"Legginess" is what is referred to when seedlings become tall and spindly. This is caused by a lack of light or light that is too weak. The stems grow spindly because they are stretching and reaching for light.
To prevent legginess, Provide direct light. Although a window seems bright from sunup to sundown that doesn't mean there is enough direct sunlight for the plants to grow. If you are growing seedlings in a window, it needs to be a window that gets a good six to eight hours of direct sunlight each day in order to prevent legginess.
Provide long light. Grow lights can help provide your seedling with the strength and amount of light they need. Using a timer will make it easier to keep track of this. This is another reason why the supplemental light is needed—daylight hours in late winter and spring are nowhere near long enough.
A seedling wants a cycle of 16 hours of light a day and 8 hours of dark. The 8 hours of dark is important to the development of the plant. So make sure you don’t leave the light on. If a seedling does not get 16 hours of intense light it will stretch and grow towards the light source. Providing a supplemental light source besides sunlight is the best method of preventing leggy seedlings. T5 grow lights are great. You can use regular florescent lights but they are not always bright enough.
Make sure your lights are 3 inches above your seedlings. Then set a timer for 16 hours to give your plants the light they need. A grow light is not very expensive and you will make back what you would have spent in seedlings in your first year.
Provide close light: Keep the supplemental light source within two to three inches of the tops of the seedlings. When the distance is more than that, the tiny plants will stretch toward the light, resulting in leggy, weak stems.