Plant lights Vegetables

How to Use a Grow Light for Tomatoes

By Alexander Tsvetkov

Tomatoes love growing in ample light and there is nothing more exciting for a gardener than to watch them grow into plump and juicy fruits with vibrant colors. Unfortunately, when the winter sets in, they’ll only be getting very little sunlight.

Depending on the vegetation or your grow space, you’ll need to prepare for growing them indoors. And for that, the most important consideration to take into account is grow lights.

In this article, we’ll discuss using grow lights for vegetables and specifically how to use grow light for tomatoes.

How Important Is Light for Tomatoes?

Nothing is more important to tomato plants than ample sunlight. And if you cannot provide it to them, then there is no other way to go than to supply artificial light. Tomatoes love to be in the sun and when you’re growing them indoors, you have to make sure to put them in the brightest part of the house such as a windowsill, sunroom, etc.

Many plants can survive without much light and growing them inside your home is therefore quite easy. However, that’s not so for tomatoes. They require all the energy from the light source to convert into energy they can use to produce fruit.

Only when there is sufficient light do tomato seedlings truly start to grow vigorously and produce tomatoes. Grow lights act as an alternative to sunlight.

What Type of Grow Light Is Best for Growing Tomatoes?

When it comes to hanging grow lights above to tomato plants, you need to decide which type to use. There are a few categories:

  • Fluorescent—these lights are the cheapest, but don’t give off the full spectrum of light nor do they penetrate deep into the plants.
  • High Intensity Discharge (HID)—there are two types of these lamps: HPS (high-pressure sodium) and MH (metal halide) and both are needed, depending on the growth stage of the plant. However, these are mostly used for a semi-professional or professional setups.
  • LED Lights—these are standard lights for indoor usage since they give the best plant growth. They consume less energy and increase yield as well as the quality of growth. Besides, these lights can penetrate the plant’s canopy and reach the understory, enhancing the plant’s ability to produce more fruits.

If you need to know more about the top grow lights for tent culture, there is a separate article for that as well.

Warm or Cool Light?

Tomatoes need a wide range of light colors to properly grow to their full capacity and both warm (red) and cool (blue) lights are especially important for this.

Cool colors (6500K) are most helpful during the early germination phase. They help stimulate the leaves and vines to grow healthily. However, they contribute little to flowering and fruit production.

Warm color lights (2711K), on the other hand, encourage the plants to produce healthy flowers and bear fruit, but they don’t help much in leaf growth.

Light Intensity Needed for Tomatoes

LED lights give you more control over the intensity and height of the panels and better confidence in your plants’ survival. For each growth stage, the intensity of full-spectrum LED is as follows:

· Germination—20–40%

· Seedling—40–60%

· Vegetative—60–90%

· Flowering—90–100%

You might be wondering about the number of lumens needed to grow tomatoes indoors. In terms of brightness, you can provide 7000 lumens for optimal tomato growth. But remember that brightness isn’t all there is to it, but color and temperature need to be considered as well to ensure good growth of the plants.

Watts for Tomato Growth Indoors

You need to figure out the number of sq. feet (0.1 sq. meter) the plants’ stems and foliage occupy. This is not the area of the entire room, rather only the area where the plants will grow. For example, if the grow room has an area of 10 × 10 feet (3 × 3 m), but the tomatoes are growing in a 5 × 5 (1.5 × 1.5 m) area, that is, 25 sq. feet (2.5 sq. meters), then you need to provide light for this sector only.

On average, you need around 40 watts per sq. foot (0.1 sq. meter) for tomato growth. Which means for 25 sq. feet (2.5 sq. meters), you’ll need 25 × 40 = 1000 watts. 

How High Should Grow Lights Be Above Tomato Seedlings?

Since tomato seedlings require light at high intensity for healthy growth, the lights should be placed very close to them. 

If the bulbs are fluorescent, then probably you might want to keep the bulbs within 12 inches (30 cm) of the plants. Fluorescent bulbs lose their intensity if placed further from the plants, so they won’t be getting enough light energy in this way.

On the other hand, if the bulbs are LED, then you have a better situation. LED bulbs can emit intense light energy for up to 30 inches (75 cm). So, if you want your tomato plants to grow plump and juicy tomatoes, don’t keep the bulbs more than 30 inches (75 cm) above them.

And finally, for HID lights, you shouldn’t place them too close to the plants. Their distance from the plants should be between 24 and 60 inches (60 and 150 cm).

Moreover, as the plants get taller, you’ll need to move the lights further away as well. You don’t want the leaves to get burnt, right?

Day Length Under Grow Lights

Different plants require different day lengths under grow lights. You need to properly balance the length of light and darkness for proper vegetative growth. The more light your tomato plants receive, the more the foliage they’ll produce.

It might seem tempting to turn on the light all the time, but that won’t work. Plants need to rest at night, just as we do! Too much light will exhaust them. So, you need to offer them at least 6 to 8 hours of darkness per day.

Since tomatoes are long-day plants, they require between 14 and 18 hours of light per day. However, if you’re using a combination of LED light and natural light, some fine-tuning might be necessary.

Final Words

Growing tomatoes indoors requires a lot of care. You need to do your research beforehand and while growing them under grow lights. Optimum exposure to these lights is necessary for perfect growth.

That’s why whether you’re a pro-gardener or just a tomato lover, you need to know how to use grow lights to succeed with tomatoes indoors.

Alex is the senior editor of and also its co-founder. He’s a passionate organic gardening teacher who holds his master’s degree from the University of Connecticut. 

He’s spent the last 18 years working in different segments of horticulture and green gardening. Running green garden centers and nurseries, teaching about them, and writing for such a long time has provided Alex with a wealth of knowledge that we need so much in this world.

Alex believes everyone has a duty to this world in reducing carbon emissions and global warming. He also wants to motivate everyone to be as self-sufficient as possible and get involved more in gardening. This, according to him, will greatly help people and their offspring to grow healthy and build a great world to live together.

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

7 comments on “How to Use a Grow Light for Tomatoes

  1. So 14 to 18 hours of light, no matter the stage, only vary the intensity not the length? Growing san marzano inside, all advice appreciated, 5 gallon bags, grow tent. Thought you could control when they fruit? Can’t find more research!

  2. Pingback: Growing Tomatoes Under Fluorescent Lights!

  3. Pingback: Indoor Tomato Gardening: Harnessing Grow Lights for a Bountiful Yield

  4. Due to the advancements in led lighting, if you use 40 percent for seedlings and fifty percent for veg etc, you will kill your plants or they will grow poorly. The lm301b diodes on the mars hydro sp-3000 for example can pull off seedlings on twenty five percent at eighteen inches. Too much light and the leaves grow smaller and curl, that is if not die. The LED estimates are old.

  5. Kathy Koch

    Good morning.
    Besides grow lights. Does the room temperature have to be considered, since they need the heat to grow.

  6. Good post for a lot of folks wondering about growing indoors during the winter months.

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