Don’t Try to Fit LEDs into an HPS Box

One of the obstacles faced by LED growers is trying to equate their LED grow lighting to systems and theories that were developed under traditional light sources such as the sun, high pressure sodium, and fluorescent lighting. This misunderstanding has been amplified over the years by LED plant lighting companies marketing arrays using product descriptions such as “replaces 400 watts of HPS.” This kind of statement is incredibly misleading and implies one can simply switch out an HPS with an LED array and expect the same results regardless of the application.

The rhetoric has become so popular that I now find myself under attack when I can’t answer the question of how many watts of HPS an LED array can replace. The fact is there is no scientific basis for a direct comparison to HPS. This isn’t to say you can’t replace HPS with LEDs – just that they are two completely different light sources with their own measurement variables and characteristics. Simply switching out the lighting systems will not give the same results. On that note, commercial growers, scientists, and educators basing their projects or utilizing methods developed through research under HPS or fluorescent lighting, should expect different results under LEDs.

The difference with the most profound effect would be temperature. The majority of research available on the effects of manipulating photoperiod or light intensity is based on a radiant (hot) light source such as HPS or the sun. Up to 80% of the light emitted from an HPS lamp is emitted as heat. With this in mind, HPS studies are not only manipulating the light, but manipulating the temperature within the plant canopy. If we were to plug-in an LED array in place of the HPS and run the same experiment, we now have more quality wavelengths being delivered to the plants, but none of the heat. It stands to reason that the outcome will not be the same.

To illustrate this point, let’s look at numbers from a strawberry greenhouse in New Zealand running HPS on one side and LEDs on the other side. The greenhouse uses climate controls to regulate the temperature, but even with these controls in place there is a stark contrast between temperatures beneath the HPS lighting and those under the LEDs. Mr. Kolovos states, “When the lights come on (6:00am) the area under the HPS lights is almost immediately brought up to max temp whereas that doesn’t happen in the rest of the greenhouse until about 10:00am. Similarly, that temperature is maintained under the HPS lights until 9:00pm when they are switched off whereas the temp under the LEDs drops off at about 4:00pm.”

Clearly, there is a significant difference between the two light sources. The LEDs do exactly what they are meant to do which is supply quality light, using very little energy, with a lifetime of 10-20 times longer than traditional light sources. In the case of the greenhouse above, heat will need to be supplemented in the morning and evening on the LED side while they are forced to pay for the heat emitted by the HPS for the entire photoperiod; essentially blowing money and resources out the ventilation system. It is far more efficient, cost effective, and environmentally friendly to add heat only when it is needed. This is a distinct advantage for LED growers.

In conclusion, while an LED array won’t “replace” an HPS much like the apple does not replace the orange, LEDs do provide a good alternative. By accepting the inherent differences between HPS and LEDs we can overcome the confusion created trying to fit LEDs into the HPS box, and start accepting LEDs on their own merits.

Angela Lundmark is an indoor gardener and freelance writer. 

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