Light is the main limit on an indoor tomato. A tomato is a sun-loving fruiting plant, and a sunny window rarely gives it enough, especially in winter. With a grow light on a timer, compact tomatoes in Kratky containers can flower and fruit indoors all year.
How Many Hours of Light
Give indoor tomatoes 14 to 18 hours of light a day, then a dark period. A plug-in timer keeps the schedule steady.
Don't leave the light on around the clock. Tomatoes under continuous light can develop yellow, mottled leaf damage; they need a daily dark period.
Outdoors, tomatoes need full sun: at least 6 to 8 hours of direct sunlight a day.
Choosing a Grow Light
- Full-spectrum LED is the usual choice. LEDs run cooler and use less power than older lights.
- Brighter for fruiting. Seedlings grow under modest light, but flowering and fruit need much more. A light that grew lettuce well may not be enough for tomatoes.
- Coverage matches the plants. One light should cover the whole canopy, not just the top of the tallest plant.
How High to Hang It
Follow the light maker's recommended hanging height, because it depends on the fixture's power. Start at that height and watch the plants:
| What you see | What it means | What to do |
|---|---|---|
| Long, stretched, pale stems | Too little light | Lower the light or add hours |
| Bleached or crispy top leaves | Too much light or heat | Raise the light |
| Short, sturdy, deep green growth | About right | Raise the light as the plant grows |
Temperature Indoors
Tomatoes grow best at about 70 to 85°F during the day, and no lower than about 55°F at night. A grow light adds heat, and heat matters twice in Kratky: warm solution holds less dissolved oxygen, which stresses the roots and invites root rot, and temperatures outside the comfortable range make flowers drop without setting fruit (see tomato flowers falling off).
Indoor plants also miss the wind and insects that pollinate tomato flowers outdoors. See hand-pollinating indoor tomatoes.
Back to the Kratky method for tomatoes.
Frequently Asked Questions
How many hours of light do indoor tomatoes need?
About 14 to 18 hours a day under a grow light, with a dark period every day.
Can tomatoes have light 24 hours a day?
No. Tomatoes under continuous light can develop leaf damage. Give them a dark period every day.
Can you grow tomatoes indoors with only a window?
It's hard. Even a south-facing window rarely gives a fruiting tomato enough light, especially in winter. A grow light makes indoor tomatoes practical.