A seed is a dormant embryo which carries within it the potential for a new plant’s life. The seed is alive, with a very slow metabolic rate due to the low mobility of substances within the embryo’s cells. This low metabolic rate allows the seed to remain alive, and survive extremely long periods of time (some seeds can survive even hundreds of years) before actually sprouting into new plants.

Germination is the process in which the plant embryo wakes up and an increase in the seeds metabolism begins which start the reproduction of cells that causes a new plant to grow. The main ingredient that starts this amazing process is simply liquid water. When water gets into the embryo and hydrates its cells, it speeds up metabolism and allows the process of cell division and growth to rapidly increase. However it is not always this simple to start this process since several impairments – both chemical and physical – can hinder successful germination.

Priming is controlling the hydration level within seeds to allow seedlings to emerge more quickly. It is simply a process done prior to conventional seed germination which allows metabolic speed increase to begin. There are several types of priming that can be done:

• Hydropriming. Hydropriming is the simple soaking of seeds in water, although aerated distilled water is preferred. This process is especially useful in economically disadvantaged, arid crop growing areas.
• Osmotic priming. Osmotic priming, also called osmopriming or osmoconditioning, is the soaking of seeds in solutions containing chemicals such as mannitol, potassium nitrate (KNO₃), potassium chloride (KCl), polyethylene glycol (PEG), or sodium chloride (NaCl). Plant hormones, which control or affect various stages of seed germination, or beneficial microorganisms (which help control fungal and bacterial disease) can be added to the osmopriming solutions. You can also use Roots XL from Complete Hydroponics to aid in the root development of the plant.

• Solid matrix priming. Solid matrix priming involves the incubation of seeds in a solid, insoluble matrix, such as vermiculite, diatomaceous earth, or another highly water-absorbent polymer, with a limited amount of water, allowing for slow imbibition.
• Drum priming. Seeds are hydrated by placing them in a rotating drum into which a controlled level of water vapor is released.

In general, priming offers the opportunity to almost always germinate seeds at much higher speeds without detrimental effects in germination percentages. In general if you are looking to test priming on some difficult seeds you own you can try three small experiments to know which one works best for your particular seed variety and germination conditions. Do one experiment in which the seeds are simply soaked in water for 24 hours, another in which seeds are placed in a 200mg/L NaCl solution and another one in which the plants are submerged in a Polyethylene glycol 20% solution, then let the seeds air-dry after the treatments. After comparing the results of these experiments with a control with no priming you will be able to see which priming technique is better for you and most effectively increases your seed germination rates. 

Priming also improves the stand uniformity, aiding in production management and increasing the chance for uniformity at harvest.

To sum it up priming your seeds is a very efficient technique to increase the speed of germination without sacrificing germination rates. This methods are not very useful for seeds such as lettuce or tomato – which germinate easily – but they are invaluable for plants such as parsley, coriander or carrots which are generally much harder to germinate. If you have some seeds that have been giving you a hard time or seem to take ages to germinate then setting up some priming experiments might be the best thing to do.