriming – Seeds are soaked in solutions of polyethylene glycol, glycerol, or mannitol. These solutions have lower osmotic potential and can control the rate of water entry into the seed. These chemicals do not enter the seed but serve to control imbibition.
Halopriming – this method uses solutions of inorganic salts such as potassium nitrite, calcium chloride, potassium chloride, and others. Halopriming is used to improve germination rates and percentages and increase the salinity tolerance of seeds and seedlings. Salts used must be matched to the specific seed type – not one salt can be used for all seeds.
Solid matrix priming – in this method seeds are placed in a wetted inert material such as sand, sawdust, vermiculite, or charcoal. This simulates seed in a soil environment and uses the matric potential of the wetted media to control water intake into the seed. The matric material used must be able to be easily separated from the seed.
Thermopriming – in this method seeds are heat treated. This is used where soils that seeds will be planted into will be hot. This controlled heating conditions the seed ahead of time to these conditions.
During priming beneficial microorganisms such as biocontrol agents or plant growth promoting bacteria can be added to seed. In addition, mineral nutrients, such as nitrogen, calcium, manganese, zinc, and boron can be added to improve early seedling growth.
One of the major disadvantages of primed seed is that it has a shorter longevity or shelf life because the seed is imbibed and then dried again. Primed seed should be used for one season only and should not be stored for future seasons.
Seed Pelleting
Pelleted seeds are used to improve seeding efficiency by making all seeds the same size, increasing the size of small seeds, and making elongated or irregular seed round. This allows for improved mechanization of seeding in greenhouse transplant production or in direct field seeding.
Pelleting is accomplished commonly by covering seeds using a rotating drum with a clay coating that easily breaks apart when exposed to soil (or growing media) moisture. Seeds will be round when coating is complete. Lime coating is commonly used for small seeded legumes.
Common vegetables available in pelleted form include lettuce, carrots, onions, and beets. Many herbs such as basil are also pelleted.
Pelleted lettuce seed.