Best grow lights for plants

Sowing seeds on a windowsill early in the year when days are short and light is low often results in weak, leggy seedlings that won’t grow into strong plants. Grow lights cost less than 2p an hour to run and make all the difference when it comes to growing strong, healthy plants.

The Which? Gardening experts share the best grow lights on the market, plus tips about how to use them for maximum benefit to your plants, including which colour grow lights to choose.

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Are grow lights for plants worth it?

In our winter and spring tests, all the plants grown under grow lights were healthier and grew more strongly than plants on a windowsill.

The grow lights plug into mains sockets, so are easy to set up. While some might be better suited to boosting houseplants than propagation, the best ones gave us crisp lettuces and, after moving plants to an unheated greenhouse in late April, some ripe tomatoes by the end of June. Used for 12 hours a day every day for eight weeks, the Best Buy Secret Jardin light cost around £4.70 to run (based on 1 Oct to 31 Dec 2023, average cost per kWh of 27.35p), and the larger Lightwave T5 around £10.50. If you want to extend your homegrown vegetable season, or just get a head start with sowing, we think they’re worth using.

How to get the best from grow lights for plants

Where to use grow lights

The highest-scoring lights are effective without natural light, so can be used on a table, suspended under kitchen cupboards or on the floor.

What colour grow lights to buy

When choosing your lights, think about what you want to use them for. Grow lights using the blue spectrum of light are better for propagation and growth, while those using the red spectrum are better for encouraging flowering. Some lights are ‘full spectrum’ so combine different wavelengths of light for all stages of growth.

How to position grow lights

For optimum growth, lights need to be kept close to the plants. Don’t try to grow too many plants under one light because the highest light levels are directly under the lights and the level drops the further away a plant is. We also found that light levels drop off significantly if lights are too far above the plants. Read instructions carefully as they should give guidance, and adjust the height of the light as needed at different stages of growth.

How long to use grow lights

The number of hours you’ll need to keep the lights on varies depending on both the light you’re using and the plants being grown. Sun-loving plants, such as tomatoes, will need at least 12 hours of light per day.

How we test grow lights

We tested LED and fluorescent grow lights, and two hydroponic sets that incorporate grow lights. We set them up away from direct natural light and used a windowsill for comparison.

We measured the energy use of the lights and the amount of light (in lux) produced.

From November onwards, we grew lettuce, herbs and coleus plants under the lights for eight weeks. 

In early March, we sowed seeds of tomato ‘Moneymaker’, bronze-leaved lettuce ‘Amaze’ and Lobelia ‘Crystal Palace‘ under the lights.

Seedlings were potted on when large enough and grown under the lights until early May, or until they outgrew the lights and had to be moved.

We assessed how easy the lights were to use, the germination rate of the seeds, the rate and sturdiness of growth, how well their roots developed, and the colour of the coleus and lettuce leaves. We also tasted the lettuce leaves to see if flavour was affected by the lights.