Grow lights for tomatoes and other fruiting plants indoors

Quick answer: fruiting plants need more light than herbs, and a bigger plant needs it for longer. University of New Hampshire Extension puts full-sun plants at 14-20 mol·m⁻²·d⁻¹ while they grow, flower and fruit. Michigan State University Extension asks for 12 or more for leafy greens and herbs. A shelf that grows good basil can still starve a tomato.

Herb and lettuce targets, the per-herb research and the temperature effect sit in DLI targets for herbs and leafy greens. The question here is different: what tomatoes, peppers and strawberries ask of a fixture, and why hours alone will not close the gap.

Why a fruiting plant needs a bigger daily total

University of New Hampshire Extension explains daily light as a bucket the plant fills each day. “Sun loving plants, like tomatoes, have bigger buckets,” and the same page notes that a pepper seedling has a smaller bucket than a pepper plant that is ripening fruit. The bucket grows with the plant. A seedling under a lamp is not a preview of what the mature plant will want.

UNH Extension’s Q&A on indoor lighting turns that into bands: 4-6 mol·m⁻²·d⁻¹ for houseplants, 6-8 for shade plants, 10-12 for part shade, and 14-20 for full sun, the last band meant for growing, flowering and fruiting. Seedlings are treated separately. Before transplant, UNH suggests aiming for about 8-14 even for full-sun plants. University of Maryland Extension lists tomatoes as a full-sun plant that prefers 8-10 hours of direct light a day outdoors, so the 14-20 band is the one that applies to a fruiting tomato. The UNH bands do not split full-sun plants by species, so pepper and tomato share the range.

Seedling-stage light is a separate job, covered in grow lights for seedlings. If you are moving seedlings on to fruiting size, expect the target to rise.

Strawberries: the one fruiting crop with a tested range

A University of New Hampshire trial reported to SARE grew day-neutral strawberries (cultivar Albion) in a hydroponic system at 14, 20 and 26 mol·m⁻²·d⁻¹, all on an 18-hour photoperiod. The plants at 26 produced significantly higher yields, more fruit and higher sugar content than those at 14. The report also states that plants at 20 produced similar yields to 26, and that a medium treatment may be more economical despite a slight loss of fruit.

Two limits matter. The trial tested one cultivar, and the authors say a repeat with higher light would show more of the relationship. Nothing above 26 was tried, so 26 is the best result in that trial, not a ceiling for strawberries in general. The three treatments ran at 216, 308 and 401 µmol·m⁻²·s⁻¹ respectively, which shows how much fixture output the upper end takes.

Hours cannot stand in for intensity

The daily total is intensity multiplied by hours, so a weak lamp can run all day and still fall short. UNH Extension says a plant could have a low-intensity lamp on for 15 hours a day and still not reach its DLI. Day length also acts as a calendar cue. UNH notes that daylength mostly relates to flowering and fruiting responses, so changing the hours does more than change the total.

University of New Hampshire Extension gives two examples from seedling work, where UNH assumes seedlings want about 12 mol·m⁻²·d⁻¹. A tomato seedling can fill its bucket in about five hours of full sun, while the same plant may need 22 hours under fluorescent tubes. One commercially available LED bar the author tested needed 8 hours a day at 8 inches above the crop but 16 hours at 20 inches. These are single products and seedlings, so they show how steeply height changes the hours; they are not run times for your fixture.

To get from a fixture to a daily total, measure the light at leaf height and convert it with the DLI calculator. The wattage guide explains why the watt rating on the box cannot answer that question, and the electricity cost guide shows how to price the extra hours a fruiting crop adds.

Signs the light is too low

UNH Extension’s advice is plain: if plants are stretched and pale, you need more light, and you can get it by going brighter, closer or longer. For a fruiting plant, the other symptom is a plant that grows leaves but flowers or sets little. That pattern fits a daily total below the target, though pollination and temperature can cause it too. The hydroponic tomato page covers hand pollination and the rest of the system, and hydroponic peppers and strawberries have their own crop pages.

Feeding changes once flowers appear; the sibling page on bloom nutrients for fruiting crops covers that.

Light stops mattering once tomatoes are mature

University of Maryland Extension states that light is not necessary for ripening mature tomatoes, and that tomatoes finish ripening on a kitchen counter. The shelf light earns its keep earlier, in growth, flowering and fruit size. A mature green fruit you pick can be moved off the shelf.

A practical order of work

1. Pick the band: 14-20 mol·m⁻²·d⁻¹ for tomato and pepper, and for strawberry the range UNH tested.
2. Measure what your fixture delivers at leaf height, at the height you will actually hold it.
3. Convert to a daily total with the calculator and adjust height or hours until the total lands in the band.
4. Raise the fixture as the plant grows, and measure again.

If the total cannot reach the band without an impractical number of hours, the fixture is too weak for the crop. That is a reason to pick a leafy crop for that shelf, not to keep stretching the day.

Sources

• University of New Hampshire Extension, Optimizing Plant Growth with Indoor Lighting Q&A
• University of New Hampshire Extension, Growing Seedlings Under Lights
• SARE final report GNE18-169, University of New Hampshire day-neutral strawberry DLI trial
• University of Maryland Extension, Growing Tomatoes in a Home Garden
• Michigan State University Extension, Lighting Greenhouse Vegetables