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Tomato Grow Lights for Seedlings: Frequently Asked Questions

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Grow lights for seedlings are an ideal way to start tomato plants and grow them indoors (or in a greenhouse) until they are ready to set out in the garden.

While natural light offers the full spectrum of color, grow lights for seedlings provide consistency to get your seedlings off to a good start. You can give plants the hours of light they need, at an appropriate intensity, so that they can grow healthy and strong.

That being said, many of us have questions about using grow lights to start and grow tomato seedlings.

Here are the most frequently asked questions about tomato grow lights.

Tomato Grow Lights for Seedlings: FAQs from Tomato Dirt

Grow Lights for Seedlings:
Frequently Asked Questions

Q. Should I use grow lights for seedlings instead of growing them on my windowsill?

A. That depends! First, note the position of your window. Only a south-facing window allows enough natural light to grow healthy seedlings during the off season. Second, if you live in a northern area (in the Northern Hemisphere) or southern area (in the Southern Hemisphere), the sun’s rays are also less intense your hours of natural light are shorter than in those areas which are closer to the equator. These factors make a good case for choosing to start seedlings with grow lights rather than on a windowsill. But if you have 12 hours of strong light in a south-facing window – and if you want to grow just a few tomato seedlings rather than an entire flat – than a windowsill might be a good option for you.

Q. What are the most popular grow lights for tomato seed starting?

A. Fluorescent bulbs are a favorite among home gardeners. They are ideal for starting tomato seeds because they give off very little heat and they provide high intensity light. Plus, they are not expensive.

Q. Why are the least efficient grow lights for tomatoes?

A. Incandescent bulbs emit too much heat, which can burn tender tomato foliage. They also lack blue spectrum light that helps seedlings become stocky and dark green.

Q. I hear that I should choose a “full spectrum” light. What does that mean?

A. “Full spectrum” refers to light that contains all colors of the rainbow. Tomato plants require the full spectrum for photosynthesis, although red and blue are most important. Natural sunlight contains the full spectrum of light. The most effective grow lights imitate this aspect of natural light the best.

Q. Can I use my shop light to grow tomato seedlings?

A. Probably … especially if it is a fluorescent light. Check its spectrum. You also need to figure a way to set it up so the light’s height is adjustable.

Q. How closely do I place the grow lights to the seed trays and seedlings?

A. Tomato seedlings need high intensity light. Fluorescent bulbs should be placed very close to the plants, no more than three inches away from the soil surface (before seedlings emerge) or foliage (once seedlings sprout.)

Q. How often should I adjust the grow lights?

A. Check the position of the lights each week and raise them accordingly to keep the distance between plants and lights to no more than 3 inches. This can be simple to do if you choose a light stand with adjustable fixtures. If your set up doesn’t have automatically-adjustable fixture, then rig the light and ballast so you can raise it easily.

Q. Should I leave the grow lights on for 24 hours a day?

A. No. Tomato seedlings need rest from light in the growth process.

Q. How much light do my tomato seedlings need per day?

A. Different kinds of plants need a different balance between light and darkness: short-day plants (less than 12 hours of light a day), long-day plants (14-18 hours of light a day) and day-neutral plants (8-12 hours of light a day.) Tomatoes are long-day plants and require 14-18 hours of light a day. You can set your grow lights on an automatic timer to ensure that your tomato seedlings get the optimum hours of light each day.

Q. Does the room temperature make a difference for tomatoes under grow lights?

A. Tomatoes seedlings germinate and grow best when room temperatures are between 65º-85ºF. If your air temperature falls below that, or if you keep your house cooler overnight, set plants on a seedling heat mat or use a space heater to keep the grow light area warm. Keep the grow light area free from drafts.

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