Leggy Seedlings: What Causes Them and How to Fix Them (2024)

If you like to give your seeds a head start on the season by sprouting them on a sunny windowsill, you may be wondering right about now…

Why are the seedlings so spindly and stretching toward the sun? This isn’t a catwalk, ladies!

Leggy seedlings often happen with seeds started indoors and any type of vegetable, herb, or flower seedling can be affected.

Tomato, zucchini, broccoli, kale, lettuce, and beet seedlings tend to get leggy because they’re started in spring when daylight is still limited. Beets, in particular, sprout multiple seedlings from a single seed ball, making them easily overcrowded and prone to growing leggy.

The seedlings desperately stretch toward the light source, so much that their stems grow too long and lean in proportion to their leaves. They end up with pale, skinny stems and fewer, smaller leaves than healthy seedlings.

If uncorrected, the condition can weaken their stems, stunt their growth, or make them defenseless against pests and diseases. They’re a little more challenging to harden off, and are less likely to withstand wind and rain once they’re in the ground.

The good news is, leggy seedlings can usually be fixed before it’s too late. I’ve transplanted hundreds of tall, floppy seedlings with success, most of which went on to recover and have normal, productive yields.

If it seems like your seedlings more often than not have trouble growing straight up or forming thick, sturdy stems, read on to figure out how to save your leggy seedlings.

What causes leggy seedlings?

Problem #1: Insufficient light for growing seedlings

Seedlings have a natural tendency to grow toward light. When the light source is too dim or far away, the seedlings kick into survival mode and grow quickly in height to try to get closer to that light.

Unfortunately, there’s only so much growing a seedling can do and what it gains in height, it sacrifices in girth, resulting in thin, fragile stems.

This is why, with seeds started in windowsills, you might notice your seedlings leaning toward the sun, sometimes to the point of bending completely sideways.

This is a double whammy for your seedlings since being bogged down in the seed starting mix, where it’s moist and warm, can make them more susceptible to damping off disease.

Problem #2: Too much heat

Overly high temperatures, such as those maintained over a heating mat or under a humidity (germination) dome, can lead to a rapid growth spurt in seedlings.

As soon as the seeds germinate, they respond to the heat by putting up tall, skinny stems before leaf production has a chance to catch up. This results in unbalanced seedlings that are “all legs.”

Problem #3: Inconsistent moisture

If you’re inconsistent with watering and the seed starting mix often dries out between watering days, it prevents the seedling from growing a strong stem and leafing out well.

Continued lack of moisture will turn them spindly and eventually kill them as they’re unable to access the nutrients they need from the soil.

Problem #4: Not enough space between seedlings

With tiny seeds, it’s tempting to simply scatter them in one large tray and thin the seedlings as they grow, but proper spacing helps prevents leggy seedlings as well.

Read more: Get a Bigger Harvest With This Plant Spacing Guide for Raised Bed Gardens

If you don’t manage them during initial development, overcrowded seedlings will try to grow taller and taller as they compete with each other for light.

How to prevent leggy seedlings

Solution #1: Provide more light

For the majority of home gardeners, the number-one cause of leggy seedlings is almost always insufficient light. Therefore, the easiest fix for leggy seedlings is to give them more light ASAP!

Even if you place them in a south-facing window, the average amount of sunlight in late winter to early spring (when seed starting usually takes place) is much less than the 12 to 16 hours needed for strong seedling development.

Factor in fancy windows with UV-blocking or reflective coatings, and even less light is transmitted.

Most vegetable seedlings are long-day plants that thrive with 16 hours of light (and 8 hours of dark). An artificial light is the easiest way to achieve this, especially for northern gardeners with short growing seasons.

Note: I’ve recently updated this post to include the latest LED grow light technology on the market. Since this article was originally written, the price of LED grow lights has come down considerably, making them competitive with or in some cases, even lower than fluorescent lights.

I now recommend integrated LED grow lights over fluorescent shop lights for their energy efficiency, longevity, convenience, and cost.

