how to fix leggy seedlings

At this time of year we get frequent calls from people who have started seeds inside and find that they've gotten too "leggy." This is most common for those who are growing seedlings on a windowsill where the light might be strong but not as direct or constant as you'd find in a greenhouse. The problem is, of course, that once the seedlings get lean and spindly they are more likely to be damaged when planted outside. Here are some of the reasons that seedlings get leggy, and what you can do if this has happened to you.

Seedlings grow leggy when they are reaching for the light. Be sure to grow your plants in as much light as possible. If you're growing under artificial lights such as fluorescent or the long, tube gro-lights, position the bulbs only about 3″ from the tops of the plants. Most people rig up a system where the lights can be raised as the seedlings grow.
Seedlings also grow leggy when they are started too early. Be sure to use the end of May as the time when most summer plants can be placed outside and work backwards from there, using the germination times on the seed packets. So for plants such as zinnias, for example, that germinate and grow quickly, they shouldn't be started before the end of April.
Once seedlings get too long and leggy many wonder if they can sink the stems lower in the soil once the plants are put outside. This works for tomato plants but most others can't be sunk into the ground in that way. Instead, use the methods below to help strengthen the plants.
Don't over-fertilize! Many people mistakenly believe that fertilizer will make plants stronger. In reality, fertilizers make plants grow larger and faster but they don't help the plants to become sturdy. Keep synthetic fertilizer to a minimum until the plants are growing outdoors.
Environmental "stresses" such as wind stimulate hormones in plants that signal the roots and stems to grow strong. So putting a small fan next to your seedlings on a timer so that the plants are blown in the breeze for a couple of hours a day will help strengthen leggy plants. Gently passing your hand over the tops of seedlings a few times every day will also stimulate stronger growth.
When it's time to move the plants outside introduce them to the "real world" gradually. Don't put tender plants out into the direct sun – either place them in mostly shade (the dappled sun through trees is good) for a few days or put them outside during a stretch of cloudy weather. If the weather turns stormy, pull those plants inside until heavy rains and high winds pass.
Most leggy plants become sturdier once they are growing outside.

Sometimes people are tempted to fertilize tiny tomato seedlings in order to hurry their growth in the spring. Don't do this too early or you'll end up with taller, weaker plants before it's time to plant them outside. Tomatoes shouldn't be planted outdoors until the night time temperatures are reliably above 50 degrees.

These broccoli seedlings are short and strong because they have been raised in a greenhouse where there is lots of light. Seedlings are more apt to become leggy when the light isn't strong enough. If your windows don't get full sun, consider starting your seedlings under lights that are placed about 3" from the tops of your plants. (Note: high-powered grow lights that get hot can be further from seedlings, but fluorescent tubes, regular or full spectrum, should be close to the plants.)