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Can You Fix Leggy Seedlings? Here's How (And What Causes It).

Leggy seedlings are a common problem when starting seeds in late winter and early spring. The good news is that these spindly plants can often recover and become lush green crops. 

What Is A Leggy Seedling?

"Leggy" is the gardening term for a seedling that appears stretched and weak. Their stems are long but thin, not stout. Leggy seedlings often have fewer and smaller leaves—they seem spindly. 

As a new gardener, I thought this was just what plants did, and I would rotate my seedlings every day or so to "keep them even." I didn't understand the problem was an overall lack of light. My tomatoes and peppers never looked like the ones I could buy at the greenhouse, and I wasn't sure why. 

My seedlings would start out gangbusters, and I'd be filled with hope and pride. Then they'd start to get a bit pale and leggy. In a few weeks, I'd have seedlings six inches tall but with only two or three tiny leaves and thin stems the size of mechanical pencil lead. I didn't understand the problem, and I'd give up and buy fat, bushy, healthy seedlings from the nursery and transplant those.

Why Are My Seedlings Leggy?

While many factors combine to create the optimum environment for young seedlings to grow, the most significant issues are light, temperature, and water.

Phototropism and Light

Plants naturally grow toward light. You may have seen trees in a forest growing tall with few branches, stretching for the canopy gap above. They're reaching for a portion of the sky—and the sunlight. Your seedlings will do the same thing. The process is called phototropism. 

Lack of adequate light triggers the plant to prioritize finding more. The plant prioritizes stem length over other growth, trying to find more light. It's similar to you when you're very hungry. You'll begin to look for food (sunlight to a plant) at the expense of other tasks like mowing the lawn or working out. The worse the condition, the more food (light) becomes the only focus. It's the reason seedlings bend toward the window and the most likely cause for leggy seedlings.

Water and Temperature

Plants are autotrophs, meaning organisms that synthesize their own food. The word breaks down into two root components: "auto," meaning self, and "troph," meaning nourishment or food. Autotroph means self-feeding. As we learned in grade school, plants need carbon dioxide, water, and light as external inputs to accomplish this feat. 

Lack of water inhibits the photosynthetic process, resulting in a lack of plant food, structural support, and nutrient transport. Outdoors in the soil, plant roots will grow toward a source of water just like the above-ground portions grow toward a source of light. We want the roots to expand their reach, so that's a good thing, and it's called hydrotropism. 

However, they can't do this in your seed-starting tray. If you don't supply the right amount of water, they'll grow slowly, weakly, and spindly. They'll look limp and unhealthy. But don't overdo it—too much water can inhibit growth by denying oxygen to the roots and causing fungal disease issues.

Temperature also affects photosynthesis. Too cold and it slows; too warm and it also slows. Fortunately for gardeners starting seeds and raising seedlings, most plants are pretty happy in the room temperature range of 65–80℉. A cold windowsill or an unheated greenhouse may dip below that range, causing slow growth.

How To Fix Leggy Seedlings

If your seedlings are off to a less-than-stellar start, you may be able to save them. Insufficient light is the most common cause of leggy seedlings. If your seedlings are getting enough—but not too much—water, and it's not too cold, try adding more light.

After providing more light, observe the soil moisture level carefully. Seedlings under light will use more water as they conduct photosynthesis faster. The warmer conditions under lights will also cause some water loss. You may find they'll need to be watered daily. 

Providing Adequate Light

While supplying enough water—but not too much—isn't difficult, and the temperature indoors is usually adequate, many home gardeners struggle with the light issue. 

In the northern hemisphere, gardeners start seeds indoors in late winter or early spring to get a jump on the growing season. Plants with longer maturity times, like tomatoes (check here for our favorites), peppers, and some flowers and herbs, can fruit earlier and longer before the frost if given a head start.

We all understand that the sunlight is brighter, or stronger, at noon than at sunrise or sunset. The level of light intensity is not even throughout the day. While your south-facing windowsill might receive bright sunlight in the early afternoon, the intensity of that light is not the same in the morning or early evening. 

If you live in the north, the intensity of light the plants receive is further reduced by the sun's path across the sky. In winter, the sun is lower in the sky—even at its zenith—than in summer. To compound the effect, the period of daylight is also shorter.

This combination means that even a sunny window likely won't provide the quantity of light needed by a sun-loving plant like a tomato. While you may get seedlings to grow in a windowsill or on a kitchen counter, they probably won't be healthy, green, stout plants. Most of us need to supply supplemental lighting.

Lighting Solutions for Starting Seeds Indoors

Providing a nurturing environment for seedlings need not be expensive or complicated. While commercial greenhouse operators use intense lighting and environmental controls to grow ripe tomatoes indoors in January, we can get by with much less for starting seeds.

Seedlings do well with about 12 hours per day of even, bright lighting, but just because it looks bright to us doesn't mean it is to the plant. Our eyes are poor judges of photosynthetically active light levels. However, we can let the plants tell us when they have enough. They have what they need if they're stout, deep green, and vigorous.

You can download an app to measure light from your smartphone for more accurate measurements. Be sure it measures photosynthetically active radiation (PAR). Apps for photography aren't suitable, but that's a topic for another blog.

Standard fluorescent bulbs in shop light fixtures are sufficient for giving seedlings the boost they need for those first few weeks of indoor growth. Inexpensive to buy new or used (look at local online marketplaces to find them), they don't draw much electricity and are lightweight and easy to hang. You can also buy a ready-to-use system.

LEDs are replacing long fluorescent bulbs, and while they are a bit more expensive, they last practically forever in seed-starting terms. You may never need to buy another one. Look for long shop light-type fixtures; they shouldn't cost much. 

Many DIY videos of how to set them up are available, but two steps make providing light simple and reliable.

  • Hang the lights on an adjustable chain or rope. Fluorescent lights should be hung only a couple of inches above the plants and moved up as the seedlings get taller. 

Light is energy and dissipates the farther it gets from the source, just like the radiant heat from a campfire, which is not as intense ten feet away as it is up close. If the lights are hung too far away, you'll get legginess again.

  • Plug the lights into a timer. Since I run several fixtures I plug them into a power strip. The strip plugs into a timer. I set it for 12 hours and forget about it.

*** Don't exceed the power strip's rating, and locate it somewhere you won't drip water on it.

While fluorescent and LED light fixtures work well, don't use old incandescent bulbs. They generate too much heat (and are inefficient), and you will likely cook your seedlings.

Check out some of our other articles for healthy, robust seed starts, including how to prevent damping off and hardening off seedlings before transplanting. 

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