I’ve spent a lot of time talking with gardeners since publishing The LED Grow Book, and the majority ask the same question: are there any good, published, side-by-side studies comparing LED grow lights to HIDs?

The unfortunate answer is, no. One reason for the lack of side-by-side trials is that they are difficult to set up in a way that allows the trial parameters to be the same on both sides, without one side unduly influencing the other and thereby undermining the results of the trial. Differences in power draw, heat signature, light scatter and other variables between LEDs and HIDs make the execution of true side-by-side trials difficult.

Watts

One of the largest promises of LEDs (light-emitting diodes) is you can use less watts to produce the same harvest quantity as HIDs (high intensity discharges). With today’s LED grow lights, it’s becoming generally accepted that 500-600 W of LEDs can replace 1,000 W of HID lighting. This presents the first question when designing an LED vs. HID side-by-side trial: how many LED watts do you use—1,000 W of HIDs vs. 1,000 W of LEDs? Or 1,000 W of HIDs vs. 500-600 W of LEDs? The former tests for how much can be grown with the same wattage, and the latter tests whether LEDs can use fewer watts while maintaining harvest size. A well-designed LED vs. HID grow light trial should test both of these options.

Grow Spaces

The next big question when contemplating a side-by-side LED vs. HID grow light trial is how to physically set up the trial. Will both lights occupy the same area, or will they be in separate physical spaces? Everyone wants to see a side-by-side experiment, believing that this physical arrangement proves all the growth conditions are the same. But there are inherent problems with a side-by-side trial, starting with the amount of heat produced by HIDs. When LED and HID lights are hung side-by-side in the same space, the LED side can end up acting as a heat sink for the HID side because it runs cooler, which artificially influences the outcome for both sides.

Another problem is how to grow plants in the same space but keep the lighting separate. LED light is focused, while HID light travels in all directions from the lamp, which is the main reason they are paired with reflectors. If the light from the HID side is not controlled, it can add to the light received by the plants on the LED side. This can be prevented by placing a physical barrier around both sides of the grow space, but these barriers will also alter air flow, which could benefit one side over the other, depending on the room’s configuration.

Environmental Conditions

Environmental conditions also affect the outcome of a trial. Take temperature, for example. HID lamps can harm plants in hot conditions. Plants that are too hot have a difficult time transpiring enough to keep themselves cool, resulting in reduced harvests. Thus, indoor gardens illuminated with HID grow lights always include equipment to ventilate excess heat out of the grow space, and may also include air conditioning to counteract the excess heat generated by HID lamps.

Active venting and cooling processes can spell trouble for LEDs as well. Because LED grow lights do not create excess heat, the heat remediation necessary for HID grow lights can cause the LED side to be too cool, which can stunt the plants and suppress yields. Even worse, LED-grown plants can really suffer in cold, wet climates. Without extra heat to keep humidity in check, molds, which are tough to identify under the pinkish glow of many LED grow lights, can thrive.

Nutrients

The next factor in setting up a trial is deciding whether the plants are going to be fed the same nutritional program, or whether custom blends will be used for each side. Most people I talk to want to see a single tank for both sides. This too has its problems. Depending on the plants and their environmental conditions, the plants under one of the lights might want a different nutrient strength.

This could be driven by higher leaf temperature, particularly on the HID side, which increases the transpiration rate of the plants and therefore their uptake of nutrients from the feed solution. Differences between how the plants absorb light from the two different light sources can also drive differences in how they uptake and absorb nutrients.

So, which side do you mix the nutrient tank to feed? If you feed to the side that needs a heavier nutrient solution, you’ll reduce the harvest on the other side by overfeeding. Conversely, if your mix caters to the light feeders, the hungry ones won’t perform as well. One option for resolving differences in nutritional requirements between the two sides is to mix the nutrient solution for the weak feeders, then supplement nutrition for the stronger eaters through foliar feeding, but this method invalidates the reason for having a single reservoir in the first place.

Location

The location of a trial might also influence its outcome. For example, in Alaska, I believe most indoor gardeners would choose to use HID lighting because the general climate is cold (in regards to plants—no offence to readers)! Gardeners in Alaska can use the extra heat produced by HID lamps to maintain optimal garden temperatures and even pump some of that heat into their living space.

Growers using LED grow lights in Alaska will most likely require a heater in their garden to maintain appropriate temperatures. This could negate the energy savings of LED grow lights and make them more expensive to operate than HIDs.

Meanwhile, gardening in Florida with LED grow lights makes great sense. Florida is hot, and growrooms in this region do not need any extra heat dumped into the garden from HIDs. Additionally, electricity is expensive in Florida, and less heat means less need for a huge air-conditioning unit, which helps save on energy bills.

Seasonality

Seasonality may also affect which type of grow lights perform better during a side-by-side trial. Some climates shift dramatically from summer to winter in both temperature and humidity levels. It’s ideal to conduct multiple trials in both summer and winter. Although it’s an expensive solution, research could determine LEDs are better in the summer, and HIDs are better in the winter. Or not.

Plant Selection

As the gardening community gains more experience with alternative lighting options, people discover plants have their own lighting preferences. Some growers tell me certain plants prefer LEDs, while other plants grow larger under HIDs. The specific varieties of plants selected for grow light trials can influence the outcome. A proper trial would include multiple plants and different genotypes within a plant family before a winner is declared.

Personnel Selection

Who will run the side-by-side trial? Few members of the indoor gardening community have enough experience (yet!) gardening under both LEDs and HIDs to properly construct and run a trial. Finding someone with enough LED experience and no bias is key to a successful comparison trial, but that person could be hard to come by.

Documentation

One of the most important and often overlooked parts of conducting any type of trial is documenting everything you do. This critical step allows others to fully understand how and why the trial was conducted in the first place. The experimental procedures you develop should be complete enough so someone else can reproduce the experiment. Make sure to include the small stuff, like feeding formulas, room temperatures and pest management. This is often where the best information hides. All assumptions should be clearly spelled out.

Goals

One of the most important things to do is document the goals of the experiment. When comparing LEDs to HIDs, what are you trying to discover? Which one grows more harvestable weight? Which one grows higher quality plants? Which one is more efficient in regards to total watts used? There are many factors that can be tested. Make sure your goals are clearly defined and are addressed by the methods used during your trial so your work will have merit.

Repetition

Whatever you do, don’t fall into the trap of thinking that one harvest from one side-by-side trial permits you to declare a winner. The experiment needs to be repeated over multiple growth cycles before you can consider it a complete trial. Be sure to test the influence of the grow spaces themselves. Switch the sides each light occupies between runs and note any variances in results. There could be unknown external influences, and changing sides can help rule them out.

My final thought on side-by-side LED vs. HID grow light trials is this: why bother? With all the variables that need to be controlled and tweaked in one side’s favor, what’s the point? Gardening, both indoors and out, is highly personalized.

What works for one gardener might not work for another, even if they are growing in the same garden. As for me, I’ve decided to stick with what 1,000 W can grow, and keep it easy. Let’s get over wanting the side-by-side trials, and focus more on seeing what we can do with the least amount of watts possible.