Are Cheap Grow Lights Bright Enough For Tomato Seedlings?

Under the right conditions, some cheap LED grow lights can provide your tomato seedlings with the extra light they need to avoid going leggy. However knowing which lights are suitable it isn’t as simple as finding the power rating in the product description, as the light intensity that reaches the plant’s leaves depends on factors such as the spread of the light and the distance from the light source.

Some may be too weak and emit insufficient light, while others that are powerful enough on paper might be used in an incorrect configuration, leading users to wrongly conclude that LED grow lights are a waste of money and pass that advice on to new growers.

Over Christmas I decided to run my own experiment to find out if cheap grow lights are bright enough for tomato seedlings. I grew six different tomatoes varieties at eight distances from the LEDs to find out how much light tomato seedlings really need, if cheap grow were powerful enough to grow healthy seedlings, and to find out which varieties performed best under cheap LED grow lights.

I’ve published the results in this article and in the YouTube video below so that you can use grow lights to get a head-start on your growing season and an earlier tomato harvest later this year.

*Some links in this article are affiliate links. Commission may be earned on purchases made through them. Click here for more information about how I use affiliate links.

Why Grow Tomato Seedlings Indoors Under Lights?

I love tomatoes! I know it’s a cliché, but I can’t get enough of them. And to get an earlier crop, it’s essential that you start plants indoors to give them a head-start on the season. This means that the will be larger come springtime and they will grow much faster when you transplant them outdoors.

But if your house is like mine and you haven’t got many windowsills, then it might be too dark inside your home to grow healthy tomato plants. They’ll think they are covered by soil or debris and grow very tall, very quickly. They will seek out more and more light and your plants will end up falling over, unable to support themselves, let alone bear any fruit.

To stop your plants growing weak and leggy you’ll need to provide some kind of artificial light. Any bulb will have an effect but a full spectrum light, attempting to recreate daylight, or a cool colour temperature light is required so your plants don’t miss out on that essential green and blue light and can carry out basic biological functions.

Therefore it’s important that you both pick the right amount of lighting and the right kind of lighting too.

What Kind of Grow Lights Do My Plants Need?

Grow lights are designed to provide the right kind of light to grow healthy plants and are marketed at indoor houseplant enthusiasts and home-growers for this reason.

They often come as either light-emitting diodes (LEDs) or fluorescent bulbs. LEDs are more energy efficient but the initial cost may be higher, while fluorescent bulbs cost more to run but can be a cheaper purchase. However due to their inefficiency, in some circumstances, fluorescent bulbs can also create heat issues for the plants underneath and cause damage.

Grow lights will often provide full spectrum light (daylight) or attempt to mimic it by balancing the output of different colour bulbs, but some will allow you to provide more red light to enhance foliage growth or more blue light to promote root development. They also come in a variety of shapes, sizes and power ratings — all of which changes the amount of useful light that ultimately reaches your plants.

Knowing which to buy and how much money you need to spend to get enough light to on your plants can be a mine-field. Unfortunately, it’s often only once you have your grow lights that the calculation becomes simple. For a given energy output rate (in lumens), the emitted light will be spread out depending on the shape of the lighting unit and that determines the rate per area – the light intensity – that both the plants receive or your eyes perceive.

For an ordinary ceiling light bulb, light is emitted almost equally in all directions which makes comparison between bulbs relatively simple, but it also means the efficiency of getting light to a given area (such as on your plant’s leaves) is very low.

Grow lights on the other hand are highly directional and they are often shaped so that the light is emitted almost entirely downwards and onto your plants. However the exact light intensity at a given distance will depend on the precise shape and configuration of the grow light.

This can make comparison between grow lights difficult unless you are provided with data on the coverage or illumination area when the light is placed at a set height.

As a rough rule of thumb, I’d recommend aiming for an absolute minimum of 20 lumens per inch of cheap full-spectrum LED strip lighting based on the results of the experiment I carried out below. For lights that specify the coverage area at a given height and the power output, you will need to calculate the light intensity and find out how much your tomato variety needs during its seedling stage. If the calculated intensity is too high or too low, you will need to adjust the height or your grow light and bring it closer or place it further from your tomato seedlings.

How Close Should A Grow Light Be To Tomato Seedlings?

