Seedlings Started Under Lights vs. On A Windowsill

In early February I started some broccoli seeds the way I normally do, under a bank of full spectrum fluorescent light tubes. At the same time, using the same seeds, the same seed potting mix, and an identical pot (a 6-cell starter tray of 72-sized-cells), I started some broccoli seeds for a friend. She took her seeded pot home and put it immediately on the windowsill of a bright, south-facing window.

About a month later, on March 8th, we compared our results.

On the left, the broccoli seedling grown under lights. On the right, the seedling grown in a window.

Notice that in pretty much all ways, the seedling on the left is better: stockier, better color, more upright growth, bigger leaf size, stronger overall growth and much better root development. In fact, the seedling on the right after a month in pot didn’t have enough root development to fully hold its soil ball together.

This picture actually minimizes the signifigant stem lankiness issue because of the positioning of the top of the root ball, but if you compare the distance from soil to first true-leaf node you will see that the windowsill seedling has a pale, spindly stem about twice the length of the light-grown seedling.

These two seedlings were both pretty representative samples of the packs we compared. Honestly, I wouldn’t count on the windowsill-grown seedling to last an evening after transplanting. It’d be slug fodder.

This is why I invested in the seed-starting set-up I did. (Read more about my indoor seed starting set up.) Pacific Northwest late winter and early spring sun is not generally strong enough, through layers of typical cloud cover and various UV-reflective-coated windows, to allow for really strong seedlings to be grown out on a windowsill.

How do you grow the strongest possible seedlings indoors?


  1. says

    I have no room indoors to start seeds. So I winter-sowed all of them and am extremely pleased with the results. Because they have been out since day one, they are very hardy and used to the elements and I don’t have to worry about hardening them off. Best of all, it cost me about $10 in total, the cost of soil, a roll of duct tape, a Sharpie Paint marker… and a lot of free plastic milk jugs from friends!

    • Sara B says

      I tried winter sowing in milk jugs last year for the first time with mixed results. I’m going to try it again this year. We’ve talked about a seed starting set up but haven’t gotten there yet.

  2. Kathy says

    What a great visual! Last year I invested in lights and heating mats for seed starting in the basement. It’s going well but I’m still struggling with tomato seeds and not sure I’ll ever grow successful tomato starts. Thank goodness for the local heirloom tomato guy, who I always run to for beautiful starts after another year of failed seeds.

  3. says

    Wow, that is crazy! I’m currently waiting for my seedling trays to germinate (they’re up against a southeast slider) and then I was planning on setting up my lights. But just seeing the difference makes me feel better about spending the $60 for the grow light set up!

  4. ~marie says

    Erica! I am sooo enjoying your gardening tips! Last week when you shared your grow-system set up – I finally got off the couch and set up our grow-system (in anticipation of getting 15 yards of good dirt this week!). I used both an aerogarden garden starter ( and a shop light with a all kinds of must-start-early seeds in a mishmash of containers (1/2 toilet paper rolls, plastic containers with lids, anything else that looked right) all set in rubber tub lids so I could water from the bottom up and covered in saran wrap. I must have some 200+ seeds in production so far. Every artichoke plant that pops up I think: hey! I just saved $3.99!
    Keep the gardening momentum going! It’s great!!!

  5. says

    Wow that comparison is actually shocking and a bit depressing. It seems that gardeners with limited indoor space are at a significant disadvantage and may just be forced to buy plants rather than raise their own.
    I wonder how this experiment would work out in my local climate…

    • says

      There are very compact full spectrum lights out there designed for growing plants that could be wedged into a closet or above a kitchen cabinet in even a small space home. The drawback is that they tend to be more expensive than shop lights and florescent tubes.

      • says

        Thanks, I’ll really have to look into this, but when I said limited indoor space, I meant I live in a static caravan. ;) In order to find space even for the smallest set up, I’m going to have to do some serious decluttering and somehow try to get the hubby on the same page.

        • says

          :) Ah, well in that case I’d use the sun (the best grow light around, even in early spring) and consider bottom heating a warm frame built outside then to get seeds to grow and germinate in a reasonable time – some sort of heating element, either a seed mat with controllable thermostat, or a heating coil in sand or a DIY equivalent, to raise soil and air temp. You’d have to look into how to make this work from an energy cost standpoint. Or go *really* old school and make a hot frame with fresh horse manure. ;)

  6. says

    You brought up a good point, the windows you have impact your seed starts. We’re the DIY type of family and on a tight budget, so we replaced our windows one at a time. I only have one bank of fluorescent lights, so I used to start some of the less finicky seeds under the lights then move them to the window sills on our south facing home to make room for my heat and light lovers. The year that one window was replaced with new low e double panes and the other had the original single pane, wood framed, circa 1940’s glass window, I had a significant difference in overall vitality between the starts in each window.

  7. says

    I’m still learning. This is my first year of using a grow light and set-up. My previous attempts involved windows. They were half ass to say the least.

    Erica – Have you seen my tomatoes?
    They are not dead. Yet. I am truly amazed. Still completely prepared for some sort of fungus/disease to strike at any time and we haven’t even gotten anywhere close to outside yet, but cautiously optimistic, might be a good phrase.

    Next year. Heating mat all the way.

