How to raise healthy seedlings inside

Tomato seedling

If you’ve tried growing seeds on a window sill inside, you might have found that they are prone to grow thin and leggy. It’s a common challenge for many of us growing in small spaces.

Why is this an issue? Healthy, sturdy seedlings raised in good light are more likely to grow into strong and healthy plants. Whereas, weak seedlings are more prone to grow into weak plants or their thin stems are more likely to snap. So it’s worth trying to grow the healthiest seedlings you can.

Seedlings need really good light levels to grow strong. However light levels inside are invariably much lower than outside (our eyes are so good at adjusting it’s not always easy to notice this). Even seedlings on a bright windowsill will receive a fraction of the light they would outside. This challenge is compounded early in the year by the short day length and low light levels. And even further by the fact that the warmer it is, the more light seedlings need. Modern central heating means many homes are warmer than in the past – and seedlings need more light.


The stems of these tomatoes are a bit tall and thin and the leaves should be greener. But they will probably come round and grow OK. With more light they could have grown into stronger, healthier seedlings.

Four ways to raise healthy seedlings inside

If light in your home is marginal (and in many homes it is), here are some ways you can improve it to grow stronger seedlings.

1. Reflect light

Put something reflective – like white card – behind the seedlings to bounce light back on to them. This won’t usually completely solve the problem but every bit of extra light will help.

2. Move seedlings outside on warm days

On warmer (not too windy) days, move your seedlings outside for a few hours. They will love the extra light – and, in my experience, this can make a dramatic improvement to their health. It will also help them adapt to outside conditions. Avoid putting baby seedlings outside on windy days (they hate wind until they have adapted  to it)  or give them some wind protection. A simple way to do this is to use a propagator lid (a plastic lid that fits on top of the seed tray) or construct a DIY shelter from plastic sheeting or similar.

Avoid moving them out on cooler days. For sub-tropical plants like chillies and tomatoes it’s best if the outdoor temperature is 10 degrees centigrade (50 Farenheit) or more. However, if it is sunny and the plants are well sheltered, slightly lower temperatures for short times can be beneficial.

Above all, remember to bring them in again before nightfall. I often set an alarm to remind me 🙂

This technique works particularly well a bit later in spring (April and May) when days are longer and warmer. But unseasonably warm days are not uncommon in February and early March. Keep an eye on the weather forecast and put your plants outside for a few hours whenever you can.

Compare these to the image above. The stems are shorter and thicker and the leaves are greener. These were started under a grow light and then moved outside regularly on warm days.
Compare these to the image above. The stems are shorter and thicker and the leaves are greener. These were started under a grow light and then moved outside regularly on warm days.

3. Use a mini greenhouse or cold frame.

A small greenhouse will not be warm enough to start the seeds of tender plants, like chillies, outside in February or March. But it can be a good option for growing healthy seedlings like squash, later in the spring (late April /May), just bringing them inside if frost is predicted. You can also move plants outside into a coldframe or greenhouse a few weeks before you would move them outside. So, if you have space, a coldframe or greenhouse is useful to help raise healthy plants. You can buy one or build your own from reclaimed materials. One of my best ever skip finds was the perspex lid in photo below.


A homemade cold frame. This has salads growing in it, but I also put seedlings in pots in it in March and April. They get the benefit of the outside light but shelter from the winds. I bring them in at night and in cold weather.
A homemade cold frame. This has salads growing in it, but I also put seedlings in pots in it in March and April. They get the benefit of the outside light but shelter from the winds. I bring them in at night and in cold weather.

4. Get a grow light.

Advances in LED technology means that good quality grow lights are more efficient and affordable than they used to be. If you want to raise strong, healthy seedlings early in the season when light levels are low (February and March), they can be an excellent investment. Certainly, an invaluable tool for keen chilli growers!

Although we have a south facing window, light levels are still marginal for seedlings inside in February and March. Getting a grow light has transformed my ability to raise strong healthy chilli and tomato seedlings early in the year.

Other options

Try to grow the strongest, sturdiest seedlings you can. But if they still grow weak and spindly, don’t give up on them. Plants that get off to a poor start can be a bit like children who get a difficult start in life: some continue to go downhill, while others will, with careful nurturing, do great. When you repot a plant with a thin, spindly stem, bury some of the thin stem under compost. This helps to sturdy the plant.

