why vegetable seedlings stretch and get spindly

MAYBE YOU’RE WONDERING this about now: Why do vegetable seedlings stretch and grow spindly sometimes, and how can you prevent such leggy seedlings? That was how I began a note to Dr. Thomas Bjorkman, Professor of Crop Physiology at Cornell, seeking an answer to a question I’m asked a lot.

He’s a botanist whose research focuses on the effects of environmental stimuli on plant growth and development, particularly in vegetables. So I asked him what’s going on—are the leggy seedlings reaching for light, or is something else at work? (I couldn’t resist sharing the mung-bean time-lapse video, above…though probably not what you’re sowing at the moment.)

Everything I’d read over the years listed a range of possible causes for leggy seedlings:

  • too little light (maybe also too much);
  • the intensity of the light source;
  • temperature, and even the temperature difference between day and night;
  • improper use of fertilizers;
  • leaving seedlings in “germination chamber” conditions (extra-warm and extra-humid) too long, under that plastic dome or on that heat mat or both;
  • …and the list goes on.
  • Turns out even the spacing between seedlings can affect the way they grow.

But all the cited research had been done in the light- and climate-controlled conditions of a lab, or commercial greenhouse—not the less-formal home seed-starting environment.  So what are the answers for home-garden types, I kept wanting to know?

Young tomato seedlings ready for transplant

preventing spindly seedlings: q&a with thomas bjorkman

Q. For a home seed-starter like myself, wanting to grow sturdy tomato seedlings like the ones above or other stout, strong transplants, what’s the likely cause of seedlings that seem to reach for the light, and stretch?

A. The stretching that home gardeners experience is almost entirely from insufficient light. There are a bunch of other factors (as you mention) that play smaller roles, but most people starting seedlings can safely concentrate on making the light brighter.

Q. But the fluorescent tubes under my seed-starter hood look so bright and the tubes are close to the seedlings, and I leave them on 12 hours a day.  How can it not be bright enough?

intensity of outdoor light in springA. Our eyes have an incredible capacity to adjust to different light intensities, which makes it easy to underestimate how dim the light really is for seedlings. Putting them by a window is rarely enough. Fluorescent lights are rarely enough, unless they are almost touching, but that can get hot. (Some of the new LED fixtures are attractive in putting out concentrated light with low risk of fire.)

When I measure the light intensity in the units that the plant uses, under regular fluorescent lights it is typically 50 to 100 µmol/m2.s [that measure, micromoles per square meter per second, quantifies the number of photons used in photosynthesis that fall on a square meter every second]. With Super High Output fluorescent lights spaced 2 inches apart, we can get 400 at about 6 inches.

On a cloudy day outside in the spring 500 to 800 is common. On a clear day in late May (when a lot of seedlings come up) the light will be 1,500 to 2,000. So even though a T5 fluorescent grow light looks bright to your eyes, it is practically dark compared to what the seedlings are used to.

Q. I have read in technical papers that one failure can be leaving seedlings too long in the “germination chamber,” which in my home setup would very roughly mean on the heating mat and under a plastic dome.

A. I have a germination chamber in which I have messed up many times. It provides humid heat at 80°F, so the seeds germinate really well. If it is done right germination is fast and uniform, and the plants do really well later.

But if the cotyledons so much as peek up above the soil surface in there, they are a lost cause. They put up thread-thin hypocotyls very fast, and those will never survive in the greenhouse air. With brassica crops, for example, I need to get them out of the chamber in 36 hours.

Parsley and Brussels sprouts seedlings ready for transplant

Q. I have read about mechanical tactics for making stems stronger, as if they were out in nature getting buffeted a bit, so I was fascinated by the “brushing” work you published some years back.

A. Brushing the seedlings is helpful from the cotyledon stage to fully expanded first leaf stage for the species that can handle it (tomatoes, brassicas). The work I did with it was all in a commercial greenhouse where the light was already high. It was helpful because we were using plug trays that had the seedlings spaced about an inch apart.