Disclosure: All products on this page are independently selected. If you buy from one of my links, I may earn a commission.

So you’re already using standard fluorescent shop lights? Maybe you’re finding that your seedlings have gotten leggier than years past. If this is the case, it may be time to replace those bulbs (with either T5 tube lights or T8 tube lights).

Fluorescent bulbs do become dimmer over time, even if our eyes can’t perceive the change. A simple replacement may be all you need to prevent leggy seedlings.

Solution #2: Adjust the ambient room temperature

While warmer temperatures between 75°F and 80°F are ideal for germination, most seedlings grow best at a high of 65°F to 70°F during the day and a low of 55°F to 60°F at night.

Be sure to remove the humidity domes from your seedling trays or take them off the heating mats within a couple days of sprouting.

Seedlings left to grow in such environments develop thread-thin stems that are far too fragile to survive in overly humid or hot conditions.

Solution #3: Water from the bottom up

Even if you’re religiously spritzing your seedlings with a spray bottle, there’s no guarantee the water is making it all the way down to the bottom (especially if you sow seeds in deeper containers).

Bottom watering ensures the entire seed starting medium is moist and encourages roots to spread downward.

To fully hydrate your seedlings, place your seedling pots in a tray, fill the tray with water, and let the pots “wick up” as much moisture as they need.

Solution #4: Give proper spacing between seedlings

To promote good plant health and keep seedlings from stretching for light, space them no less than an inch or two apart once they develop the first true leaves.

Overcrowded seedlings will compete for light, and lack of proper airflow also makes them more vulnerable to pests and diseases.

Build stronger stems on your seedlings by brushing them

Leggy seedlings can sometimes be saved by gently brushing your fingers back and forth along the tops of the plants every day. This simple motion simulates an outdoor breeze and tricks the seedlings into thinking they need to grow thicker stems to hold up against windy conditions.

You can also circulate a small oscillating fan near your seedlings for the same purpose (and it does double duty by reducing the chances of damping off).

Just be sure to keep an eye on your seedlings if you have a fan running, as they may be prone to drying out more quickly.

Transplant seedlings outside as soon as conditions are ideal

If the soil is warm enough and the seedlings are mature enough to be transplanted, harden them off and get them outside in the sun to keep them from growing leggier.

Sometimes, seedlings live indoors longer than they should because of bad timing with frost or slow turnover with older plants in the garden. Use a planting calendar that’s customized to your zip code so you can better time your seed starting and transplanting.

If you jumped the gun on your seed starting this year or got some unexpected weather, there’s still hope: Rather than leaving your seedlings in their trays or tiny pots to continue growing, repot them individually into larger containers and move them to a space indoors that gets plenty of sunlight.

How to fix leggy tomato seedlings

If you have leggy tomato seedlings, the best way to correct them is to repot the seedlings (or transplant them) and bury the stems up to the lowest set of leaves.

Not only will this fix any problems with legginess, it’s a recommended practice to strengthen tomato stems and help their roots form more mass.

This is because a tomato stem has the ability to grow adventitious roots: tiny root-like nubs that sometimes seem to grow in the air. These adventitious roots (also called root initials or tomato stem primordial) are the earliest stage of development of the plant’s roots. If any part of the stem comes in contact with soil, the adventitious roots can develop into full-blown roots, anchoring the stem and further strengthening the plant’s overall root system.

I personally like to repot tomato seedlings a couple of times before I transplant them in the garden. Don’t worry about digging a super deep hole at this point, as you can simply plant your tomato in a shallow trench and save your back!

Read more: Why and How to Transplant Tomato Seedlings (a Second Time)

By the way, this method of partially burying stems also works for tomatillo, eggplant, and pepper seedlings (whether they’re leggy or not).

Common questions about leggy seedlings

This post updated from an article that originally appeared on February 21, 2017.

View the Web Story on how to fix leggy seedlings.