Success With LED Grow Lights

By Angela Lundmark
Published: May 29, 2018 | Last updated: May 4, 2021 09:31:57
Key Takeaways

Thinking of using LED (light-emitting diode) technology in your grow room? Here’s what you need to know to make the most of this light source.

Source: Nikkytok/

Over the years, I’ve seen gardeners of every skill level attempt to plug new LED technology into legacy methods and apparatuses, not realizing how using LEDs will change their system. The most notable of these environmental changes has to do with radiant heat.


Indoor gardeners have become accustomed to battling excess heat emitted from traditional light sources that run from several hundred degrees to more than 1400°F at the bulb’s surface. This excess heat limited the amount of light you could add to your environment—placing glass bulbs too close or packing too many into an area could literally burn your plants.

LED technology has made it possible to saturate plants with light wavelengths that target photosynthesis without worrying about overheating. A 5-mm LED grow light adds (on average) 0.1 BTUs per square foot over crops. To put this number in perspective, standing close to plants to contribute your own body heat would provide 600 times more BTUs than a 5-mm LED plant light. We won’t see LED grow light manufacturers adding heat to their arrays any time soon because heat is the enemy of LED lifetime, which is one of the primary advantages of the technology.


What we are left with is one light source creating too much heat, and one light source that doesn’t provide any. When LEDs are used as supplemental lighting or to extend photoperiod in a greenhouse or other area receiving natural light, radiant heat is provided by the sun. When gardening indoors, without sunlight, many applications are best served by a combination of LEDs and HID lighting. To be clear—plants can grow under LEDs only, but they will grow much slower without the radiant heat provided by the sun or HID lighting.

Using LEDs and HID lighting together, we can maintain the growth rates indoor gardeners have come to expect and still dramatically decrease the operating costs while increasing the efficiency of an indoor garden. Run LEDs for the entire photoperiod to provide quality light, and use a reduced amount of HID lighting turned on periodically to provide only the heat you need and no more. To make the most of LED technology, one should consider how all aspects of the environment are affected. As someone who has been using LED technology since its inception, here are some tips:

  • Design your controlled plant growth environment around your LED light source. LEDs will likely be your biggest investment and most permanent fixture. High brightness 5-mm LEDs are rated for 100,000 hours of life (20 years if you use them 12 hours a day). If you already have an environment designed around high-pressure sodium or metal-halide lighting, for every lamp you switch over to LEDs, there will be a corresponding decrease in environmental temperature, water use and ventilation requirements that should be considered.
  • Enclose your grow space to reduce effort spent on climate control. If you have a 4 ft. by 4 ft. garden in a 10 ft. by 10 ft. room, it’s going to be easier and will require less resources to control the 4 ft. by 4 ft. area than to control the whole room. Using a reflective material (white) gives your plants more than one opportunity to absorb the light.
  • Keep your plant pots as far from the floor as possible to keep root zones at optimal temperature. Heat rises and the floor will be the coldest area in the room.
  • Spend at least 30 minutes per day in your garden. It doesn’t matter if everything is on a timer, in self-watering buckets with state-of-the-art climate controls. A garden needs a gardener. Use the time to inspect the plants for signs of disease or pests, rotate or turn them, and all the while your breath will be increasing CO2 concentrations.
  • Monitor and control temperature/humidity levels to stimulate healthy plant growth. Transpiration happens on the leaves for transfer of water molecules back and forth from humid air to maintain healthy water levels in plants. Ideal humidity and temperature levels vary by plant species, but a general rule of thumb is to make the daytime humidity around 60 to 65%, and daytime temperature between 73 and 85°F, allowing the temperature to drop 10 to 15°F at night. When the lights turn off, exchange the air inside the garden for cooler air.
  • Look out for overwatering. Without the heat, we have little evaporation taking place under LEDs. LED gardeners should develop a new watering schedule. Water thoroughly and then wait to see how long it takes for the plants to completely dry out. Use this information to develop the schedule. Grow mediums designed for water retention should be mixed with other mediums that encourage porosity (hold air). Remember, whatever heat you add to your environment is to make the plants drink water.
  • Add some heat. Photosynthesis increases (up to a point) with increased temperature. For vegetative stages, raising ambient temperature is fine. For fruiting stages, use a radiant light source to spray heat directly on the leaves. The goal is to add exactly enough heat and humidity to keep your plants drinking (from the roots and from the air).


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Written by Angela Lundmark

Profile Picture of Angela Lundmark
Angela Lundmark is the editor of the LED Gardener, a monthly columnist for Garden & Greenhouse and CEO of LED Grow Master Global, LLC.

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