That raises the issue of spacing. Putting the seedlings out to 1-1/2 or even 2 inches apart once they have the first leaf will keep them shorter. Bigger distances tend to stunt the small vegetable seedlings. Overplanting and then thinning is just asking them to stretch.

about the cornell lab of dr. thomas bjorkman

ON THE CORNELL website, Dr. Bjorkman’s faculty bio page explains his background and work. A main focus of his is in developing broccoli varieties more suited to an Eastern climate (most is grown in the very different conditions out West); more about that here.

my takeaways on preventing spindly seedlings

  • I long ago replaced my old traditional shop-light fluorescent hood with a small one that’s reflective and holds four T5 tubes (no longer sold), which are up to eight times brighter but not nearly equivalent to outdoors. The hood was not adjustable in height; you have to rig up S-hooks and chains or some kind of pulleys (thankfully good ones are easily purchased). I also have a 4-foot “Jump Start” T5 stand.
  • I’d been watching LEDs as they got more competitive in price, to select bulbs that I can retrofit my T5 hoods with for even more light output, or just get a new setup. The LED strip lights by SunBlaster are recommended (and come in different lengths), so that’s my next generation. More on growing indoors under lights here, and on shopping for seed-starting lights here.
  • I’m being careful to remove the heat mat and clear plastic dome as soon as germination occurs.
  • I’m spacing things more carefully, slowing down when seeding to get it right in the first place, or as farmers say, “planting to stand.”
  • When the weather allows, I’m giving my seedlings time in natural light (such as in a bright protected porch–not out in the open right away) to increase their overall exposure to light.
  • I don’t feed too soon; seeds are pre-loaded with the nutrients they need to germinate. I never feed earlier than when the cotyledons are showing, and if then, only with a very dilute solution of seaweed and/or fish emulsion. Mostly I start at first true-leaf stage, with half-strength dilute seaweed/fish emulsion, and work up to following the label directions for seedlings as the plants approach transplant stage.
  • It seems like a little hands-on TLC in the form of brushing can’t hurt with certain crops, as Dr. Bjorkman mentioned–and hey, it feels good to me, too, especially after a long winter.
  • Need to know when to start seed? My handy seed-starting calculator will help with vegetables, and common herbs and flowers, too, by creating a custom chart of when to start seeds indoors for your area.
  • Need to upgrade your gear? All my basic seed-starting equipment is on this page.
  • For a crash course in growing from seed: My 18 Confidence-Building Tips.

(Video of mung bean germination by Wjh31, CC-BY-SA-3.0, via Wikimedia Commons license. Disclosure: Purchases from Amazon affiliate links yield a small commission.)

  1. A.Z. says:

    I have used the same styrofoam with wicking mat system you show in your pictures. Do you ever reuse them from year to year? In my perhaps vain attempt at frugality, I soaked all the components in a med/weak bleach solution. Not sure if that will work this year. Maybe I’d better lower my risk by buying some new ones or use Jiffys?

  2. charles lien says:

    can last years trays be used again or do i need to have new ones for this year

    can old ones be cleaned enough to be used again, if so with what

    i have about 25 different types of trays from past plantings

    1. margaret says:

      You can re-use trays, Charles, is you wash them well. Some people use a 1:10 bleach:water solution, but hot soapy water (like you’d wash pots and pans) is fine.

    1. margaret says:

      Hi, Kevin. Learn more about how Dr. Bjorkman experimented in his lab with brushing the tops of the seedlings regularly at this link on the Cornell site. Commercial nurseries use this technique with poles that move across the benches of seedlings and brush (touch gently) the tops, stimulating strong growth as wind and other factors do outdoors.

  3. carisa says:

    Can the spindly plants be slowed down and brought back? Or do I scrap and start over. I want to save these ones. Can I prop them.and slow growth down. I think it’s to warm in my house and to cold outside in Michigan right now. How do I save them now?

    1. margaret says:

      Hi, Carisa. If they are very leggy and spindly I’d just start over. Remember: The Number 1 reason for failure with seedlings is insufficient light. Don’t know what light source you are using, but with my T5 tubes and then carrying things outside on each day later on when the weather is favorable, I avoid the stretching.

  4. Joe GLIDDEN says:

    On one of Mr. Lamp’l’s recent segments, he suggests resolving the spindly seedlings by keeping a fan blowing over them. I’m hoping it works for me!

