Answer: The most common reason for leggy seedlings is insufficient light. For this reason supplemental light in the form of fluorescents is almost always necessary for home gardeners to produce good quality seedlings. Leggy seedlings can happen if the seedlings are too crowded, or because they are too far from the lights (a few inches is how close that light should be!) or possibly because your light bulbs are old. (If they show blackening at the ends then they need to be replaced. You should have one cool and one warm bulb in each hood.)
The next range of problems can be in timing. Just as soon as the seeds germinate they should be placed in very bright light. Waiting a day or so can cause them to elongate, so you do need to keep a careful watch on the seeds. Make sure you pot them up to the next larger size pot as soon as they need it. This will help keep them in active growth and prevent stress. It will also preclude crowding and reduce any competition for light. Do not start your plants too soon -- they never grow as well under home conditions as they will in the garden -- so only start them as soon as required to reach transplant size at the target date.
When you plant the seeds, make sure you are leaving space in between them so that they do not come up like a tightly packed forest. You need to be able to get in around them for transplanting without damaging them and they need room for air and light to reach them. They also need room for their roots to develop, so in all ways crowding is a bad thing.
Finally, do not overfertilize becuase this can lead to overly lush growth, but do make sure you are fertilizing enough. Use a complete water soluble fertilizer and/or compost tea and/or seaweed products to try to provide a broad range of micronutrients as well as the big three.
But to go back to the beginning, the biggest reason for leggy seedlings is lack of light. Set your plants close to the bulbs, place the hoods (if you use more than one) close together so that the light is directed over the entire group of seedlings. Rotate the pots or trays so they are evenly lit -- the lights are brighter in the middle than at the ends. Finally, leave the lights on for about 16 hours a day.
I hope this helps.
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