Growing plants with and without additional light basically comes down to three main variables: balancing heat, light and sowing time:

  1. Too much heat and too little light and a plant is leggy
  2. Too little heat and too much light and a plant is flat/stunted, or dies
  3. Plant at the wrong time and a plant will either go to seed, or grow too big/small before you are able to plant it (for more info on sowing times, see my individual growing guides)

<aside> 💡 I recommend the iPhone app SOL which amongst many other features makes it easy to see what the daylight hours are for any day/month of the year.

‎Sol: Sun Clock

Or just use this:



<aside> 💡 There’s one more variable - light length - which I will cover later in the section on experiments


I rarely use grow lights to accelerate growth, instead I use them to improve my control over these variables and improve consistency of results, for example:

  1. When light levels are low in late autumn, winter and early spring LEDs can create conditions that closely resemble September or April. I can then duplicate those conditions to a reasonable degree by using cold-frames, low tunnels and a high tunnel when planting
  2. When heat is high in summer, too high for onions, lettuces etc I can turn the grow lights on at night and off during the day (keeping day dark) and hence allow the plants to grow cooler, duplicating the conditions of spring/autumn, rather than July.
  3. When plant growth slows down in winter, I can increase it so that I can keep to my usual 3-4 weeks from sowing to planting, this consistency makes planning so much easier.

<aside> 💡 Fundamentally I'm always trying to fool plants, by creating the natural conditions (heat, light and day length) that they expect in - for example - March/April, but in January/February. It's also possible to use grow lights at intensities that plants never experience in nature, I don't do that, mainly because my plants almost never live out their lives under lights, after 15-60 days they will be in natural light and then planted out.
What this means for the plant - if I get it right - is that they just experience - for example - March like conditions that actually lasts from January until April, they seem to cope with that just fine.


Light intensity

I generally start seedlings at about 60% intensity, because most seedlings expect to be growing in early spring when light levels are lower. As the seedlings mature I step up the intensity by 10% a week, eventually arriving at 100% for perhaps their last week under lights. Always watch your seedlings though as some plants really don’t like these higher intensity levels and it’s not easy to predict. Many lettuces like high intensity once they are growing strongly with good roots, radish loves high intensity, onions and turnips not to much, most kales like high intensity, but not cauliflowers, so you need to experiment.

<aside> 💡 I recommend getting a light meter app for your phone, it’s very useful to compare different growing environments. I use Lux myself, it’s free


‎Lux Light Meter Pro


I almost never germinate seedlings under grow lights. Instead I will allow them to germinate in a propagator, window sill or cool bedroom floor, depending on the plant. You can imagine that this is the first week of life.

<aside> 💡 In 2022 I stopped using a propagator and just grew on a warm windsill and/or chitted my seeds


Unheated (cool) grow lights (starter environment)

After germination I will then normally prick seeds out into their individual modules, although sometimes they are direct sown into their modules. At this point they often - but not always - go under my cool-lights, typically for 2 weeks, although slow growing plants like some brassicas might go under for longer.

This grow light environment is very bright and quite cool, the plants focus is on growing leaf, not stem because of this. This environment is in an unheated workshop, it has no additional heat.