Providing a constant heat source from underneath can be very beneficial to seedlings. Temperatures in the potting mix of indoor containers can be as much as 5F lower than indoor air temperatures. Seeds of most plants started indoors germinate sooner and produce healthier roots when the potting mix is warm, and bottom heat can help to prevent "damping off," the death of tiny seedlings due to pathogens at the surface of the potting mix. Electric heating mats specifically for seed starting are available from many garden centers and mail-order suppliers. If you use a timer for lights above the seedlings, don't plug the heat mats into it!
Keep the potting mix moist while the seeds are germinating. A spray bottle to water the surface gently without washing the potting mix out of the containers may be useful, or water can be added to the tray and allowed to move up into the mix. In either method, drain excess water that remains or accumulates in the tray, to keep roots healthy.
Seedlings draw energy for germination from nutrients stored in the seed. They don't need fertilizer until they have several sets of true leaves. Seedlings grown in a soil-less mix will benefit from a weak general purpose water-soluble fertilizer mixed 1/4 strength. Fertilize only once a week. Water as needed the rest of the week with plain water.
Transplant seedlings that outgrow the cell packs into larger containers. Larger peat pots or styrofoam or plastic cups with holes punched in their bottoms are excellent. Lift seedlings by the rootball, using a spoon or plant tag for support if necessary. Never hold the seedling by its stem, as you may crush it, or harm the growing tip. If you feel the need to steady the plant from above lightly hold the plant by a leaf. A seedling that has lost a leaf can grow another, but a seedling that has lost its growing point cannot survive. Larger seedlings in larger containers will require more space and often another set of lights.
Plants started indoors will not have been exposed to full sun, wind, or widely fluctuating temperatures. If they are not gradually accustomed to the outdoor environment, a process called "hardening off," their leaves may be scorched by sun or wind; they may even wilt and die.
About two weeks before planting outdoors, start hardening off the seedlings by moving them outside for increasingly longer periods each day. Start by putting them outside for a few hours in the shade during the warmth of the afternoon. Choose a spot protected from wind. Bring them back inside for the night before temperatures start to drop.
Each day, leave the plants out a little longer, and expose them to a little more direct sunshine. By the end of two weeks, unless freezing temperatures are forecast, the seedlings can stay outside in a sunny area until you are ready to transplant them into the garden.
An easy way to harden plants off is to place them in a coldframe, a temporary mini-greenhouse. Commercially produced coldframes are available in many designs. Once they have been hardened off, seedlings can be set out in the garden. Transplant on a cloudy day or late afternoon when the sun has passed its peak. Even hardened off plants may wilt when first exposed to full sun, but they generally recover within a day or so.
Row covers and other types of plant protectors can help even plants get off to a good start in the garden by reducing damage from wind and temperature fluctuations.
When transplanting seedlings grown in peat pots, newspaper pots, cow-dung pots, or any other containers made of organic matter, trim the pots down to soil level. The collars of these pots, exposed to drying air, will wick water away from the root zone.
To encourage roots to spread out into garden soil, carefully cut or tear holes in the bottoms of these pots, because they usually don't break down completely in the soil and may inhibit root growth.
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