At an indoor grow facility, measuring ambient room temperature doesn’t reveal the vitality of the plants in the room. This is why tracking leaf surface temperature (LST) can give growers insight into how plants respond to their environment—allowing the grower to make adjustments to the environment to produce the highest, healthiest yields.
It must be noted that leaf surface temperature is not the same as ambient air temperature. Air temperature affects LST in the growroom, but it is not the only factor that affects LST.
Why does leaf surface temperature matter (in short)? A too-warm room temperature stresses plants out, while too-cold temperatures halt growth. For maximum yields, it is important to find the perfect ambient temperature. There are also different optimal LST ranges that maximize growth of secondary metabolites such as cannabinoids, terpenoids, sterols, flavonoids, and others—so it is important to consider the purpose of your cultivation when measuring LST.
What Affects Leaf Surface Temperature
- Plant Species/Variant
- Ambient Air Temperature
- Light Spectrum
Yield-optimizing LED grow light systems have been proven to benefit ambient air temperature in growhouses.
What is the Ideal Leaf Surface Temperature?
Background: The metabolic reactions that keep a plant alive each operate at a different optimal temperature range. Photosynthesis is the most important of these metabolic systems, as it is vital to the plant’s survival. Optimal temperature ranges depend on which process needs to be enhanced.
For example, ideal temperature is subject to:
- Species/variety of plant
- Overall environmental conditions
- Purpose for cultivation (resins, pigments, enhanced flavonoids, etc.)
In general, growers should adhere to these guidelines:
- The leaf surface temperature of most cannabis plants should be between a range of 72-86 degrees.
- LST varies depending on the native climate of the cultivar. For example, plants that have evolved in cooler climates can generally tolerate in cooler temperatures, and the same for those evolved in temperature climates.
- CO2 generally raises the ideal temperature for photosynthesis.
Bottom Line: LST depends on the plant species/variant and concentration of CO2. And in any given growroom, trial-and-error can help determine the ideal range for LST.
What Affects Leaf Surface Temperature?
A more in-depth look at the factors affecting LST:
- Ambient air temperature
- Relative humidity
- Leaf physiology and pigmentation
- Genetic/metabolic differences
- Light spectrum all affect LST
Leaf Surface Temperature and VPD
Air temperature sets the foundation for leaf temperature, warming the leaves that are cooler than the air, and cooling leaves warmer than the air.
Leaves typically cool themselves by evaporating water through stomata (a process called transpiration)—much like sweat flows through pores to cool the body. The efficacy of transpiration is determined by the vapor pressure deficit (VPD) , which refers to the relationship between the relative humidity in the ambient air to the relative humidity in the leaf. Higher relative humidity typically increases leaf surface temperature by reducing this evaporative cooling. The spectrum of light the leaf is receiving also affects leaf surface temperature.
Blue and Red Light Most Useful for Photosynthesis
According to extensive research, chlorophyll, the primary driver of photosynthesis, most efficiently uses blue and red light. This does not mean that green or yellow light is unusable, it means that plants use that spectrum of colors less efficiently. Optimizing the light spectrum, by utilizing light that emits a high volume of blues and reds, will increase photosynthesis while also keeping the leaves at a cooler temperature.
Efficiency of the Light Spectrum Indirectly Measures LST
A less efficient spectrum will tend to heat the leaf more, while a more efficient spectrum keeps the leaf cooler—because more light energy is being converted to plant-usable energy instead of heat.
Light spectrums optimized for plants will therefore require a warmer ambient air temperature to keep the LST in the ideal range than spectrums not optimized for plants. Since heat management is generally a concern in indoor gardens using artificial light, these higher ambient temperatures can result in significant savings.
How To Measure Leaf Surface Temperature
There are many tools (probes, IR thermometers) that give an accurate reading of leaf surface temperature at a single point.
Issue? Temperature can vary dramatically across the leaf, depending on which areas of the leaf are fully exposed to light or covered in the shadows.
Solution: A forward-looking infrared camera (FLIR) provides a more accurate depiction of leaf surface temperature and light efficiency.
Note to Grower: Measure the top, middle, AND bottom of the plant. This will give you a complete survey of the conditions of your crops, which will tell you where you need to make changes.
How Grow Lights Can Be Used to Optimize LSF
The different types of grow light fixtures create different light spectrums. LED grow lights differ significantly from other forms of electric grow lights because the spectrum of the light can optimize certain wavelengths, or colors, over other colors—enhancing plant-usable light.
In comparison, other light sources produce much of their light as an unintended byproduct of their operation system, producing wasted energy and heating up plants. LED grow lights without optimized spectrums will also produce wasted energy while heating up plants.
In certain cases, an LED yield-optimizing spectrum can require ambient temperatures of up to 10 degrees warmer to keep the LST at the ideal range for a maximized yield.
HPS Produces More Infrared Light and Less Blues and Reds
High pressure sodium (HPS) grow lights convert a large portion of energy consumed to non-visible infrared (IR) light in the 810-830 nm range. This IR light can be perceived by humans (and plants), because of the warmth it creates, but it does not have enough energy to fuel photosynthesis.
Additionally, HPS lights produce primarily yellow light, which is the intermediate in terms of energy between blue and red light.
Optimizing Growth Under LED Grow Lights
When growing under LED lights with a red- and blue-dominant plant-optimized spectrum, the lack of excess infrared and other directly usable light causes the leaves to remain cooler, meaning that ambient air temperature needs to be significantly warmer than for the same plant grown under any light (natural or electric) which is not optimized for plant growth.
LED lights with a high concentration of “white” LEDs produce mainly green and yellow light. This range warms up the leaves more than reds and blues, but because the white light lacks the 800+ nm infrared output of most HID lights, the ambient air temperature may still need to be slightly warmer to get the ideal LST.
If you are switching from HID to LED, you will need to amp up the room temperature slightly with white LEDs, but much higher if you are using a plant-optimized spectrum grow light. If you don’t adjust the room temperature according to the spectrum change, your plant growth will be limited.
How do LED grow lights stack up to other grow light types?
Again, when using optimized-spectrum LED grow lights, ambient air temperature MUST be kept higher in comparison to other lighting types, to achieve the same metabolic rate.
It also must be noted that when testing LED lights against other types of lights, such as HPS, the leaf surface temperature should be kept consistent under each light—to enable identical metabolic rates—ambient air temperature aside. This is because with plants grown under MH and HPS lights, the leaf surface temperature differs enormously.
LEDs Promote Energy Savings
Running a warmer indoor growhouse, as required for spectrum-optimized LED grow lights, can lead to significant reduction in costs associated with cooling systems, such as ventilation or air conditioning.
Reduced Need for CO2 Supplementation
When using spectrum-optimized LED grow lights, the reduced need for cooling also reduces the need for CO2 supplementation. This is in comparison to a growroom needing a cooling system, such as ventilation, which results in the loss of CO2.
LED technology allows complete control over the light spectrum, which means cooler leaves for your plants at the same ambient temperature.
Using spectrally optimized LED grow lights makes achieving ideal leaf surface temperature easier and more cost-effective – resulting in higher yields and higher returns.