Seedling germination is often not conducted directly in the greenhouse. There are many reasons for this including space use efficiency of the greenhouse and the improved climatic control achieved in a germination room or seed nursery. Germination rooms and seedling nurseries serve two different purposes and require different infrastructure. A germination room can be a place where flats are stacked until emergence of the cotyledons and needs only temperature control. A nursery is more complex as plants will be grown until transplant stage and thus requires supplemental light.
Light
Light is required for the germination of some seeds. Seedlings should be exposed to light shortly after germination so that they don’t stretch. If using supplemental lighting, it may be necessary to remove the heat generated by the lamps.
Temperature
Germination rooms and seedling nurseries allow the use of temperatures that are slightly higher than the CEA/MEA set point. This higher temperature will accelerate seedling emergence.
Moisture
Follow moisture recommendations for each seed type to determine how wet to make your media. Remember that media that is too wet can rot the seeds. If using rockwool as a medium be sure to flush it with water before planting as it can have pockets of limestone left over from the manufacturing process that can create areas of locally high pH. All flats should be covered with either commercially available clear plastic humidity covers or plastic bags until seedling emergence to maintain media moisture levels. Once the seedlings have emerged, the covers should be removed to prevent the plants from stretching due to inadequate light intensity.
Germination
Seed should be purchased from a commercial source in large batches. The supplier should supply a germination percentage but it is a good idea to conduct your own germination experiment. Evaluate what percent of seeds planted have emerged after some fixed number of hours. Late emergence is not useful as those plants will be behind the others. Each cell in a flat can be double or triple seeded and the seedlings thinned to the appropriate density to select for a uniform crop. Seedlings that emerge early should be thinned so that they do not shade their neighbors and create a large discrepancy in plant size.
Saving seeds
Be careful about saving seeds. Often cultivars are hybrids which may not produce a seedling that was identical to the parent plant. Furthermore, many cultivars are patented and saving seed is prohibited by law. If you do save seeds or have extra seed from a large order, be sure to store them in a dark, dry and cool environment. Often a refrigerator is used for this purpose.
Avoiding Transplant shock
The transition from the constant environment in a growth room or nursery to full and variable sunlight in the greenhouse or high tunnel can be too abrupt for some crops. One strategy is to transplant as soon as possible so that plants will be exposed to the harsher conditions of the greenhouse/high tunnel from a very young age. Often, however, it is desirable to move somewhat older and larger plants to maximize growing space use as efficiently as possible. It is a good practice to avoid excessive light intensity (direct sunlight) gradually by utilizing shade cloth over the area where new transplants are to be moved at any plant age.
When to transplant
The optimal time to transplant depends on what seeding method was used and what the next stage of grow out will be. Often plants are transplanted at the cotyledon stage if seed was broadcasted as the planting method. Plants that were started in plugs or rockwool cubes are often transplanted after they have produced the first true leaf.
