A Grower’s Guide to Leaf Surface Temperature: Finding the Sweet Spot for Your Crops
Air temperature tells you part of the story. Leaf surface temperature tells you the truth.
Most growers already know leaf surface temperature (LST) matters. If you’ve switched from HPS to LEDs, you’ve probably noticed your room feels cooler, and overheating isn’t the same constant battle. But here’s the catch: under LEDs, leaves often run cooler than the ambient air — meaning that trusty wall thermometer doesn’t always tell the whole story.
Dialing in the right LST isn’t just about avoiding stress. It’s about unlocking faster photosynthesis, more consistent canopy growth, and ultimately higher-quality yields. With the right tools, measuring and managing leaf temperature is easier than ever — and the payoff is massive.
Why Leaf Surface Temperature Matters
Think of Leaf Surface Temperature as the control dial for your plant’s metabolism. Photosynthesis works best between 15 °C and 30 °C (59 °F–86 °F) under standard CO₂ conditions. That’s the general baseline — but every crop, and every growth stage, has its own sweet spot.
Here’s what that looks like in practice:
- Cannabis growers aim for 78–80 °F during stretch, and 80–84 °F during bloom under high light (PPFD).
- Many cultivators swear by 82–83 °F as the “gold standard” for peak cannabis performance, especially under LEDs.
- CO₂ supplementation shifts the ideal range even higher, letting plants handle and even thrive under warmer conditions.
- Don’t forget: leaves can run cooler than air due to transpiration and stomatal cooling — or warmer depending on how efficiently they absorb specific wavelengths of light.
Bottom line: if you’re only tracking air temp, you’re not really tracking what your plants are experiencing.
How to Measure Leaf Surface Temperature (Without the Guesswork)
Not all tools are created equal. If you want accuracy, skip the wall thermometer and look at what’s happening on the leaf itself:
- FLIR thermal cameras: The Cadillac of Leaf Surface Temperature tools. They give you a full heat map of your canopy so you can see hot spots, cool spots, and everything in between.
- Infrared (IR) thermometers: Affordable and effective. Just take multiple readings across the canopy and average them out. One quick zap won’t cut it — consistency is key.
Leaf Surface Temperature Crop Guide: Finding the Sweet Spot
Pro Tips to Lock In the Right Leaf Surface Temperature
- Forget the “air temp = everything” mindset. LEDs often require higher ambient air temps or thermal IR to achieve the same Leaf Surface Temperature you were used to under HPS.
- Invest in an IR thermometer. Cheap, quick, and accurate enough to make a huge difference.
- Match temperature to your goals:
- Faster veg growth and higher yield → aim for 78–84 °F LST.
- Late flower and finishing → drop temps slightly to enhance flavor, aroma, and cannabinoid content.
- Know your crop. Lettuce taps out above 32 °C, tomatoes thrive in warmer ranges, and cannabis shines in the mid-80s.
- Balance LST with VPD. Leaf temperature affects transpiration, nutrient uptake, and CO₂ efficiency — so keep humidity and airflow dialed in alongside temp.
If you’re only measuring air, you’re flying blind. Leaf surface temperature is where precision meets performance. Get it right, and your plants won’t just survive — they’ll thrive. From lettuce to late-flower cannabis, dialing in LST is the difference between “good enough” and consistently better harvests.
About Aelius LED
Aelius LED is a global supplier of premium Horticultural LED products for all stages of a plant’s life cycle. Aelius aims to help cultivators worldwide realize the true genetic potential of their crops by providing results-driven LED Light Fixtures designed to master commercial cultivation. Aelius LED is dedicated to the discovery of new technology and contributing to growth of the controlled environment agriculture industry. Since its inception in 2018, Aelius LED has expanded to hang above a variety of crops in over 10 countries and is now on the hunt for distributors to bring even better customer service to the #AeliusFam across the globe.
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