Leggy seedlings: what causes them and how to correct them
Garden of Eatin', Seeds & Seedlings

Leggy Seedlings: What Causes Them and How to Correct Them

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

Leggy seedlings commonly occur with seeds started indoors. They have the telltale long, skinny stems and sparse sets of leaves and, if left uncorrected, the condition can weaken their stems, stunt their growth, or make them more susceptible to 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.

Luckily, legginess can often be remedied before it’s too late and I’ve transplanted hundreds of leggy seedlings with success, most of which went on to have healthy, 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 the root of the problem.

Leggy brassica seedlings

The Causes

Insufficient Light
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 as being bogged down in the seed-starting mix, where it’s moist and warm, can make them more vulnerable to damping off.

Beet seedlings leaning toward the light

Too Much Heat
Overly high temperatures, such as those maintained over a heating mat or under a 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.”

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

Not Enough Space Between Seedlings
With tiny seeds, it’s tempting to simply scatter them in one large tray and thin them as they grow, but proper spacing helps prevents leggy seedlings as well. If you don’t manage them during initial development, overcrowded seedlings will try to grow taller and taller as they compete for light.

Spindly seedlings

The Fixes

Provide More Light
For the majority of home gardeners, the number-one cause of leggy seedlings is almost always insufficient light. 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 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.

For a budget DIY, you can pick up some fluorescent shop lights to supplement or provide all of the light your seedlings need. A standard 2-light T8 shop light fixture runs around $15 and a 10-pack of 32-watt T8 daylight fluorescent bulbs runs around $30. Hang two fixtures (four bulbs) a couple inches above your seedlings and you’re in business. For tight spaces, you can use this narrower fixture with a pair of 17-watt T8 daylight fluorescent bulbs, but the total cost for these non-standard sizes comes out nearly the same as high-output grow lights.

Sometimes seedlings have to live inside for a while due to weather or other circumstances. If yours typically don’t make it outside until they’re at least 6 inches tall, a good set of grow lights will ensure all the leaves continue to get enough light. Consider investing in a T5 light fixture with four daylight fluorescent bulbs to help your plants grow stockier and stronger. Got grow lights but notice your seedlings growing lankier than years past? It may be time to replace those bulbs. Fluorescent bulbs do become dimmer over time, even if our eyes can’t perceive the change.

Lights are typically hung from the ceiling, on a jump start stand, or below shelves like these. For either lighting system, you can use a simple programmable timer to switch the lights on and off as needed.

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 germination 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.

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.

Promote Stronger Stems by Brushing Them or Running a Fan
Leggy seedlings can sometimes be saved by gently brushing your fingers back and forth along the tops of the plants every day. This 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 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.

Brushing tomato seedlings

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.

Transplant Outside
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.

Bury the Stems of Tomatoes
If you have leggy tomato seedlings, the best way to correct them is to repot them (or transplant them) and bury the stems up to the lowest set of leaves. (This method also works for tomatillo, eggplant, and pepper seedlings.)

Bury the stems of leggy tomato seedlings

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  • K-Sea

    If I can’t save them do I just throw them in the trash? That seems so wasteful. Can they be composted or broken up and mixed with some type of soil for be reused? I’m very new to gardening so I don’t know all the terminology.

    • You can eat them, if they haven’t died. They’re basically just sprouts. Or yes, you can compost them. Leggy seedlings aren’t diseased, they’re just lanky and less certain to grow into strong plants.

  • Beryl

    As an ultra-budget friendly suggestion from the UK – add a high tinfoil collar to your windowsill seed trays. The foil reflects light back onto your seedlings as the sun travels across the window. It’s not quite as good as lights, but it’s usually enough to prevent legginess and keep the seedlings growing & happy till the days are warm enough to put them outside.

    • I should caution that if the foil has any crinkles, it won’t reflect evenly and could cause hotspots. A flat sheet of white paper (or white board) would work better in this case.

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