The typical garden wisdom is “transplant seedlings when they have at least 2 sets of true leaves.” This is quite true, but there are other indicators as well. For instance, seedlings are definitely ready for the garden when roots grow through the bottoms of their containers, regardless of how many sets of leaves they have. (Some varieties grow a long taproot very quickly. These are best direct-sown outdoors, but if you do start them inside, transplant them as quickly as possible.) Another rule of thumb is that when seedlings are 2 to 4 inches tall, they are ready for garden or outdoor container.
That said, you can continue to hold most types of plants indoors for several weeks after they have visible roots, several sets of true leaves, or a few inches of growth. It is more important for the success of your plants to wait for safe transplant weather. If you set them out too soon, a frost or severe rainstorm may kill them.
If you have to hold your seedlings so long before setting out that they begin to bud or even bloom, go ahead and pinch off the buds or blooms before transplanting the seedlings. A young garden plant needs its energy to grow roots, not flowers. It will establish much more quickly and easily without buds or blooms.
Your transplants will grow best and reward you the most if they are set out into well-prepared garden soil. If your soil needs fertilizer or pH correction, take care of these tasks before you turn or rake the soil for the last time.
Before planting, smooth the surface of your bed with a garden rake. Level out any depressions and remove hard clods, rocks and sticks.
To get proper spacing, place your transplants on the surface of the garden bed in an arrangement that pleases you. DO THIS IN SMALL GROUPS . . . YOU DON’T WANT PLANTS TO WILT BEFORE YOU CAN PLANT THEM! Space far enough apart so that each plant can grow to maturity without overcrowding its neighbor. Some plants need more space than others; your catalog and packet instructions recommend optimum spacing.
Some plants, especially those with base branching habit, do not like to have their stems buried any deeper than they were as seedlings. These include: Ageratum, Begonia, Dahlia, Gerbera, Gazania, Pansy, Petunia, Primula, Salvia, Zinnia, Cabbage, Lettuce, and Pepper. Be sure that the soil line outdoors is the same as it was indoors for these types of plant.
Other species, such as Marigolds, Chrysanthemums, Coleus, Cosmos, Impatiens, and Tomatoes do not mind if the lower part of their stems is buried, and will root readily along the buried portion of the stem. If you have stretched-out seedlings (quite common when indoor light is not as strong as it could be), you can actually set these seedlings almost horizontally in the ground, with only a small portion of stem visible above the soil line. They will grow new roots along the buried part of their stem, and you will have short, stocky, strong plants instead of tall, spindly ones!
Do not bury the leaves of a seedling. If you do plant part of the stem underground, remove the leaves from that part of the stem first.
Avoid planting any debris such as sticks or leaves along with the root system of your plants. Such debris interferes with the necessary contact between root and soil.
If your plants are set out closely in beds, soak the beds immediately after planting. Soil should be wet to a depth of several inches below the roots. Run a gentle sprinkler or soaker hose as long as it takes to achieve this. Plants that are set out individually should be watered so thoroughly that a temporary mud puddle forms around the base of each plant. This will eliminate air pockets and bring about good root/soil contact.
Avoid splashing soil onto the stem or leaves of your young plants. For this reason, it’s best to avoid having water from your hose flowing at full force. Plants should be watered early enough in the day so that they do not go into the night with wet leaves.
Mulching is essential to conserve soil moisture, help keep down weeds, and maintain a more constant belowground temperature for the plant. Mulch your beds as soon as you have planted and watered. Use material that has weathered for several months so as not to deprive the soil of nitrogen. Old leaves, bark, dry grass clippings, wood shavings or any other loose, light material will do. If you do not have organic material on hand, use a plastic mulch, being careful to leave enough open space around the plant for water to penetrate.
Spread mulch several inches deep over the soil between the plants, pressing the mulch gently around the base of the stems. Take care not to break the stems or bury the leaves.