Greenhouse Grow Light Calculator
Find out whether your greenhouse plants get enough light in winter, and how many grow lights it takes to fill the gap.
How it works
How many grow lights does a greenhouse need?
Plants respond to the total light they get in a day, called the daily light integral or DLI. It is measured in moles of light per square meter per day. In winter, a greenhouse often gets only half of what fruiting plants need.
Light to add = Target DLI − Sunlight DLI inside the greenhouse
We spread that extra light over the hours your lights are on to get the light intensity (PPFD) at the plants. Then we divide by each fixture's output (PPF), allowing 20% for light that misses the bench.
| Plants | Target DLI |
|---|---|
| Orchids and flowering houseplants | 8 to 12 |
| Seedlings and starts | 10 to 14 |
| Lettuce, leafy greens and herbs | 12 to 17 |
| Citrus and potted fruit trees | 15 to 20 |
| Tomatoes, peppers and cucumbers | 20 to 30 |
Winter sunlight figures are typical January values. Clear glass lets through about 75% of the light and polycarbonate about 65%.
Questions
Grow light FAQ
Do I need grow lights in my greenhouse in winter?
In the northern half of the US, usually yes for tomatoes, peppers and seedlings, because short days give them too little light. Leafy greens and overwintering plants often manage without them, especially in the south.
How many hours should grow lights be on in a greenhouse?
12 to 16 hours a day total, counting daylight, suits most plants. Run the lights in the morning and evening to stretch short winter days, on a timer.
What is DLI for plants?
DLI, or daily light integral, is the total amount of light a plant gets in a day. Lettuce needs about 12 to 17, and tomatoes need 20 to 30.
How high should grow lights hang above plants?
Most LED grow lights work best 12 to 24 inches above the plants. Hang them higher over seedlings to avoid stretching and burning, and lower for fruiting plants.
More tools
Other greenhouse calculators
- Heater Size CalculatorHow many BTUs it takes to keep your greenhouse warm on the coldest night.
- Heating Cost CalculatorWhat a winter of heating will cost with electric, propane or natural gas.
- Fan and Ventilation CalculatorThe exhaust fan CFM and vent openings that keep summer heat down.
- Snow Load CalculatorHow much the snow on your roof weighs and when to brush it off.
- Foundation CalculatorConcrete, gravel, pavers and lumber for your greenhouse base.
- Greenhouse Size CalculatorHow big a greenhouse you need for the plants you want to grow.
- Shade Cloth CalculatorThe right shade percentage and cloth size for your greenhouse.
- Rainwater CalculatorHow much water your greenhouse roof can collect for your plants.
Want a second opinion?
Tell us about your greenhouse and where you live, and we will help you get it right.
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