Leggy Seedlings Are Asking for More Light

leggy seedlings
bellemarematt/Flickr/CC BY-SA 2.0

Starting seeds indoors is a great way to get a head start on spring planting, and it is a good deal of fun when it goes well. In cooler climates with short growing seasons, starting seeds early indoors can ensure that slow-maturing plants such as tomatoes will be mature enough to produce fruit by midsummer. And flower gardeners will find that they can save lots of money on unique plants by ordering seeds, starting them indoors, and transplanting the seedlings into the garden once they are established. 

Indoor seed starting, though, can be fraught with difficulties, and one of those problems is legginess. Every seed-starting gardener has experienced this: seeds that are carefully sown in the proper soil, watered precisely, placed in a south-facing window, watched diligently until they germinate and sprout—only to see them grow freakishly fast with tall, spindly legs that collapse under the weight of just a few leaves.  

This tall, spindly growth is what is known as "legginess," and it is very unhealthy for your seedlings.

The Cause of Legginess

Leggy seedlings are caused by lack of light, or by light that is too weak and indirect to meet the plant's needs. The stems grow thin and spindly because the plant is desperately reaching out for light. Even if we place the seedlings in a window with the best light available, most seeds are started in the late winter or early spring, when the sun is still low in the sky. The sunlight at this time of year is barely direct enough to allow plants to grow at all, much less thrive. 

Luckily, preventing leggy seedlings is a pretty straightforward affair.

Preventing Legginess in Seedlings

Here are three surefire ways to prevent your seedlings from becoming leggy, and they all involve light, light, light: 

  • Provide direct light. Just because a window seems bright from sunup to sundown doesn't mean that there is enough direct sunlight to grow plants. Your plants crave the direct rays of the sun, not reflected or refracted light. If you are growing your seedlings in a window, it needs to be a window that experiences a good six to eight hours of direct sunlight each day to have any chance at all. Plants that grow fine in partial shade during the heat of the summer still need much more direct sunlight in the winter and early spring, when the rays of sunlight are oblique due to the low position of the sun in the sky. It's likely that your window location will need to be supplemented with an artificial light set-up. A variety of specialty grow lights are available, but all you really need is a basic fluorescent shop light.
  • Provide long light. Provide your seedlings with at least 16 hours of light per day. 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.
  • 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. 

Once the seedling has become established with many leaves, you can pinch off the longer stems, which will keep the plant compact and cause it to bush out, rather than grow straight up. 

Preparing for Transplant Outdoors

A very common mistake is to move those lovingly tended indoor seedlings directly out into the garden once the key planting date has arrived. This can be a disaster, as the sudden change in environment kills most seedlings unless they have been gradually acclimated.

As the outdoor planting date approaches, begin "hardening off" your seedlings by giving them a bit of time outdoors on early spring days with mild temps and low breezes. Bring the plants indoors at night to protect them from cold. Gradually increase the length of time they spend outdoors during the day. By the time you're ready to plant them in the garden, your seedlings will be tough and ready for anything.