The Relationship Between Leaf Surface Temperature and Lighting Spectrum

Growing your plants in ideal temperatures has long been recognized as critical to maximizing yields; keeping plants too warm stresses them, and too cold slows down growth—either will reduce yields. To that end, most growers try to maintain an ideal air temperature in their growing areas to keep the plants growing vigorously.

However, the temperature of the plants themselves, the leaf surface temperature (LST), is not the same as the ambient air temperature in the growing area. Measuring the ambient air temperature in your garden is like measuring the air in our living spaces, it is important for us to have a proper temperature to live in.

Measuring LST is like taking a person’s temperature with a thermometer under their tongue. The actual LST is determined by several factors: plant type, air temperature, humidity, and light spectrum all influence leaf surface temperature.

Only with recent technological advancements has it been possible to provide plants with completely optimized light spectrums, so many growers are unaware of how dramatically light spectrum can change the ideal ambient air temperature in an indoor growing area.

In some cases, a plant-optimized spectrum can require air temperatures 10°F warmer to keep the plants’ leaves in the ideal temperature range to maximize yield.

Why Does Leaf Surface Temperature Matter?

Most biochemical reactions only operate within a certain temperature range, and have an even narrower range in which the reaction proceeds most efficiently. If temperatures are too low or too high, the reactions proceed more slowly or not at all.

The metabolism of most plants occurs within the leaf; for any given plant, there is an optimal leaf surface temperature range that maximizes growth as well as production of other desirable secondary metabolites such as resins, pigments, flavor-enhancing compounds, and vitamins.

Note that the leaf surface temperature is affected by, but not equivalent to, the ambient air temperature in the growing environment. Leaves can be cooled through evaporation occurring in open pores in the leaf (stomata) that allow gas exchange, and are warmed by absorbed but unused light, whether from artificial or natural sources. Leaf surface temperature is almost always different than ambient air temperature.

What is the Ideal Leaf Surface Temperature?

So, what is the ideal leaf surface temperature for plants? This is unfortunately a question without a simple answer as many factors influence the ideal.

Multiple types of metabolic reactions exist within every plant, and each has a different optimal temperature range. Primary metabolism (photosynthesis) is obviously the most important; without it the plant will not survive. Optimal temperatures for desirable secondary metabolites must be considered as well, especially if the plant is grown specifically for the secondary metabolites.

The optimal leaf temperature range for photosynthesis depends on the type of plant and concentrations of CO₂. Arctic- and alpine-adapted plants typically require cooler temperatures, while desert-adapted and plants using C4 photosynthesis prefer it warmer. (There are two slightly different chemical reactions for photosynthesis, called C3 and C4; the variant a plant uses is determined genetically.)

Most growers also know they can turn up the temperature when running CO₂, since CO2 supplementation will generally raise the optimal photosynthesis temperature. Thus, the ideal LST for photosynthesis is dependent on environmental conditions as well as the type of plant.

Secondary metabolic reactions can have a huge range of optimal temperatures; many plants have even evolved responses specifically triggered by exposure to cold or hot temperatures to better adapt to their surroundings. For example, some plants produce proteins with anti-freeze properties when exposed to cold.

In short, the ideal leaf surface temperature depends on the species/variety of the plant, overall environmental conditions, as well as what the plant is being grown for. Only experimentation can determine an ideal range for LST for a specific plant variety in a specific set of conditions.

What Affects Leaf Surface Temperature?

Ambient air temperature, relative humidity, leaf physiology and pigmentation, genetic/metabolic differences, and light spectrum all affect LST. Air temperature sets a baseline for leaf temperature, providing warmth to leaves cooler than the air, and cooling leaves warmer than it.

Many leaves can cool themselves through evaporation of water through stomata; higher relative humidity typically increases leaf surface temperature by reducing this evaporative cooling. The spectrum (colors) of light the leaf is receiving also affect leaf surface temperature.

Many studies have shown that chlorophyll, the primary driver of photosynthesis, most efficiently uses light in the blue and red areas of the light spectrum. This does not mean that plants cannot use green or yellow light for photosynthesis, just that it is less efficiently used.

We can slightly modify the popular game of Bags and use it as an analogy to understand how photons can fuel photosynthesis. Imagine a Bags board representing the leaf, but instead of one hole near the center there is one near the top and one near the bottom.

The top hole represents the higher energy blue photons and the bottom hole the lower energy red photons that chlorophyll can utilize. Each bag thrown at the board represents a photon of light. If you throw a bag through the blue hole it efficiently fuels photosynthesis. Same for a red photon—it will go into the red hole and fuel photosynthesis.

If a photon hits the board that is a different color such as yellow or green, it will slide down the board, creating heat from friction and changing color as it slides down and loses energy. It may slide down and fall through the red hole which will fuel photosynthesis, or the photon could slide off the bottom of the board, not fueling photosynthesis and only creating heat.

This analogy demonstrates that optimizing the spectrum of light (targeting bags at holes, not just the board) can increase photosynthesis while also keeping the leaves cooler (less sliding down the board).

Therefore, measuring leaf surface temperature indirectly measures the efficiency of the light spectrum mix for growing plants—a less-efficient spectrum will tend to heat the leaf more, while a more-efficient spectrum mix will heat the leaf less, as more of the original light energy is being converted directly to chemical 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 mitigation is generally a concern in indoor gardens with artificial light, these higher ambient temperatures can save significant money.

Artificial Grow Lights

Various artificial grow light technologies create different light spectrums. LED grow lights differ significantly from other forms of artificial plant lights in that the spectrum can be tailored to any specification, eliminating unwanted excesses of light wavelengths (colors) while providing light plants can use most efficiently.

