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When plants that are supposed to be easy keep going pale or stop growing, check the light before blaming the plant. A grow light can supplement or replace inadequate window light. This guide covers how to tell whether light is the limiting factor, what grow light specifications mean, how to position and time the light, and which plants benefit most.

Signs your plants need more light

  • Variegation fading to green. A variegated golden pothos (Epipremnum aureum) reverts toward all-green leaves when it can't photosynthesize efficiently enough to "afford" the non-chlorophyll portions of its leaves; NC State Extension notes that low light can cause loss of variegation. See pothos care.
  • Stalled growth. The plant stays alive but produces little or no new growth.
  • Small, widely spaced leaves. New leaves on a plant like monstera come in small with long gaps between them. This is etiolation: the plant stretching toward insufficient light.

UMN Extension and MU Extension list these among the signs of too little light. To check how much light a spot actually gets, the light meter calculator converts a phone-app lux reading into a plant light category.

Grow light basics

PAR vs. lux. Lux measures light intensity as perceived by the human eye. PAR (photosynthetically active radiation) measures the portion of the light spectrum that plants actually use for photosynthesis, the 400 to 700 nanometer range, per MU Extension. A light can look very bright to your eye (high lux) while being poor for plants if it's weighted toward wavelengths outside the PAR window. Grow light makers typically list PPF, the PAR a fixture emits each second; PPFD (photosynthetic photon flux density) is the PAR that actually reaches a given area of leaves, per Iowa State Extension. You don't need to become an expert in this; the practical point is that lux ratings on box packaging are less meaningful than PPF or PPFD ratings or spectrum charts.

Full-spectrum LED. "Full-spectrum" in the grow light context means the light covers both the blue and red portions of the spectrum. Oklahoma State Extension notes that most plants make especially good use of blue light at about 440 nm and red light at about 660 nm. Per Iowa State Extension, blue wavelengths help regulate chlorophyll production and control excessive stem elongation, while red wavelengths are used extensively for photosynthesis and promote flowering and fruiting. For foliage plants, UMN Extension rates blue or mixed bulbs as suitable for non-flowering houseplants, and white or balanced bulbs as suitable for most plants at any stage of growth. White LEDs give leafy foliage plants a usable spectrum without the purple/magenta glow of red-and-blue horticultural LEDs.

Color temperature basics. Color temperature is given in Kelvin (K): lower numbers are redder ("warm white") and higher numbers are bluer ("cool white"), per Iowa State Extension. Iowa State recommends a full-spectrum fixture, sometimes sold as "daylight," which has some of both red and blue, or a mix of warm white and cool white bulbs. UVM Extension gives natural light a color temperature of 6500K and calls a bulb rated near 6500K ideal for grow lights. A Kelvin rating gives only the approximate color of the light, not the exact wavelengths the plant receives.

Timer is essential. Plants use light duration (photoperiod) to regulate growth cycles. Iowa State Extension suggests 12 to 14 hours of light a day for most indoor plants, and no fewer than 10 or more than 16; UMN Extension gives 12 to 14 hours for foliage houseplants. More is not automatically better: plants still need a period of darkness to grow properly. A mechanical or digital outlet timer is inexpensive and removes the variable of remembering to switch lights on and off.

Distance matters more than wattage. Watts measure the energy a fixture uses, not how much light it gives off, per Iowa State Extension. Distance, by contrast, changes what the plant receives: the intensity of light drops rapidly as the distance from the light increases, per MU Extension, so a modest light close to a plant can deliver more than a strong one far away. Iowa State Extension suggests 12 to 24 inches from the foliage for most LED fixtures and 6 to 12 inches for fluorescent tubes; UMN Extension gives 12 to 24 inches for foliage houseplants and 6 to 12 inches for flowering houseplants.

What to look for in a grow light

LED grow lights have extremely high product turnover on consumer marketplaces, and model names don't reliably map to consistent specifications. Judge any unit by its listed specifications rather than its name. Prioritize:

  1. Documented spectrum coverage rather than just wattage claims.
  2. A full-spectrum or "daylight" rating for foliage plants; UVM Extension calls a color temperature near 6500K ideal.
  3. A mounting system that lets you set the distance the manufacturer recommends, which for most LEDs is 12 to 24 inches from the foliage per Iowa State Extension.
  4. Either a built-in timer or a separate outlet timer. A light with a built-in timer and dimmer also lets you adjust intensity without changing the light's physical position.

