How to Handle and Transplant Seedlings
If seeding did not occur in individual containers, the resulting seedlings must be transplanted when they are small and resistant to transplant shock or slowed growth. This it typically about the time that they develop their first real leaves above the cotyledons.
You can purchase growing mixes and containers designed specifically for transplanting seedlings. These contain more plant nutrients than those used for germination. Some of these already contain fertilizer. If not, you can use houseplant fertilizer diluted as recommended by the manufacturer and applied every two weeks after the seedlings are established.
When transplanting, carefully dig up the seedlings with a knife, spoon, or other tool. Gently lay the clumps of seedlings down and allow them to fall apart so that you can pick out the individual seedlings. Take care not to damage the roots while separating or handling them. Always pick them up by their leaves, not their stems or roots. Punch a hole in the planting medium deep enough that the seedling was growing previously in the seed flat and insert the seedling. Once they have been transplanted, the seedlings should be watered and kept in the shade or under fluorescent lights for a few days away from direct sunlight.
Containers for Transplanting
There are many types of containers that can be used for transplanting seedlings. Selection should be based on the type of plant being transplanted and its growing conditions. Containers chosen should be cost effective, durable, and make effective use of available space.
Commercially, you can get containers made of pressed peat moss in lots of sizes. They are available individually or as strips of connected peat pots. When plants grown in peat pots are eventually planted in the garden, be sure the peat pot itself is completely covered by tearing off the top edge of the pot. If you fail to completely bury the peat pot, it could pull water away from the plant.
Compressed peat pellets are also available. When soaked in water, they expand into individual pots. These do not fall apart as badly as traditional peat pots.
Strips of connected individual plastic pots called cell packs are also available. They are frequently used by commercial growers because they do not fall apart when handled a lot.
Hardening-Off Seedlings Grown Inside
Before transferring plants from a greenhouse or home to an outdoor garden, they should undergo a hardening-off period to prepare them for the changes in environmental conditions. This can be done by lowering temperatures, reducing water, and reducing fertilization. This hardening-off process causes the plant to store carbohydrates and thicken its cell walls resulting in a firmer, harder plant.
This hardening-off period should begin at least two weeks prior to planting outside. The plants should be placed in a shady location either inside or outside where the temperatures range from 45° to 50° F. Initially, they should spend the day shaded. Then each day they should be exposed to more and more sunlight. Avoid exposing the seedlings to temperatures below 45° or to lots of wind. Slow watering, but do not allow the plants to wilt.
The purpose of hardening-off seedlings is to slow plant growth. Do not take it to an extreme, as this can stop plant growth and do significant damage to certain plants and crops.