The other option is simply to buy your chilli and tomato plants later in the year.  Sometimes this is the easiest and best solution, particularly if you lack space and time. If buying plants, the key is to find a good supplier. Some community growing projects raise tomatoes in greenhouses and then sell them at community plant sales – a great option if you can find one.

Your Turn

What’s your experience of raising seedlings inside your home? I’d love to learn about your challenges and what works for you in the comments.

19 thoughts on “How to raise healthy seedlings inside”

  1. What can you do if you have been working with plants for weeks and one night you forget to turn on the blue light which dropped the temperature
    How can I save what I messed up due to temperature and get back onto schedule?

  2. What was your opinion on the propagation light in the end?

    I keep considering one, but they vary in price so much, and now there are heaps of LED ones to complicate things.


    1. Hi Paul

      Yes, in the circumstances (very poor light from windows) it was worthwhile for raising healthy seedlings. I’ve also tried an LED light (not one of the cheapest models – a mid price one) since which is even better. I think they are a good investment if you are growing quite seriously, particularly if you want to start chillies and aubergines early in the season. But their is a lot that can be grown without a growlight so they are not essential.

      Hope this helps.


      1. Thanks for the advice Mark!

        I have a similar problem in that my kitchen gets very little direct light, due to some huge trees in my garden.

        I already tried growing seedlings but they end up very spindly, and just desperately reaching for the windows to get more light.

        I’m happy to hear LED lights have helped with this… I think I’ll bite the bullet and get a mid range one like you suggest 🙂


  3. Thanks for the info
    I’m a Canadian gardener, my seedlings are precious and healthy as can be .
    I have tomatoes, peppers, leeks, onions, and kale on the go .I look forward to reading your information, I enjoyed what I’ve seen so far.

  4. Where would be use best ie cheapest place to buy propagators in the UK? Homebase or b and q or is there an online company that anyone knows is better?

  5. Boy this came just at the right time I’ve just waiting for my propagator and seeds to be delivered and it must be about 25 to 30 years since I’ve used one, thanks for all the tips everyone

  6. I’ve bought a propagator this year, and it has made an enormous difference to my tomatoes, aubergines and cucumbers.
    However, one tip for fixing spindly tomatoes etc is to repot them once they have a couple of true leaves. Bury them up to the first true leaf and they will grow roots along the buried stem. By this point they are less suceptible to etoliation and should cope better with low light.

  7. Thanks for the tip on reflective foil, that should help me to overcome the darker corners in my porch for my seedlings. I might have a leftover somewhere from an underfloor insulation with an reflective aluminumside on, that should help me out.

    As for the cut open juiceboxes: I’ve done that to last year but it doesn’t give a remarkable result, however better than nothing, but the foil used tends to turn less reflective when exposed to light.
    An other alternative might be reflective sunshades for carwindows (

    Another little trick is placing mirrors in a certain angle to redirect sunlight, I did this in my garden last year to give more light to the first red beets I’ve planted because they had shade from the neighbour’s high grass (it’s a small urban garden I’m having) I guess you could use it indoors to.

  8. The council has built up scaffolding around the house and that is so useful now! In the morning I’m taking my trays with tomatoes, peppers, chillies and lettuce outside, leave them until the sun passes and bring them back in in the afternoon. It made a tremendous improvement in the tomatoes, they are developing leaves much faster now!

  9. Do you leave the light on 24/7 or do you simulate night and day?
    Also, would it be beneficial to use an older style of lightbulb that provides heat to start them off (in this case the heat is valuable).

      1. High powered daylight balanced energy saving bulbs that are intended for photographers could be useful. The 105 watt bulbs are equivalent to about a 450 watt traditional bulb. The lifespan is thousands of hours, they have a standard screw fitting and cost about £7 each.

  10. You can also pop you seedlings into a cut away box that has been lined with foil – so light coming in at the front is reflected back. It’s a low cost option and helps seedlings grow much straighter.

    1. Hi Emma, that’s a great tip, and one I forgot to mention – so thanks very much for posting it on here. Also, some cartons (fruit juices sometimes?) come lined with reflective foil and I’ve seen people cut them open and use the back as a reflector in a similar way. Wish I knew which products came with this reflective foil, any ideas?

      1. I guess you could use coffee packets. These are usually foil, so you could cut out a piece and stand it at the back of your seed tray. I always feel guilty throwing them out. Actually, since these packs are usually quite sturdy, they might even hold up quite well as seedling trays themselves, cut up in the right way.

  11. Pingback: What To Do In March | The East London Garden Society

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