What Are Grow Lights for Greenhouse | Supplemental Light for Year-Round Growing

Grow lights for greenhouses are electric fixtures, most commonly LEDs, that supply the specific light spectra plants need when natural sunlight falls short, allowing growers to cultivate vegetables, herbs, and flowers through any season.

Even the best-oriented greenhouse loses light during short winter days, heavy overcast stretches, or when the sun sits low on the horizon. A greenhouse grow light system fills that gap — not with generic brightness, but with measured wavelengths and intensities that actually drive photosynthesis. Whether you are starting seedlings in February or pushing tomatoes into December, the right lighting setup determines whether your greenhouse stays productive or goes dormant.

How Greenhouse Grow Lights Work

Plants use specific portions of the light spectrum for different jobs. Violet and blue wavelengths (400–500 nm) drive vegetative growth and leaf development, while red light (600–700 nm) triggers flowering and fruit production. A greenhouse grow light delivers these targeted ranges, usually in a red-to-blue ratio suited to the crop. Leafy greens and herbs thrive on a balanced 3:1 red-to-blue ratio, while flowering and fruiting plants need a heavier 6:1 red-to-blue ratio to set fruit.

Full-spectrum lights rated between 5,000 and 6,500 Kelvin mimic bright midday sunlight and work well for general-purpose growing where you rotate crop types. The critical measurement for plant lighting is not lumens (which measure human-brightness) but photosynthetically active radiation (PAR) and daily light integral (DLI). Leafy greens need a minimum DLI of 12, while high-light crops like tomatoes and peppers require 20–30.

LED vs. Other Grow Light Technologies

LEDs are the dominant choice for greenhouse supplementation, converting about 90% of electricity into usable light while producing minimal heat. That efficiency translates into lower electricity bills and less heat stress compared to older technologies. Fluorescent T5 or T8 fixtures still work for small propagation areas and seedlings because their lower intensity and cooler operation let you place them just 12 inches above the canopy. High-intensity discharge (HID) lights remain viable for large commercial operations in cold climates where their waste heat is actually a benefit, but their higher operating costs and shorter lifespan make them a poor fit for most home greenhouse uses.

Wattage and Coverage: How Much Light Do You Need?

The amount of light required depends on how much natural light your greenhouse already gets. For full supplementation (replacing sunlight entirely), budget 30–50 watts per square foot of growing area. A 100-square-foot greenhouse under total supplementation would need 3,000–5,000 watts. For partial supplementation where sunlight provides about half the needed light, drop that to 750–1,250 watts for the same space.

Quality LEDs last over 50,000 hours — roughly five to seven years of continuous use — and many offer 0–100% dimming so you can dial intensity up or down as crops change. The best LED grow lights for greenhouse setups include models with separate spectrum control, which lets you adjust blue and red output independently through a single fixture.

Greenhouse Grow Light Specifications at a Glance

Setting Up Your Greenhouse Grow Lights: Height, Schedule, and Spacing

Fixture height above the canopy directly affects light intensity. Low-wattage LEDs under 300 watts should hang about 12 inches above the plants. High-wattage fixtures over 1,000 watts need a 36-inch gap to avoid leaf burn. Fluorescents stay at 12 inches, while incandescent fixtures need 24 inches to spread heat.

Day length matters as much as intensity. Run lights 14–16 hours daily for seedlings, 12 hours for leafy greens and herbs, and 12–16 hours for flowering vegetables. A minimum dark period of 8 hours is essential — plants need uninterrupted darkness to metabolize energy. Using an automated timer eliminates the risk of forgetting.

Common Mistakes That Waste Time and Money

Cramming too many plants under a single fixture reduces airflow and invites fungal disease. Overhead light only penetrates the top layer of dense canopies, so space plants so each one gets direct exposure. Placing LEDs closer than 12 inches on low-power fixtures still causes leaf tip burn — the leaves curl and crisp at the edges within days. Running lights 24 hours a day seems productive but actually halts the plant’s energy-use cycle, reducing growth over time. And selecting a 6,500 K “daylight” bulb for flowering misses the mark entirely — flowering plants need the red-heavy spectrum that 2,700–3,000 K bulbs provide.

Greenhouse Grow Light Schedule by Growth Stage

Sizing Your System: What to Do First

Start by measuring your actual growing footprint, not the greenhouse floor. If you have a 10×10-foot space but only use two 4×8-foot benches, your light needs are for 64 square feet, not 100. Decide whether you are supplementing existing sunlight (partial) or replacing it entirely (full), then calculate wattage from the ranges above. For a 64-square-foot space needing full replacement, that is 1,920–3,200 watts. For partial supplementation, drop to 480–800 watts.

Match your fixture type to the crop cycle. A single programmable fixture like the TekSupply 4′ LED that dims from 0–100% handles both the low-intensity seedling phase and the high-light flowering phase without changing hardware. Track your plants’ leaf color and stem stretch — leggy growth means they need more intensity or closer placement. Compact, darker green growth with short internodes means light levels are spot on.