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 it? That was how I began a note to Dr. Thomas Nils Erik Bjorkman, Associate 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 spindly 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 the “germination chamber” conditions (extra-warm and extra-humid) too long…and the list goes on. Turns out even spacing can affect the way seedlings 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?

Cellpack of young tomato seedlings ready for transplant.

my questions and dr. bjorkman’s answers

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?

A. 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 1500 to 2000. So even though a T-5 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 dr. bjorkman

Dr. Bjorkman’s faculty bio page from the Cornell website 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 from the q&a and other reading

  • I recently replaced my old traditional shop-light hood with a small one that’s reflective and holds four T-5 tubes, which are at least brighter if not nearly equivalent to outdoors. I’ll look into LEDs as they get more competitive, because the plants would be less overheated.
  • 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.
  • 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 mentiond–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.

(Video of mung bean germination by Wjh31, CC-BY-SA-3.0, via Wikimedia Commons license.)

27 comments
March 30, 2013

comments

  1. says

    I’m curious how important heat is after the seeds have germinated. Should they go outside every day, sun or clouds, as long as they won’t be washed out by rain or blown away by wind? Outside under a hoop cover for protection? Only brought indoors when the weather goes below X degrees?

  2. says

    I like seed hacking. My windows hang full of zip lock bags filled with seeds germinating in the spring. They seem to get a good sturdy start that way receiving a fair amount of heat and light! Thanks for posting, I have had the same trouble and thought fluorescent was enough.

  3. Handy Andy says

    Thank you so much for this timely post, Margaret. I have been starting my own seeds in a small greenhouse for several years. Have advanced from, “Wow, they germinated!”, to “Why aren’t my plants stockier, greener, healthier, etc?”
    Now I have some answers.

  4. Martha says

    Thanks for the good info. A couple of points:
    Fluorescents are fine for seed starting. The regular ones, you don’t need special grow lights. However, they should be on 16 hours a day, not 12, and very close to the seedlings (move higher as seedlings grow). Also, they lose their strength over time, so testing or replacement every so often is necessary.

    • says

      Hi, Martha. I have always used fluorescents, and sometimes fared perfectly well and sometimes not. What Dr. Bjorkman is saying is that they simply don’t come close to the light that a seed would have germinating outdoors, so you will get a stouter seedling with a stronger “trunk” so to speak if you can beef up the light. In all the lab research (Cornell and many other universities) the question of how many hours a day to provide light is also up for grabs, and because all the factors coincide (what crop, the temperature, the kind of light, how close, etc.) but the precise combination differs for each of our set-ups, we have to experiment. Sounds like 16 works for you!

      Very funny, Dahlink. (I often buy seedlings of basic things, but can’t get special varieties that way.)

      Hi, Donalyn. The fan helps prevent things from getting overly warm, too, under the lights, and can be a preventive element in fungal issues that might arise in a still, too-humid condition. The brushing work was done years ago, and I was reminded of it recently somewhere and went and dug it out. Fun!

  5. says

    i had never heard of brushing before – we have used a small fan, blowing across the seedlings to help strengthen them, and that seemed to work well. I will definitely try the brushing technique this year – assuming we ever do begin to see signs that we will have a spring here in Upstate NY!

  6. Laura says

    My gosh, what perfect timing. As I carefully bring all me seed starts from the porch outside for sun and a bit of ‘real outdoor life’ each day, I was dismayed at a broccoli start of mine that is long and scraggly (as are a few others I must admit). This information was so helpful. I must say I share Dahlink’s thoughts! I will keep trying.

    Another valuable lesson I have learned is to not train my large dogs to Sit and Wait near my seedlings. My garlic starts did not appreciate my 100 pound dog obeying me with such enthusiasm with such a perfect sit right on top of them! Fortunately they were very forgiving!

    • says

      Hi, Laura — and yes, pets and plants can be a tricky combo! :)

      Hi, Sally/Michael. I think the big temp swings are not ideal, but I assume you are venting and moving air somehow? 90 seems too hot for some babies that prefer it cooler once they sprout!

      Glad to help, Joanna, and welcome. Nice to see you.

  7. mikeinportc says

    I’ve always ” brushed them, but didn’t know that anybody researched it. Originally it was just because the aroma of tomato plants smells like summer. ::) It seemed to make them better, so kept doing. About 15 years ago, I heard Dr Nina Basuk, of Cornell, speak about her research on newly planted trees. Moving in the wind resulted in stronger trees, with heavier caliper, sooner. If trees, why not vegetable seedlings? Brushing would be like wind gusts, in addition to the fan

    Here’s something I ran across 2 yrs ago, to consider ( next year?) : winter sowing ( with protection) of some crops. I haven’t gotten around to trying it, but someone I know did, with good results. Last November, I found some cilantro ( self-sown), tomatoes & ground cherries ( both chipmunk-sown ;) , that came up in protected locations, and grew well despite 20F nightly lows. Of course they died later, but gives some hope for spring, if timed correctly.
    http://www.wintersown.org/wseo1/index.html

  8. Mary Withrow says

    I tend to always try to start early indoors and put outdoors as soon as frost date has passed. But I was running late one year and realized that my plants all did so much better and seemed to have less diseases and problems during that growing season. Since then, I get my seedlings going and put them directly out at later dates and have had much more success.

