It’s Not Your Fault Your Seedlings Are Leggy – It's All in the Light-and-Heat Balance (and It’s Easy to Fix)

“Leggy” is one of those words that can have positive or negative implications depending on the context. In old detective novels, it was considered a compliment when referencing a woman. But if you are talking about a plant, it implies an excessively long and straggly stem, which is not such a good trait.

Leggy seedlings are similar to leggy plants. They are both weak and stretched-out plants with thin stems. But since seedlings are very young, they have a better chance of recovering from this condition.

Don’t waste your time feeling guilty about why your seedlings are leggy and your other gardening pals all have stout seedlings. This is an extremely common problem when you start seedlings indoors. Instead, learn what causes leggy seedlings. Hint: it's all about the light and heat balance. I'll explain exactly what that means so you can make the necessary changes to grow strong, healthy plants.

What Are Leggy Seedlings?

Some seed starting mistakes are difficult to recognize and require a level of expertise. But figuring out if your seedlings are leggy doesn't require a horticultural degree.

Anyone can recognize leggy seedlings. Are stems long and thin? Do they look fragile? Do they lean over, unable to support their own weight? Is there too much space between leaves? Do they have less leaves – or smaller, paler leaves – than they should?

If the answer to any of these questions is yes, then you have leggy seedlings. And leggy seedlings are stressed seedlings. Determining the root cause behind this common problem will help you grow stronger, more successful plants.

The Most Common Cause of Legginess

Just as legginess in seedlings is easy to recognize, the causes are fairly standard. There are two main culprits that can work alone or in tandem. Seedlings become leggy because of light or temperature or a combination of the two. This usually comes down to too little light or too much heat, or both.

Not giving seedlings the right amount of light is the most common cause of leggy seedlings. As your seeds germinate, they require sunlight to produce energy and grow. If they don’t get enough sunlight, they can’t produce healthy foliage.

This happens most frequently when you start seeds indoors. Seedlings in a garden are likely to get all-day sun, while seeds grown on a windowsill may get far less.

Another risk of growing seedlings on a windowsill is uneven sunlight. If the light comes from one side and the plant positions are never rotated, the young seedlings will stretch and lean. Plus, if the air temperature is too warm, it can stunt the seedlings.

You may have heard that seeds germinate faster and seedlings grow faster in warm weather, but that’s not entirely true. Very warm temperatures encourage rapid, weak growth.

How to Grow Strong Seedlings

Leggy seedlings frustrate even the most dedicated gardener. Yet the condition is quite common when seedlings are grown indoors during late winter while natural light levels are low.

So, is it really possible to grow strong seedlings indoors? It is, if you keep this formula in mind: lots of light and cool to mild temperatures. Here are some steps to take.

If you plan to grow seedlings indoors, make adequate light a priority from day one. Check the light available in your windows before deciding on the best location for your plants. If you live in the north, winter months may not offer sufficient sunlight to grow healthy plants in the window. So starting seeds in March or later is a good idea.

Alternatively, you may need to use grow lights for a few months to supplement regular sunlight. If the sunlight slants in from one side, make sure you rotate your plants regularly to prevent seedlings from leaning into the light.

Grow lights are often a great alternative when there is little sunlight. But you can still get leggy seedlings if the artificial lights are too far away or too weak. Check the strength of the light bulbs you are using and increase them, if necessary. Position lights about 2 inches (5 cm) above the tops of your seedlings. They will need at least 14 hours a day of artificial light.

Be sure you keep the air temperatures cool to mild in any room where you have seedlings growing. If you’ve heard that seeds need warmth to germinate, it’s entirely true. That’s why some use heat mats, like this one from Amazon, and plastic domes during germination.

Once your seeds sprout, move the seedlings off the heat mats, take off the domes, and move the seedlings to a location that is cooler and brighter.

Tips to Fix Leggy Seedlings

If you have already started your seeds indoors and are facing leggy seedlings, don’t despair! In many cases, you may be able to help them get strong and healthy before it's time to transplant seedlings into the garden in spring.

One fix to try is replanting seedlings deeper in the soil. Try repotting so that the soil level hits right below the lowest leaf. Roots can form along the stem. You can even try trench planting tomatoes, as they form roots from their stems very easily.

Another option is to up the amount of light to 14-16 hours per day. If the seedlings have been grown in a north- or east-facing window, move them to a west- or south-facing window. Alternatively, bring in LED grow lights, like these ones from Amazon.

Bringing in a small fan can help, too. This affordable one from Amazon is a good choice. Place it so that it blows gently on the seedlings for several hours a day. This can strengthen the stems and help them thicken up over time.

What You Need to Grow Strong Seedlings