  8. Just Nick says

    Part of the issue is the windows themselves. Modern windows are designed to do a few things – reflect infrared radiation (in most climates or on South facing windows) to help keep things cool in summer, reflect UV radiation (to stop fabrics, books, art, and paint from fading), and to present a pretty and not-too-bright view of outside. Great stuff, but as a result there are lots of treatments applied to the glass and even different compositions of glasses used.
    And note that “help plants grow” isn’t on the list of window designer’s objectives!
    Plants care most strongly about light in the 680-700 nanometer range – a little to the red end of the part of the spectrum to which our eye is most sensitive (circa 550 nm). A plain glass window – like you might find in an old house or a greenhouse – will transmit 80-85% of the light in this photosynthesis-friendly region. Modern residential and commercial windows, depending on the degree of low-e coatings, tints, and emphasis on heat rejection may transmit as little as 25-50% of the light in the 680-700nm region. Plants have a secondary affinity for light around 400nm (there is some species dependency at work) – but since light from 300-380nm is what causes sun bleaching, windows often start to get opaque in this region as well.
    So while our eyes are seeing bright, cheerful, full-sun streaming in through the window, the chemical reactions that are powering that seedling might be struggling along with half (or less) of what they’d be getting outside. The whole issue of what spectral content different types of plants need and/or prefer is wicked complicated – and varies depending on the phase of the plant’s life (e.g. while setting or ripening) and may help trigger certain seasonal reactions. But in a nutshell, their needs are not quite the same as ours – and just because we think it is bright doesn’t mean they are getting what they need.

  9. says

    Good to know! We have mostly new windows now as well, epecially the glass door in front of which my seedlings sit. And I confess my broccoli seedlings look exactly like the one on the right. Maybe I could repurpose an old fish tank and replace the light bulb? I’m still looking for ways to gow my own without spending more money than I would by just going to the farmers’ market.

  10. Arrianne says

    Great pics, thanks for the comparison.

    I didn’t think I had room for seedlings either! We live in a little 900 sq ft house with 3 kids, I used to have a plant shelf in the kitchen before it leaked and we had to replace the window. I found a little corner that holds the cat box and a waste basket. I built a simple PVC shelf (so I can take it apart it store it) to go above the cat box. It’s only 24″ wide and holds two light fixtures side by side, but I’ll be able to start about 50 little seedlings under it.

    A little tip I picked up for preventing damping off in seedlings where there isn’t good air circulation is to make a strong tea of chamomile and spray them with it regularly. Chamomile tea has a little sulfur in it which acts as a fungicide. It works!

  11. says

    I am so jealous you started your seeds in February! I totally slacked this year and just got some planted last weekend! I stick mine in the oven until they sprout then under a light they go. I learned two years ago that seedlings not grown under a light did not work out so well for me. I think I paid less than $30 each for my lights, I keep my seedlings on a wire shelf with the light attached to the top shelf. Works great!

  12. says

    It’s so interesting to see the comparison! We have a set of wire shelves in our office, and we’ve affixed 2 3-foot florescent lights hanging from the bottom of one of the shelves. Hook up a timer so they get 6 hours of darkness a day, turn on the overhead fan every once in a while to encourage their stems to grow strong, and voila! Baby planties!

  13. says

    I haven’t yet got into grow lights, though I want to badly, but I haven’t felt like I’ve needed them that badly. For several years, I have had pretty good luck in the window. It’s an old house (1946) and the windows aren’t even ‘real’ windows (according to a carpenter friend): they’re just huge sheets of glass doubled for insulation, framed into the external walls and capped on the outside. Fortunately, our hugest window faces the neighbour’s white siding which reflects the light beautifully when it’s not shining on that side of our house -that window gets bright light all day long.

    Things that I definitely do to ensure healthy seedlings:
    *turn the trays at least once per day, if I remember, I’ll do it twice
    *rotate each tray on the shelf once per day (so the ones furthest from the window get a turn being right up against it
    *set an oscillating fan blowing across them for at least an hour each day, more often for 2 hours -this helps surfaces dry to reduce fungal infection AND the plants are stimulated to grow stronger, tougher stems to deal with the wind pressure

    I’ve had great luck with tomatoes, brassicas, squash and others. BUT I’m in central Alberta and apparently we get more hours of daylight annually than the rest of the country so I can count my blessings :)

  14. dr. dave says

    For some reason my indoor seed germination has been questionable at best this year. I am now having much better results laying tiny seeds on top of the soil instead of lightly covering them with moist sphagnum moss. Is anyone else having indoor germination problems ?

  15. Kimberly C says

    I just slapped together a grow light set-up last week. I ended up choosing a 4ft hanging shop light [ light&storeId=10051] with 2 daylight (6500K) fluorescent tubes for a total of about $28. It isn’t very pretty, but I’m not fussed. I’m most worried that the two bulbs won’t be enough to cover my seed trays, which I think are 5 x 10 cells.

    I started a tray of seedling by window-light about 3 weeks ago, just lettuce & chard, to see if window light would be sufficient. The seedlings looked very sad, lanky and flopped-over, with only a few chard looking even vaguely healthy. I’ve moved them to the light set-up, but I doubt they’ll survive long-term. I’m still going to use the window location for germination, because it’s in a much warmer area of the house, and then move them to the light set-up when they’ve sprouted.

  16. Tanya Lopez says

    More poring over all sorts of cool old posts.

    But if you have time (ha!), you misspelled fluorescent above (missed the u) and I couldn’t find this post when I searched your site for fluorescent.

    As always, thanks for the blog! You rock.

  17. Melissa says

    I am new at this and am sprouting lettuce using fluorescnt light. My seedlings are just emerging. I know lettuce needs to stay moist. Should I be using a fan on them at all, or will this dry them out. At the same time, I don’t want them to die from fungus. Any tips for leaf lettuces? Should I buy a spray fungicide? Thanks.

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  1. […] should be. Even if you don’t live in the Northwest, there is loads of info on starting seeds, why you should grow them under lights, and how to save money on those seeds in the first place. This blog has been a lifeline for me when […]

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