Using Grow Lights for Tomatoes & Any Other Seedlings

Each spring, I can be found using grow lights for tomatoes and other seedlings. We start almost 16,000 seedlings per year in our grow room, so an efficient grow light setup is a must!

Grow lights are a wonderful tool that can help you grow more food, help you get an earlier start to your seed starting season, and help you achieve healthier plants in the long run. This guide will help you learn how to use them optimally.

Rookie Mistake

When I was still a novice gardener, I made the mistake of spending a ton of money on what I thought were the best grow lights. But they didn’t really improve my gardening or seed starting indoors, partly because I didn’t understand how to use grow lights effectively. 

I’ve seen other gardeners make similar mistakes with grow lights, seedling heat mats, seed starting trays, and even greenhouse kits. I hope this blog post will save you from making the same mistake.

When to Use Grow Lights

All plants need light energy to grow! Grow lights are commonly used for indoor seed starting and for growing plants when the available natural sunlight is insufficient.

You can start seeds without grow lights, as long as plants can get at least 4 to 5 hours of strong, direct sunlight. But for most indoor growing applications, grow lights are the ideal solution.

For years, I started seedlings in a sunny south-facing window. It can work, but even in a south-facing window, a supplemental grow light will help you achieve optimal growth.

How Long Should Grow Lights Be On

The truth is, I cannot give you a set time for how long to leave your grow lights on because it depends on many variables. I can tell you that 10 hours is the minimum threshold; don’t go below that daily exposure when using grow lights.

I can also tell you that you should never need to exceed 18 hours. If you need to go beyond 18 hours of lights on per day, something isn't working, and you need to adjust your setup.

To really dial in the efficiency of your grow light setup, pay attention to these variables:

• Plant Type

• Light Output & Efficiency

• Distance From Plants

• Grow Room Environment

Light Requirement Based on Plant Type

All plants need a minimum threshold of light to grow, but this threshold varies by plant type. Outdoors, leafy greens may be fine with just 4 hours of direct sunlight a day, while tomatoes need at least 5 to 7 hours. Growing indoors under grow lights, I would start lettuce with 10 hours of light and tomatoes with 16 to 18 hours, and adjust those durations based on other variables.

If a plant gets extra light hours above and beyond what it needs, it will not negatively affect the plant. Plants will be better for it and may even grow faster. 

That being said, you want to simulate day and night for your seedlings to prepare them for life outside and give them a rest period. Pro-Tip: Don’t spoil your seedlings with endless rays! It's tempting to keep grow lights on as long as possible to boost growth; however, they should not be left on all the time!

Light Output & Efficiency

The light output of your specific grow lights will influence how long they should be left on. Based on the type of grow lights you have, you may need a longer duration of exposure than if you had a more powerful or efficient grow light.

As far as how much photosynthetic energy each light produces, high-intensity discharge lamps (like metal halide or high-pressure sodium lights) put out the most growable light, compact fluorescent lights put out the least, and LEDs are in the middle. This means that you will need to leave your compact fluorescent grow lights on longer to achieve the same results as LEDs or metal halides. (For more information, read Choosing the Best Grow Lights.)

If your grow light is weak and far from your plants, you are creating unfavorable growing conditions, which means you will need a longer exposure time to compensate.

Optimal Grow Light Distance From Plants

When grow lights are close to plants, the light energy has to travel a shorter distance to touch these leaves. If your lights are farther away, more light energy is wasted and never reaches your plants. 

The minimum threshold for a grow light is 2000 lumens at a distance of 6 inches away from the plant. For every 1000 lumens you add, you can increase the distance by about an inch. That means my 30,000-lumen grow light setup can be about 34 inches from my plants and still be adequate. 

If your lights are too far away from your plants, it may not be the worst thing in the world, but you will have to run them longer to get good results.

Grow Room Environment

You may be surprised to learn that the environment in your grow room impacts the effectiveness of your grow lights. The more wasted light you have, the longer your lights will need to be on.

A room with white walls will reflect light back towards plants, increasing efficiency. Colored surfaces absorb light; black absorbs the most light, so it decreases efficiency. You can also add reflective or metallic surfaces to bounce the light back towards the plants.

How To Use Grow Lights for Tomatoes & Other Seedlings

Using grow lights for tomatoes, seedlings, or even growing plants in grow bags is not an exact science. There is no formula that will tell you to keep your lights on for a specific amount of time at a certain distance.

The most important tool you have is observation. Keep an eye on your plants and watch for cues that they need you to change the variables. 

You will know that your seedlings are happy and getting enough light if they are not leggy or stretching for the light, are not turning yellow, and continue to grow each and every day.