Wattage on grow light packaging measures energy use, not light output, so compare PPF figures where the maker lists them. Look for user reviews that mention actual plant response, not just light appearance. For specific models, see best grow lights.

Setting up the light

Position. Mount the light at the distance the manufacturer gives for that light; for most LEDs, Iowa State Extension suggests 12 to 24 inches above the foliage. Position relative to the light source matters a lot: "under the grow light" doesn't mean much if a plant sits well below and to the side of it. MU Extension measured less light to the side of fluorescent tubes than directly beneath them, with the brightest spot under the center of the tubes. Put the plants you most want to improve closest to the light.

Shelf units. A single light mounted at the top of a shelf unit, aimed down, mainly serves the top tier. The lower shelves get only diffuse spillover light, and because light intensity drops off fast with distance, they receive much less than the top tier. Two ways to even it out:

  • Place a reflective surface behind the shelf to redirect some of the spillover light toward the lower tiers.
  • Mount a T5 fluorescent or LED bar fixture to the underside of each shelf. This is more expensive and more complex to wire, but it gives each tier consistent light rather than a gradient from top to bottom.

Timer. Set an outlet timer to a consistent schedule. Iowa State Extension suggests 12 to 14 hours a day and no more than 16. MU Extension suggests 12 to 14 hours for plants that also get some window light and 16 to 18 hours for plants that get none, and advises running the lights at the same time the plants receive window light.

Running cost. To estimate it, multiply the light's actual power draw in watts by the hours it runs per day and divide by 1,000. That gives kilowatt-hours per day; multiply by 30 and by your electricity rate for a monthly figure.

Which plants benefit most

Low- to medium-light tropical foliage plants such as pothos, snake plant, heartleaf philodendron and ZZ plant can get sufficient light for healthy growth from a grow light (see the FAQ below). Snake plant (Dracaena trifasciata, formerly Sansevieria trifasciata) is genuinely low-light tolerant; NC State Extension says it "will tolerate very low light." It is not going to turn into a high-growth plant under grow lights.

Grow lights set up for tropical foliage plants are not ideal for succulents and cacti, which have different light and dark cycle requirements. High-light-demanding plants, including many cacti and succulents, may underperform even under a quality LED; MU Extension notes that most cacti and succulents are not satisfactory for growing under artificial lights.

FAQ

How many hours a day should I run my grow light?

Iowa State Extension suggests 12 to 14 hours a day for most indoor plants, and no more than 16. The key is consistency — the same on/off schedule every day, which is why a timer is effectively required. Running lights 24 hours is not recommended; plants still need a period of darkness to grow properly.

Does color temperature matter for grow lights?

Yes, in the sense that it tells you something about the light's spectrum. Lower Kelvin values are redder and higher values are bluer, per Iowa State Extension; blue light helps control stem elongation, and red light promotes flowering and fruiting. For indoor tropical foliage plants — pothos, monstera, ferns, calathea — a full-spectrum ("daylight") LED is the usual choice, and UVM Extension calls a rating near 6500K ideal. That said, color temperature is not the whole story; PPFD at your plant's canopy distance is a more accurate measure of what the plant actually receives.

Can grow lights replace a window entirely?

For most tropical foliage plants, a quality full-spectrum LED can replace or supplement inadequate natural light reasonably well. It cannot replicate the full spectrum and intensity of direct outdoor sunlight, so high-light-demanding plants (many cacti and succulents, fruiting plants, orchids) may still underperform. For low- to medium-light tropicals — pothos, snake plant, heartleaf philodendron, ZZ plant — a grow light at the manufacturer's recommended distance can provide sufficient light for healthy growth even in a windowless room. MU Extension suggests 16 to 18 hours of light a day for plants that receive no outdoor light.

Sources: Iowa State Extension — Important Considerations for Providing Supplemental Light to Indoor Plants, Iowa State Extension — How to Set Up Supplemental Lights for Indoor Plants, UMN Extension — Lighting for indoor plants and starting seeds, MU Extension — Lighting Indoor Houseplants, MU Extension — Understanding Grow Lights, UVM Extension — Tips for Choosing Grow Lights, Oklahoma State Extension — LED Grow Lights for Plant Production, NC State Extension Plant Toolbox — Epipremnum aureum, NC State Extension Plant Toolbox — Dracaena trifasciata