    1. margaret says:

      Yes, simulating “wind” outdoors that naturally would be present if the seeds sprouted on their own is the idea…but not right on them or too close. On low across the room a bit.

  5. shelley mai says:

    Some of my plants germinated when I wasn’t looking.. Can I just bury them in more soil, or should I just start over?

  6. Lucas McCain says:

    I have had decent results after starting tomato and pepper seeds indoors on a window sill without purpose-specific lights. After 2-3 weeks following germination I pot the seedlings up (which are spindly) almost to the seed leaves in deep pots (I use full size Party Cups* and make holes in the bottom). This seems to mitigate the effects of a spindly seedling and promote deep root growth out from the long stem. By this time in my growing zone they’re about ready for hardening off (don’t start hardening off immediately after potting up) which begins to provide the seedlings the full spectrum, high intensity light they need to really take off. It’s also about the time that true leaves appear and I can fertilize with diluted fish emulsion while they’re outside – and even if I have to bring them inside at night they don’t stink.

    I understand that this is not a perfect solution but I think it’s adequate for people without the space and budget for a more elaborate setup.

    * I do not recommend peat pots which in my experience wick moisture away from the seedlings extremely quickly. This is especially the case when you’re also using a peat-based, soil-less seed starting product. Indoors, the peat pots tend to saturate and become moldy, and outdoors they seem to respire moisture away from the roots and you can wind up with a parched, often wilted seedling in the matter of a few hours while you’re away for the work day.

  7. Tyler says:

    I am having this problem with my 5 and 6 inch broccoli and tomatillo seedlings. I have a T5 grow light about a foot above them, they are to the point where some just are so long they just are falling down. So it’s too late to save them?

    1. margaret says:

      Hi, Tyler. I like to keep the lights within a couple of inches of the tips of the young plants, and adjusted as the plants grow — but always close to them. Temperature can also play a factor…the ambient temp in the room, but also sometimes people leave the heat mats on under the seedling flats/trays, which is a no-no. Can you get those broccoli out in the garden ASAP and see if they recover? Don’t know where you live, but they are pretty cold-tolerant and may just (at 5 or 6 inches) be well ready to go outside.

  8. Sathya says:

    I have some questions about a point covered where one of the reasons seedlings could get spindly is leaving seedlings in the “germination chamber” conditions (extra-warm and extra-humid) too long.

    • I’m being careful to remove the heat mat and clear plastic dome as soon as germination occurs.

    Where are your seedlings then left to develop to continue their rapid growth? Are your seedlings then left to develop indoors under lights with a temperature still warmer than outside(Would you roughly know what sort of temperature)? I’m wondering as wouldn’t the seedlings still need a warmer environment in order to continue their rapid development, so as to get the real benefit of hardy seedlings ready to be planted outside when the weather gets warmer to extend the cropping period?

    1. margaret says:

      It depends what I am growing — a tomato or melon might want it a little cozier than a cabbage or lettuce would care about, for instance. But the shed I usually start things in is like maybe 70 by day and 55 or 60 at night I bet, and yes, that’s where the lights are. I keep them under the lights without the cover on. Or…if the weather is semi-settled, I carry them outside by day to a sheltered bright but not baking (and not windy) spot). Penn State has a helpful temperature chart for day-night temps for seedling development (versus warmer germination temps) if you go to this link and scroll down.

  9. LR says:

    Where I live the problem is:
    Excessive light/too intense
    Soil temperature/too warm
    Average air temp too warm/too humid

    These are all outside conditions.
    It is literally impossible to grow vegetables where I live. The effort and loss is not worth the results. The place is too hot, and yet it isn’t a desert. Winter lasts about 3 weeks, summer temperatures the rest of the time. It’s the confusion of no distinct seasons and mismatch of growing stages with weather conditions that confuses the plants. I guess there are places where humans simply can’t expect to grow crops, but can live there if they have a modern supermarket trucking in vegetables.

  10. For those of you using real sunlight to start your seeds (e.g. on a window sill or in a cold frame), an Android app called “Sun And Shade Analyzer” will tell you how many hours of direct sunlight you will average on your seeds. Eliminates the guessing.

  11. Viola Purdy says:

    I have leggy seedlings . Wanting to change lighting system as soon as I can afford it. Hopefully before next sowing season.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.