Since the introduction of the first UFO lights, there have been diehard proponents and detractors of growing with LEDs. While we can attribute much of the performance increase in recent years to technological improvements, growers have come a long way in learning how to adapt their plants and environment to growing with modern LED fixtures. With a few of the following factors in mind, yield and quality produced under LEDs can meet and even significantly exceed those made under traditional HID lights.
The first thing that is essential to consider when evaluating any grow light is the dispersion pattern of the particular fixture in question. Just as different HID reflectors are designed with various footprints, today’s LEDs come in a wide array of beam angle options for multiple scenarios. The first LED grow lights directed all the photons downward, often using lenses that focused the light in narrow patterns of 60 or 90 degrees to minimize light wasted on walls and floors. While suitable for hanging in tall greenhouses, these early products failed to provide a uniform light footprint for indoor scenarios. The results were uneven gardens and unhappy growers.
Modern LED producers have ditched secondary optics in favor of diodes with a native beam angle of 120 or 180 degrees. These low-power, high-efficiency diodes are laid out in large arrays on bars or boards to provide extremely high light uniformity at the plant canopy. When two or more fixtures are used together, crossover lighting from the LEDs’ widespread further improves uniformity and canopy penetration.
Before upgrading from HID to LED, it’s important to measure your current light levels to provide a baseline. Throw out your lumen meter and buy a PAR meter from a reliable company. Lumen meters only measure the brightness of a light source based on the sensitivity of the human eye. PAR meters give a more accurate representation of light intensity in relation to photosynthesis. Take readings from every part of your garden and make some notes. Once you know the intensities you’re accustomed to growing under, you’ll have a better idea of what you’ll need to expect from a new LED fixture. For larger spaces, many LED manufacturers will create a custom light plot tailored to your specific needs.
For decades, indoor growers have known that the success of their gardens depends heavily upon environmental conditions, most notably temperature. Users of high-pressure sodium lights knew that they grew the healthiest plants whenever their room temperature was between 70°F and 75°F. Surprisingly, rooms outfitted with LEDs need to be in the 80°F to 85°F range to achieve similar performance. This shift in ideal ambient temperature comes down to the differences in how HID and LED light sources produce heat.
HID light sources, whether we’re talking about high-pressure sodium, ceramic metal halide, or anything in between, all work on the same principle. An electric arc heats and evaporates a mixture of metals that emit photons of a spectrum determined by the specific materials involved. Only a small fraction of the electricity used is converted to visible light, while the rest is released in the form of infrared energy. This invisible range of the electromagnetic spectrum heats surfaces directly, which is why sunlight feels warm on your skin, even on a chilly day. Plants grown under a 1000 watt HID light receive massive amounts of infrared energy, and their leaf surface temperature might be 10 to 20 degrees warmer than the surrounding air.
Most LED products designed for growing plants produce little to no infrared energy, which is why these lights don’t heat surfaces the same as HID. Almost all the waste heat from an LED fixture is released into the air, either passively or via built-in fans. Circulating fans and air conditioners easily remove this type of heat, so plants directly under the light might only be 2 or 3 degrees warmer than the rest of the room.
What’s important here is leaf surface temperature. This is where photosynthetic reactions take place, and they happen most efficiently in a specific temperature range dependent on plant variety and other environmental factors like humidity and CO₂ concentration. In addition to using a thermometer to measure room temperature, try using a non-contact thermometer to measure and control the temperature at the leaf surface. 84°F is a good starting point.
Another effect of the far-red heavy-spectrum of HPS lighting involves the plants’ shade-avoidance response. In nature, plants can sense when they are being shaded under taller plants by measuring the ratio of far-red vs visible light they receive. Since far-red light (700-800nm) penetrates through plant tissues better than visible light, a high ratio of far-red to visible red indicates shading.
Studies have shown that plants exposed to high ratios of far-red tend to stretch out, growing longer leaves and stems to outcompete taller plants. Conversely, plants exposed to higher proportions of blue light grow shorter and stockier. LEDs, with minimal far-red and abundant blue wavelengths, tend to produce shorter plants compared to the same varieties grown under HID lighting. This factor should always be taken into account when comparing or planning HID and LED gardens.
Heating plant canopies with infrared-heavy lights, then cooling them off with fans and air conditioners set up a scenario for very high rates of transpiration, and in turn high water usage. We’ve all seen how quickly plants grown under high-pressure sodium will suffer when an irrigation system malfunctions. Growers switching to LED will notice that their plants don’t need to be watered as often, but they’ll still need just as much fertilizer to avoid nutrient deficiencies. If you plan to feed with hydroponic nutrients, you may need to dial up the strength (EC/PPM) of your solution to account for less-frequent waterings.
There’s one final environmental concern that should be addressed. Well-designed LED grow lights are more electrically efficient than the best HID lights on the market, so you won’t need as much air conditioning to fight all that excess heat. Also, the absence of infrared energy we discussed earlier means that grow rooms can be run warmer, and as a result, air conditioners will kick on less often. Many HID grow rooms rely on air conditioners to handle the bulk of dehumidification. With air conditioners doing less work, it might mean you’ll need to install additional dehumidifiers to keep humidity under control in an LED grow room.
Although there are many challenges to transitioning a complex, established grow operation from HID to LED lighting, the long term benefits are beginning to outweigh the costs, and ongoing technological advancements mean that LED grow lights are here to stay. Through careful observation and practice, highly skilled growers are using LEDs to push the limits of what was once thought possible in their gardens. It all comes down to understanding the attributes of the light source and the needs of the plant.