Do plants need infrared light? Not much. Infrared light for plants matters mainly at the far-red edge, about 700–750 nm, which boosts leaf expansion and photosynthesis when combined with regular light. Longer infrared from heat lamps only warms plants and air. For indoor growing, a full-spectrum LED with a little far-red beats any infrared heat lamp.
Key takeaway: When people say infrared light for plants, the useful part is far-red. Treat heat lamps as heaters, not grow lights.
| Band | Wavelength | Effect on plants |
|---|---|---|
| PAR (visible) | 400–700 nm | Main driver of photosynthesis |
| Far-red (near-infrared edge) | 700–750 nm (often defined up to 800 nm) | Leaf expansion, stem stretch, adds to photosynthesis with PAR |
| Near and longer infrared | Beyond about 800 nm | Mostly heat; no direct growth benefit |
What Is Infrared Light for Plants?
Infrared light for plants is radiation with wavelengths longer than visible red light. In horticulture, the interesting slice is far-red. Michigan State University’s Erik Runkle describes far-red as roughly 700–800 nm, with 700–750 nm of most interest because it can increase growth. Photosynthetically active radiation (PAR) is defined as 400–700 nm, so far-red falls just outside it. Beyond that, infrared mainly carries heat, which is why incandescent and heat lamps feel warm.
Does Far-Red Infrared Light Help Plants Grow?
Yes, alongside regular light. Zhen and Bugbee (2020) at Utah State found that far-red photons from 700–750 nm, added to shorter wavelengths, increased canopy photosynthesis about as much as the same number of traditional PAR photons. A follow-up study by the same group found far-red substituted for part of PAR gave equal canopy quantum yield and raised growth largely by enlarging leaves so plants captured more light. Runkle notes the same: far-red raises growth indirectly through larger leaves and directly through photosynthesis, particularly at 700–730 nm. Some scientists propose extending PAR to 750 nm, sometimes called ePAR.
Is Far-Red Infrared Light for Plants Useful Alone?
No. This kind of infrared light for plants works by interacting with shorter wavelengths. On its own it drives very little photosynthesis. A Design Lights Consortium review also cautions that predicting far-red’s effect is complicated, because it depends on the rest of the spectrum and the crop.
What Did MSU Find When Adding Far-Red to Lettuce?
Meng and Runkle grew Rex and Cherokee lettuce and Genovese basil under red and blue LEDs at 180 µmol·m⁻²·s⁻¹, with and without 30 µmol·m⁻²·s⁻¹ of added far-red, at 72°F (22°C). Their results show the trade-offs of infrared light for plants:
- More weight: Far-red raised Cherokee lettuce fresh weight by 17% at a 30:150 blue:red ratio and by 48% at 90:90.
- More stretch: Hypocotyl length of Cherokee lettuce and basil rose 33–37% in the 90:90 treatment.
- Less color: Far-red reduced the red pigmentation of Cherokee lettuce and cut lettuce relative chlorophyll by 10–19% in some cases.
The authors concluded that blue and far-red act in opposite directions, so the best spectrum depends on whether you want size or color.
What Does Too Much Infrared Light for Plants Do?
Too much far-red makes plants stretch. Runkle explains that a low ratio of red to far-red signals shade, triggering longer internodes and leaves, upward leaf angles and less branching. That’s why seedlings stretch under incandescent bulbs, which he notes emit about 52% of their 400–800 nm output as far-red. For compact seedlings and herbs, keep far-red modest. Our article on leggy seedlings covers other causes of stretch.
How Much Infrared Light for Plants Do Grow Lights Have?
Usually little far-red. Runkle reports that most horticultural LED fixtures emit under 2% of light as far-red, and many purplish fixtures under 1%. For comparison, he lists about 25% far-red in sunlight’s 400–800 nm range, 6% for high-pressure sodium, 8% for metal halide and 2% for cool-white fluorescent. Many newer full-spectrum LEDs add a far-red channel; check the spectrum chart on the product page.
| Light source | Far-red share of 400–800 nm (Runkle) |
|---|---|
| Sunlight | About 25% |
| Incandescent | About 52% |
| Metal halide | 8% |
| High-pressure sodium | 6% |
| Cool-white fluorescent | 2% |
| Most horticultural LEDs | Under 2% |
Can an Infrared Heat Lamp Be Used as a Grow Light?