  9. SallyMichael Photosensualis says

    We germinate on heat mats in a greenhouse. We’re now a little concerned about getting them off the mat ‘as soon as germination occurs’. Sometimes all the seeds in a tray don’t germinate simultaneously. So, get ‘em off the mat as soon as the first ones germinate, or hold out until most of the tray has ‘popped’? hmmm… Also, our greenhouse drops to 57-60 in the evening, but often soars to 90+ on sunny days, even when it is still in the ’40s. Is that worrisome?

  10. Anne Allbeury-Hock says

    Hi Margaret….My studio is full of seed pans, fibre pots and 3′planter boxes. The only failure I have had are the Zinniias. Probably should put them outside later.
    I have three kinds of tomatoes, some letturce, peas, kale, marigolds, calandulas, etc. Now come the transplanting to larger pots or outside. I am putting i n raised beds made out ot cement blocks, Ugly at first, but I can plant flowers in the open parts of the blocks. I get this seed planting ferver every few years. When I lived on Beverly Rd. I had a huge set up in the basement with lights. then and now I left the lights on day and night. It seems they need lots of light. Love your site and feel very happy to know what you are doing! Best from Anne

  11. Caley says

    I visited a large commercial greenhouse this weekend. I wanted to move in there ! They use specially-made high-intensity lights in the winter in addition to the natural light that comes through the glass. They also have a heated floor and grow the plants on it all the way to the finished stage.

  12. Jo says

    Love this video! Interesting to see that some germinate much more quickly than others. I’d like to find more high-quality slow-mo plant videos.

    On another note, is it just me or is this (below) puzzling to others?

    “That raises the issue of spacing.”

    If shorter is good (shorter = stockier & healthier, yes?) then 1.5-2 inches apart is good. Got it. But the next sentence puzzles me: presumably bigger distances would mean more resources (light, nutrition) for each seedling, so why would they get stunted?

    And, how could thinning be a problem — is it a matter of technique (eg pulling up whole seedlings, roots and all, may damage nearby roots? If so, what if you just snip the top part at the soil line? Or is the issue timing, so that if you don’t thin early enough then those that germinate will start stretching soon?

    I tend to overplant so I can ensure germinating a sufficient quantity (which is less predictable for those of us who save seed from year to year and there are only so many germination tests you can do each year); and, in a northern climate, I’ve got to get a sufficient quantity germinated in the spring bec there isn’t enough time to do it again in many cases, thus I have relied on overplanting.

    Thanks for any clarification you can provide,
    Jo

  13. Angela L. says

    Last year was my first adventure into seed starting, so, as most beginners do, I just used natural light from a big south-facing window. I had limited, but slow success, with growing them indoors and then putting them outside for better light, but once they got into the garden, they finally took off.

    So this year, I got a 4-foot T5 grow light rig, and I rotated my 80+ seedlings under it in the same south-facing window inside my house. This year I had an opposite problem from last year though, I started them way too early! I didn’t anticipate how quickly they would grow under the new lighting conditions, so I started them 6 weeks before last frost date. So, what happened, was they ended up getting WAY too big and tall and they were just getting taller and taller because they were trying to outgrow one another in the 2″ spacing environment. They were the perfect size about 3-4 weeks after seeding to be placed in the garden, so I was really struggling figuring out what to do with all those plants at the 8 week mark! Every year another lesson learned. Thanks for sharing the helpful advice. I do wonder why it isn’t better to space them out a bit more though.

  14. says

    You picked all of my favorites too! I also like the Plant Talk podcast as it is based in ohio and on my zone level. I love the british ones as they have such a great sense of humor about gardening! Very entertaining!! I am definitely hooked!

  15. Brett says

    I’m grateful for the information but this borders on making it sound impossible (or at least expensive and difficult) for the average home gardener to start seeds indoors. One of the problems with our stuff-driven world is that there’s always the pressure to buy more stuff. I’ve started lots of plants successfully indoors using only florescent lights. Although this isn’t ideal for the plants (as the good Doctor points out), I’m not about to buy expensive new equipment. Gardening is expensive enough as it is.

    I would think that one of the best, easiest and most cost effective ways to beat leggy seedlings is to start them at the right time and get them outdoors as soon as possible.

  16. says

    I didn’t read every word of the comments, but most. I’m looking for any information about once you have spindly seedlings is it too late or can they be salvaged somehow?

    • margaret says

      Hi, Margaret. Nothing you can do to “fix” it. Better to start over typically, if there is time. Or buy some if it’s something you don’t have time to start again. A weak seedling just won’t perform over the long haul, so it’s best to start with a stout, strong one.

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