Other artificial lighting technologies produce much of their light as an unintended and unavoidable byproduct of how they operate, ultimately wasting energy in heating up plant leaves. Of course, an LED light with a spectrum not optimized for plants will also waste energy heating up plant leaves.

High pressure sodium (HPS), in particular, converts a significant portion of the energy consumed by the bulb directly to non-visible infrared light in the 810-830-nanometer (nm) range, peaking at about 819nm.

This infrared light is perceptible to you (and plants) by the warmth it creates, although it does not have enough energy for photosynthesis.

Additionally, much of the visible light HPS bulbs produce is yellow, intermediate in energy between blue and red light most efficiently utilized by plants, warming up the leaves.

Observing How Spectrum Affects Leaf Surface Temperature

There are several tools available for measuring actual leaf surface temperature, from probes placed on the leaf to infrared thermometers.

Many of these give an accurate reading of leaf surface temperature at a single point, but looking at only a single point on a leaf provides an incomplete idea of the temperature, since it can vary significantly over the surface of a single leaf.

A forward-looking infrared camera (FLIR) provides a complete picture of leaves’ temperature and a much better understanding of how light spectrum affects leaf surface temperature.

The FLIR images here were taken with the same plants on the same day under otherwise identical conditions, with only the ambient air temperature and light spectrum changed:

Garcinia xanthochymus (Yellow Mangosteen)

The images below show the same leaf under two different light spectrums (both provided by 240W LED lights), under two different ambient air temperatures. In the warmer ambient environment of 84°F, the leaf surface temperature difference between the 89.2 and 92.3 shown in the image is over 3°F.

Note the leaf is cooler with the plant-optimized spectrum at both ambient air temperatures. This shows that it isn’t LED technology but purely spectrum that is responsible for the different leaf temperatures. leaf surface temperature still increases with a less-efficient spectrum, even when the light is from LEDs.

240W plant-optimized LED, red- and blue-dominant spectrum

75°F ambient air temperature
84°F ambient air temperature

240W all-white LED

75°F ambient air temperature
84°F ambient air temperature

^{Purple: plant-optimized LED spectrum | Black: all-white LED spectrum | Green: relative photosynthetic efficiency by wavelength}

Garcinia xanthochymus (Yellow Mangosteen)

The images below shows the same plant under two different light spectrums and two different ambient air temperatures. In the 75°F ambient garden the hottest part of the leaf is 86.5°F, but in the same environment under the 1,000W HPS, the leaf hits 102°F. Note the leaf is again cooler with the plant-optimized spectrum at both ambient air temperatures.

750W plant-optimized LED, red- and blue-dominant spectrum

75°F ambient air temperature
84°F ambient air temperature

1,000W High Pressure Sodium

75°F ambient air temperature
84°F ambient air temperature

^{Purple: plant-optimized LED spectrum | Orange: HPS spectrum | Green: relative photosynthetic efficiency by wavelength}

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 artificial) which is not optimized for plant growth.

LED lights utilizing primarily “white” LEDs, which are actually optimized for human eyes, are dominated by mostly yellow and green light output. This lack of plant efficiency will warm up leaves more than a red- and blue-dominant spectrum, but because they still lack the 800+ nm infrared output of most HID lights, the ambient air temperature may still need to be a little warmer to get the ideal leaf surface temperature.

If you are switching from traditional HID lighting to LED, you can turn the temperature up a bit with white LEDs, but you need to turn it up more if you are using a plant-tuned spectrum. Not adjusting ambient air temperature to account for the spectrum change can limit your plants’ growth, and ultimately your yields.

Comparing LED Grow Lights to Other Grow Lighting Technologies

When growing with plant-optimized LED lights, it is important to realize that ambient air temperatures need to be kept higher compared to other lighting to achieve the same metabolic rate.

Side-by-side tests of LED lights versus other lighting such as HPS, where ambient air temperature is kept the same, are not particularly informative or accurate as to the lights’ relative performance—the tests should be run so that leaf surface temperature is being kept the same under each light to enable identical metabolic rates.

As the data from FLIR camera observations shows, this becomes more critical especially when comparing plant-optimized LED grow lights to MH and HPS, as the leaf surface temperature difference is much higher with these lights.

Relative Energy Savings

Heat mitigation is required in most indoor gardens using artificial lights, whether it is achieved with ventilation or air conditioning. Plant-optimized LED grow lights can offer substantial energy and cost savings in cooling. LED lights already contribute less heat to the growing environment than HPS, metal halide, and fluorescent lights due to higher efficiency of light generation.

Running a warmer indoor garden, as required for LED with a plant-tuned spectrum, can lead to a profound reduction of costs associated with cooling—not just running the cooling equipment, but also in sizing it as well.

Even growing areas currently relying on excess heat from conventional grow lights can easily reduce costs with plant-optimized spectrum grow lights.

In most indoor grow areas, something as simple as plastic sheeting can provide sufficient additional insulation to avoid the need for other forms of heat.

In greenhouses, natural gas or propane heaters are usually much more cost-effective than relying on electric lights for heat.

If grow room cooling is achieved partially or entirely through ventilation, and CO₂ supplementation is being used, the reduced need for cooling when using LED plant lights will also result in less loss of CO₂ further increasing savings.

Every grower knows how important it is to keep your plants happy at the right temperature for maximum yield. LED technology allows better control of the light spectrum than ever before, and this translates into cooler leaves for your plants at the same ambient temperature.

Unless you raise the air temperature with plant-optimized LED grow lights, you are keeping your plants too cool, harming your yields, and costing you more in unnecessary cooling.

With non-plant-optimized “white” LEDs designed for human eyes, you may still need to raise the temperature a few degrees, but not as much as with red- and blue-dominant, plant-optimized LED grow lights.