Grow lights replicate the sun’s energy indoors by converting electrical energy into light for photosynthesis. The answer to whether they keep plants warm is a nuanced “yes,” as heat is an unavoidable byproduct of this conversion. All light sources operate under the laws of thermodynamics, meaning a portion of the energy input is always lost as heat. The degree to which a grow light warms the environment depends entirely on its specific technology and efficiency.
Energy Conversion: Why Lights Generate Heat
Grow lights generate warmth because no electrical system is perfectly efficient at converting power into usable light. Electrical energy is split into two primary outputs: Photosynthetically Active Radiation (PAR), the light plants use, and waste energy, released as heat. The more efficient a light is, the higher the percentage of energy it converts into PAR rather than heat.
Waste heat manifests in two distinct forms. Radiant heat (infrared light) travels directly to the plant canopy and heats the leaf surface upon contact. Convective heat is transferred from the hot components of the fixture to the surrounding air, raising the ambient room temperature. This distinction is important because radiant heat can cause immediate leaf burn, while convective heat affects the overall temperature of the grow space.
Heat Output Differences Among Grow Light Types
The amount and type of heat produced varies substantially across different grow light technologies.
High-Intensity Discharge (HID) Lights
HID lights, including Metal Halide (MH) and High-Pressure Sodium (HPS) fixtures, are notably inefficient at energy conversion, converting roughly 75% of their electrical input into waste heat. A large fraction of this heat is radiant, concentrated in the infrared spectrum, and directed straight down onto the plant canopy. This intense, focused heat significantly raises the leaf surface temperature, making it a powerful warming source. However, this necessitates robust ventilation and placing the fixture far from the plants to prevent heat stress and canopy burn.
Fluorescent Lights
Fluorescent lights, such as T5 tubes and compact fluorescent lamps (CFLs), represent a moderate middle ground in heat output. They operate at lower temperatures than HID lamps and generate more heat through convection rather than direct radiation. While these lights contribute to warming the ambient air, they are generally not powerful enough to significantly heat a large grow space. Their moderate heat profile allows them to be positioned closer to the plant canopy with minimal risk of burning.
LED Lights
Light Emitting Diode (LED) grow lights are the most energy-efficient option and produce the least amount of waste heat relative to their light output. The small amount of heat they generate is mostly convective, transferred away from the light-emitting surface and dissipated upwards through integrated heat sinks. This design results in very low radiant heat directed at the plants themselves. Consequently, the LED light provides minimal direct warmth to the plant canopy, often requiring a separate heating source in cooler environments.
Managing Light-Generated Heat for Optimal Plant Growth
Since different grow lights offer vastly different heat profiles, managing the temperature requires tailored strategies.
Mitigation for HID Systems
For high-heat sources like HID systems, mitigation is the primary focus to prevent damage to the crop. Growers must ensure proper fixture height, utilizing air-cooled reflectors and powerful exhaust systems. These systems actively remove hot air and radiant heat before it reaches the leaves.
Supplemental Warming for LED Systems
Managing heat from high-efficiency LED lights often involves the opposite approach, focusing on supplemental warming. Because LEDs deliver low radiant heat, the leaf surface temperature can be cooler than the surrounding air, which slows growth processes. In cold environments, this necessitates using supplemental heating, such as space heaters, to maintain the optimal temperature range for the plants.
Temperature Monitoring
Accurate temperature monitoring is necessary regardless of the light source to ensure optimal plant health. Relying solely on a thermometer measuring the ambient air temperature can be misleading. This is especially true with HID lights that deliver high radiant heat or LEDs that deliver very low radiant heat. Using an infrared thermometer to directly measure the temperature of the leaf surface provides the most accurate data for making temperature adjustments and preventing heat-related stress.