Four tricks to ensure your seedlings get enough light


If you’ve tried starting seeds on a window sill inside, you might have found that they grow thin and weak looking?

If so, this is a common issue, particularly in urban areas. Surrounding buildings or trees cast shade, reducing the light. This creates a problem if you’re trying to raise seedlings inside as they need plenty of light to thrive.

In our new home, I’ve put a table next to the brightest window. But, because the sun is cut out by surrounding walls, it is still too gloomy to raise healthy seedlings. These rocket seedlings are verging on being too thin and spindly – but I’m moving them outside on warm days to give them a dose of bright light. I think they’ll be OK.

Affect of temperature

Seedling growth is also affected by temperature. The warmer your home is, the more light your seedlings will need. So a warm home that is not very bright will be the most challenging to raise healthy seedlings in.

Four ways to get more light to your seedlings

The best strategy depends in part on whether you want to start ‘tender’ or ‘hardy’ crops from seed.
Tender crops include tomatoes, chillies and aubergines – these do not grow well in temperatures below 10 degrees centigrade (50 Fahrenheit) and are killed by frost.
More hardy crops include leafy veg like kale, rocket, and most Asian leaves, root crops like beetroot, turnips and carrots, and peas and broad beans. These can survive colder weather once established, but a hard frost when they are tiny seedlings can be fatal if they are not protected in some way,

1. Wait until it’s warm enough to sow outside

This is the simplest and easiest solution. For the more hardy crops it’s also a very good solution. You can sow these outside about a month later than you could inside – but they’ll still have plenty of time to grow and do well.  This is also a good solution for tender crops like courgettes and runner beans – as these need a shorter growing season than aubergines and chillies. Starting these outside in late April or May is fine. But it’s not so good for tomatoes, aubergines and peppers. If you wait to sow these outside, you may not get a crop at all.

2. Get a small greenhouse or cold frame.

This will enable you to start your seeds a few weeks earlier than you could by sowing them directly outside. For tender crops you should keep an eye on the weather forecast, moving them inside on cold nights.

I found this perspex lid in a skip and made the box to fit. Here I'm growing winter salads in it but I'll also soon use it to keep seedlings protected.

I found this perspex lid in a skip and made the box to fit. Here I’m growing winter salads in it but I’ll also soon use it to keep seedlings protected.

3. Start your seeds inside and move outside on warm days

Moving your seedlings outside – even for a few hours – can dramatically improve their health, as long as the temperature difference is not too great and the seedlings are reasonably well protected. A simple and effective way to protect them is to use a propagator with a plastic lid. This will help insulate the plants from the cold and protect them from the wind. Be selective about which days you move plants outside on, choosing the warmer days. And don’t forget to bring them in again at night!


The lid on this propagator is secured with masking tape to help it prevent it blowing off. I learnt the necessity of this when growing on a balcony when the wind would sometimes blow them off into the neighbours garden!

The lid on this propagator is secured with masking tape to help it prevent it blowing off. I learnt the necessity of this when growing on a balcony when the wind would sometimes blow them off into the neighbours garden!

This strategy works really well for the more hardy crops listed above. (It’s my own preferred option). You can also use it successfully for the more tender crops – you just need to be more selective about which days you move your plants outside. Even a few hours of bright light a day will make a difference.

4. Invest in a propagation light.

If you’re serious about growing chillies, peppers and aubergines, you could consider investing in a propagation light. Energy efficient, compact fluorescent bulbs, are now available, delivering the necessary wavelength of light (you want ‘blue light’) for seedlings.

I’m trying one for the first time this year. It’s a bit early to pass judgement but so far the tomato seedlings are doing well.

This is the window where I'm starting the seedlings. It's really too dark so I've rigged a propagation light, under which I'm starting tomatoes and chillies and aubergines. The more hardy crops I'm starting here but then moving outside on warmer days.

This is the window where I’m starting the seedlings. It’s really too dark so I’ve rigged a propagation light, under which I’m starting tomatoes and chillies and aubergines. The more hardy crops I’m starting here but then moving outside on warmer days.


Or, you can simply buy the plants

As you’ll have gathered, it’s the tender plants that also need a long growing season, like chillies, that present the greatest challenge. Another excellent solution is simply to buy these as plants later in the year.

Look out for local community plant sales: a great place to buy good value plants and to meet other growers in your community at the same time.

18 comments… add one

  • 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?

  • 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.


    • 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.


      • 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 :-)


  • 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.

  • 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?

    • Are you looking for a heated one or just one with a lid? I’d probably check out Poundstretcher and the other pound stores, they often have a selection of gardening stuff.

  • 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

  • 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.

  • 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.

  • 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!

  • 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).

    • This is not something I’m an expert on. But I think that wavelength of light (you want blue I think) is more important than warmth – so choosing a light bulb with the right wavelength is most important. I just leave the light on to simulate night and day – that seems to work fine. There’s more info on Wikipedia here:

      • 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.

  • 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.

    • 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?

      • 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.


Leave a Comment

Previous Post:

Next Post: