HOW TO PREVENT LEGGY (“ETIOLATED”) SEEDLINGS
Or, if you’re already at that point, here’s why it happened and maybe how to fix it…
It’s that time of year when a young gardener’s fancy turns to the miracle of seed germination and growth. And with that often comes a lesson in “etiolation,” the process in plants grown in partial or, heaven forbid, complete absence of light. The collective signs of etiolation: long, weak stems; smaller leaves, longer internodes (the spaces between the “joints”); and a lack of rich green color (the chlorophyl). When seedlings are grown in low light (called “skotomorphogenesis,” a good word to throw around at fun parties), it leads to etiolated — stretched — seedlings. In nature, such stretching is a survival mode, a way to get to wherever the sunlight is (or might be) in a quicker manner. In the home seed-growing set-up, such seedlings are weak, prone to diseases and physical breakage, and generally hard to handle.
Insufficient light is the primary reason for etiolation. It should be noted, though, that the lighting is for the seedlings, not so much for the seeds. Very few seeds need light to germinate (it’s the very tiniest of seeds that do and those are the ones that should be pressed against the soil surface but not covered; all other seeds must be covered at least lightly).
It’s the seedlings, immediately upon germinating, that need the light. They need strong light, they need it in the right wavelengths, and they need it for a long period. “Strong light” is comparable to full, noonday sun outdoors. But since you’re not growing your seedlings outdoors (at least not most seedlings), you’ll need artificial lighting that comes close. In reality, even the strongest bulbs can’t match outdoor lighting, but we do what we can. South-facing, sunny windows come close to strong outdoor light but they, too, fall a bit short and there’s the danger of tender seedlings sitting in a frosty window.
There’s a ton of info out there about which is the best type of bulb/lamp to use for the “right wavelengths.” And too much of it is old info, complicated and confusing info, even contradictory info. So, I’ll keep it simple; I personally like “T5 HO.” There are several brands.
To make sure the seedlings get the “strongest light” possible, the bulbs must be set REALLY close to said baby plantlets. REALLY close, as in just 3 to 4 inches above the seedlings. As the seedlings grow, the bulb/lamp is raised. Hence why seed starting gardeners resourcefully devise all manner of lifting contraptions. See photo below.
Again, the seeds (with exceptions) don’t need the light; it’s the seedlings that do. So, the bulbs must be on after the seeds have germinated. How much is that “long period?” 14 to 16 hours per day. Use a timer.
Insufficient lighting is almost always the one and sometimes only reason given for etiolated seedlings. But there are other reasons, usually as a complex in addition to poor lighting, that lead to stretched, weak plants.
One of these secondary reasons is inadequate air circulation. When the air around them moves, seedlings develop cellular material that supports them better. It’s easy to set up an oscillating fan nearby. Nearby, not in the poor seedlings’ faces. As a bonus, good air circulation also helps to keep down the dreaded “damping off” disease complex (whereby seedlings turn dark at their bases and then flop over).
Another reason is too much fertilizer. Most seeds will produce seedlings that can grow quite a bit without supplemental feeding. And most commercial seedling mixes contain built-in fertilizer to help seedlings survive a few weeks into their growth. If you make your own seedling-growing mix from scratch and you’re not using a good homemade compost as the organic component, you’ll need to add the oomph for the slow-growing sorts. Also, if you are producing the kinds of plants that are really slow to get going — that is, they take their time getting to a transplantable stage — you will need to supplement. But aside from such exceptions, seedlings are fine without additional fertilizer until they are transplanted and when they are transplanted, it’s almost always into a medium that already contains a bit of fertilizer (or simply good compost).
Thirdly, temperatures for the seedlings may be too high. A heating mat is a great way to get seeds to germinate. That’s when you have it set for somewhere between 65°F and 80°F-ish, a good temperature for the soil mix. But once the seeds have germinated and seedlings are actually growing, it’s the air temperature that is more important. Turn the heating mat off (or at least down) and do what you can to keep the air temperature around the plants at no more than 65°F to 70°F for the heat lovers and 60°F to 65°F for the cool stuff.
A fourth reason for tall, skinny seedlings: too many seeds in too little space leading to overcrowding. Germination of commodity packet seeds is very high, usually in the 90-95% range (it’s marked on the packet somewhere). Even at 70-80% germination, there’s no need to plop a gob of seed into a tiny potlet or tray cell. Use a tweezers or seed-sowing device (there are many cheap ones available) and put but two or three seeds per tiny pot or cell. If sowing tiny in rows, mix the seed with a bit of light-colored sand and sprinkle carefully.
When the seedlings have shown you they really want to grow and there is a clump (more than one), thin them out. The practice of thinning is not the same as transplanting. This is not the time to divide a clump; this is the time to “cull” out the extra. That means taking a small, sharp scissors and cutting off all but the strongest single seedling. I know it’s tempting to SAVE every single baby plant (after all, it’s a life, right?) but seeds are cheap (relative to gardening in general) and you almost always get more seed than you can use (hence why so many “seed trades” on social media platforms).
Finally, leaving seedlings in their trays, cells, pots too long under artificial conditions will eventually lead to disproportionately tall plants, regardless of whether the other preventative measures were employed. This is an all-too-common issue. Time your plant production to coincide with appropriate “planting out” dates for your area (including an essential “hardening off” period) and/or make sure you have the pots and potting soil to “move up” continually and the room to do it.
Transplant beginning with when the youngest seedlings are ready — when they’ve developed their first “true leaves” (the ones that look like actual plant leaves; versus the first “umbrella” of faux-leaves, which are technically cotyledons and not true leaves). (Caution: even after development of “true leaves,” some seedlings are still too tiny to handle easily. Let them grow a bit more.) Subsequent transplantings are made just as the seedling/little plant has filled its pot, whether you’ve planned on “moving up” or you haven’t planned it but the seedlings have decided to fill their little pots and it’s still too soon to put them out.
Keeping these six guidelines in mine, THIS can be your tomato plants:
For additional info on sowing seeds, in general, there’s a “slide show” link available in my article on “SEEDS: Basics of Sowing.”
© copyright Joe Seals, 2025
So helpful ! Thank you!