It is important to keep the seedlings growing and not to let them remain overlong in the germinating medium. Root systems develop rapidly and if they get too large they are easily damaged in transplanting. Move seedlings when the first true leaves appear. You can usually see the difference between the first and second sets of leaves. If you are in doubt about the second set, wait for the third, but then delay no longer. You can transplant directly into a greenhouse bench, into other seed flats, or into pots, depending upon how you intend to use your plants. Let them go a little dry for several days beforehand; then water well just before shifting. Moist soil clings better than dry soil to tiny roots.
If you plan direct benching, prepare a well-aerated soil rich in humus; this will stimulate vigorous root development. With a dibble or pencil, make about 2-inch-deep indentations in regular rows, spacing the indents according to directions on seed packets. If space is at a premium in your greenhouse not an unusual condition put plants a little closer than recommended. Just keep in mind that it is important for sunlight and air to reach each seedling.
When your seedlings have been well watered and allowed to stand while you prepare the bench soil for them, they are ready for transplanting. Be gentle, work slowly, first plant the areas farthest from you across the bench nearest the glass. Then work forward, row by row.
To remove seedlings from the seed-bed, use any handy item: dibble, pencil, orangewood stick, or plastic fork. Press the tool down to the side and slightly behind each seedling, and with a scooping upward motion lift it up supported between thumb and forefinger. If seedlings are too tiny to handle in this manner, lift them with tweezers. Lower the rooted seedling into the prepared hole and to the same depth it stood in the flat; hold it steady while you firmly press soil around it. Take care not to bend roots; rather, suspend them into holes that are big enough for them. When all seedlings have been transplanted, water them thoroughly but gently to settle soil compactly about the roots.
Cover the transplants with newspapers or translucent plastic for a few days to protect from direct sun until they have become established in their new quarters. When it is evident that growth continues, remove the covering and give an application of liquid fertilizer. Repeat at two-week intervals. Fish emulsion, diluted according to directions, is excellent for transplanted seedlings.
If you are moving seedlings to a flat, work the same way as for bench planting but save time by making a number of holes at once with the dibble board. Spacing on the dibble board will be for plants of average size; if your seedlings are fairly large, set them in every other space instead of in each space.
You can water flats by sub-irrigation. Set the flat in a large container of water until the soil absorbs enough to feel moist on top. Then promptly remove the flat from the water. Sub-irrigation assures even distribution of moisture without wetting the foliage, which sometimes encourages disease.