Tomato seedlings need to be close enough to the grow lights so that the light intensity meets their biological needs, yet not so close as to cause stress to the plant. The exact distance depends on the grow light as well as the variety and the age of the plant. For cheap LED strip lights emitting about 20 lumen per inch of strip light, I found that the maximum distance between plant and light should be around 10 centimetres or 4 inches, but this will vary depending on which grow light you are using.

A general method for finding the right distance is to measure the light intensity and adjust the distance from the light accordingly. You can use an app like Photone or Lux to measure the intensity at a set distance and get an instant value in lux or, more helpfully, a Daily Light Integral (DLI) which is a measure of all photosynthetically useful light energy provided over the course of the day for a given area. This light intensity value can then be compared to reference values for your plant.

But what value of light intensity should you aim for? I decided to find out for myself…

How Much Light Do Tomato Seedlings Need?

To find out exactly how much light tomato plants need, I conducted my own experiment. I sowed tomato seeds from six varieties (Sungold, Red Cherry, Black Cherry, Tigerella, Costoluto Fiorentino and Roma) into pots, filled with my signature seed starting mix, making sure to weight each component to ensure consistency between batches, and I used my DIY-growing station (pictured above) using these grow lights from Amazon to create eight different lighting conditions. I then let the seedlings grow for three weeks to try and find out at which light intensity was the cut-off point between healthy plant and leggy seedling.

At the end of the three weeks, I took each plant and measured the height and width of each of the stems and calculated a ‘legginess’-factor — the ratio of the two values. I then plotted these values on a graph against light intensity and used a basic exponential decay (plus a constant) to find the best fit and determine the intensity where shining more light on the plants wouldn’t significantly reduce legginess i.e. the plant had enough light to be growing healthily at this light intensity.

That data is plotted below and I have also marked where, visually, the plants appeared to be growing in either a healthy or leggy manner.

I found that tomato seedlings need a daily light integral (DLI) of 5 mol/msq/day to avoid going leggy. From the data, you can see that each of the lines begins to flatten off at a DLI of about 5 mol/msq/day and that the data points at a DLI of 5 and 6 have similar legginess to those at a DLI of about 10. This would suggest that tomato seedlings need somewhere around 5 mol/msq/day of useful light to avoid going leggy. For my grow lights, this meant that the healthiest seedlings were growing no further than 10 centimetres or 4 inches away from the lights. For your grow lights, you should use a light meter to find the distance at which the DLI is 5 mol/msq/day or greater and start your tomato seedlings no further away than that distance from the lights.

What Tomato Varieties Grow Best Under Cheap Grow Lights?

Of the six varieties that I tested, Sungold, Red Cherry and Black Cherry performed exceptionally well at light intensities of 5 mol/msq/day or higher or when placed within 10 cm of the lights. Sungold performed the best overall, having some of the lowest legginess scores at all light intensities, while Black Cherry grew well at all light levels and none of the plants looked leggy. However varieties like Roma, a plum-type, performed poorly at all tested light levels with every plants looking leggy. In fact, at the lowest light intensity the Roma seedlings were already falling over after only three weeks of growth. The only beefsteak or slicer tomato that was tested was Costoluto Fiorentino and, while plants at low light levels were rather leggy, those at the very highest light intensities (10 DLI) appeared healthy. On the whole varieties that produced smaller tomatoes, such as cherry tomatoes, performed better under grow lights than salad tomato plants or those that produce larger tomatoes like the plum and beefsteak or slicer-types.

If you’re growing your own tomatoes at home and want to get a head-start on the season and start growing your plants early in the year, then you can do so using cheap LED grow lights like mine as long as your plants get light for 12 hours each day and are within 10 cm of the light, so that they get a daily light integral (DLI) of at least 5 mol/msq/day.

And for the best results, be sure to pick cherry tomato varieties like Sungold, Red Cherry or Black Cherry which grow best under artificial lights. For larger tomato varieties, my data suggests that it would be best to provide even more light – a DLI of around 10 mol/msq/day – by using stronger lights or placing plants closer to the light.

Of course, once your seedlings have grown up and their light requirements start to increase, you’ll need to think about moving them outside during the day. On warm days you might be able to rely on the heat of the sun but, for much of the winter and early spring, you might want to try a plastic greenhouse which is less sturdy than a regular greenhouse, but much cheaper.

Happy Gardening!

Are Plastic Greenhouses Any Good?