No. Infrared heat lamps are built to emit heat, with most output beyond the useful far-red band and very little PAR. They can scorch leaves at close range, dry out media and raise fire risk around water and plastic. If a grow space is cold, use a thermostat-controlled space heater or seedling heat mat and a real grow light. See our guides to seedling heat mats and electrical safety for indoor hydroponics.
Does Infrared Light for Plants Affect Flowering?
Sometimes. Runkle summarizes that red plus far-red light used at night is most effective at promoting flowering of long-day plants, while far-red alone isn’t effective, and far-red has no consistent effect on day-neutral plants. The benefit for long-day plants shrinks once the daily light integral (DLI) reaches about 12 mol·m⁻²·d⁻¹. For short-day crops, stray light at night is the bigger risk; see light leaks and night interruption.
How Do You Use Far-Red Infrared Light for Plants at Home?
- Start with enough PAR. Hit the right PPFD and DLI first; see PPFD and DLI.
- Choose a full-spectrum LED with a modest far-red channel if you want faster leaf growth.
- Keep far-red a small share for compact seedlings, herbs and microgreens.
- Use more far-red for lettuce if bigger, longer leaves are welcome; MSU research added far-red to red and blue LEDs and saw longer lettuce leaves and more biomass.
- Watch plant shape. Stretching means too much far-red or too little total light.
- Use a timer so photoperiods stay consistent; see light schedules.
Is Infrared Light for Plants Safe for People?
Far-red LEDs at grow-light intensities are generally low risk, but far-red looks dim, so your eyes may not register how bright a fixture is. Don’t stare into any fixture at close range, and follow the manufacturer’s safety guidance. Heat lamps pose burn and fire hazards; keep them away from water, plastic and plants.
Do You Need a Separate Infrared Grow Light?
Most home growers don’t need separate infrared light for plants. A quality full-spectrum LED delivers what plants need, and many now include a small far-red share. Separate far-red bars are useful mainly for experiments or specific flowering control. For choosing a main fixture, see our grow lights guide.
FAQs
Is far-red the same as infrared?
Far-red sits at the near-infrared edge, about 700–800 nm. Most heat-lamp infrared is longer.
Can I see far-red light?
Barely. Runkle calls it marginally visible.
Does infrared light for plants raise yield?
Far-red can, mainly via bigger leaves, when PAR is adequate.
Should microgreens get far-red?
Small amounts at most; stretch can make them floppy.
Do red heat bulbs help seedlings?
No. Use a heat mat and a grow light.
Sources
- Runkle, E. — Far-red Light in Greenhouse and Indoor Farming (Michigan State University / GPN, PDF)
- Meng, Q. & Runkle, E. — Far Red is the New Red (Michigan State University, PDF)
- Zhen, S. & Bugbee, B. (2020) — Far-red photons have equivalent efficiency to traditional photosynthetic photons, Plant, Cell & Environment
- Zhen, S. & Bugbee, B. (2020) — Substituting far-red for traditionally defined photosynthetic photons, Frontiers in Plant Science
- DesignLights Consortium — Limitations of Predicting Far-Red’s Effect on Photosynthesis (PDF)
- Photo credits (Wikimedia Commons, edited with titles): Goat kids under infrared lamp (2) by Cjp24 (CC BY-SA 4.0); Infrared HDR St Bartholomew’s church Hanworth Norfolk (7531334490) by David from Colorado Springs, United States (CC BY 2.0); Red leaf lettuce J1 by Jamain (CC BY-SA 3.0); Low-key photograph of light bulb, Straume, Norway julesvernex2 by Jules Verne Times Two (CC BY-SA 4.0); Terramera scientists working in a climate-controlled growth chamber located in Vancouver, BC. The space uses LED lighting, which supports plant growth. (50188965171) by Simon Fraser University – Communications & Marketing (CC BY 2.0)