Seedlings, like children and puppies, need constant attention. By providing the right amount of light, heat, air, and moisture you'll have a healthy litter, er uh, crop of seedlings that will thrive in the great outdoors. Here are some helpful pointers:
Light
Lack of light is the single, most common problem when raising seedlings. And it's probably the #1 reason why people become discouraged over starting their own seeds. Seedlings need more intense light than full-grown plants--14 to 16 hours a day is ideal. If they don't get enough light, or if the light isn't intense enough, they become spindly, leggy and weak.
Windowsills are popular for starting seeds indoors, but they don't provide enough of the right light for healthy seedlings. Direct sun from a southern exposure can be too harsh, resulting in wilted plants. In northern latitudes, light might be in short supply, or the windowsills are too cold.
I prefer low-cost fluorescent shop lights over the coslty grow lights for raising seedlings. Suspend the lights from chains, keeping them 2 to 4 inches above the tops of the seedlings, adjusting the lights as they grow. You can also add aluminum foil reflectors. If you do use a sunny windowsill, remember to rotate your plants every couple days so they don't have to stretch and reach for the light. Seedlings that are pale and weak are light-starved.
Moisture
Water is the highway that delivers nutrients and seedlings need a steady supply of it. Since dry air in a house or office can draw moisture right out of soil, check your seedlings often. Poke your finger into the soil or lift the containers. A container with dry soil weighs noticeably less than one that's well watered. Also, when seedlings have matured, water from the bottom to encourage roots to "reach for it." Last but not least, nobody likes a cold shower, so use tepid rather than cold water!
Temperature
Seedlings thrive in an average room temperature of 60 to 70 degrees during the day and dropping by about 10 degrees at night. Cool weather plants such as lettuce, parsley and cabbage prefer cooler temperatures after they reach 2 to 3 inches in height.
Air and Ventilation
Proper ventilation is important for a number of reasons. It helps plants "breathe" by circulating carbon dioxide and oxygen and it prevents damping-off disease. Moving air also keeps pests at bay and it helps strengthen plant stems, getting them ready for outdoor breezes. If necessary, set up a fan. Generally a small one is all you need. Finally, if your containers are covered with plastic or other covering, remove it as soon as the seeds germinate.

Seedling showing second set of true leaves
Fertilizer (no junk food served here!)
When seedlings have developed their second set of leaves, also called their true leaves, this is a good time to give them small doses of plant food. Organic fertilizers such as fish and kelp emulsions, or compost tea are ideal. Apply fertilizers half-strength directly to the soil or as a foliar spray.
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Tickle your plants! Brush your seedlings occasionally with your hand to strengthen your plants and get them accustomed to air movement.
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