Avoid this common mistake when comparing LEDs to HPS

So, you’ve decided to take the plunge and upgrade your grow room to LEDs. Simply unplug the HPS lights and install the LED lights. Better yields and lower electric bills – you are going to be a hero. Well, you might be a hero if you take into account how the switch to LEDs impacts your leaf surface temperature and the corresponding vapor pressure deficit (VPD).

It is well-established in plant biology that leaf surface temperature must be kept within a specific window to optimize primary metabolism (photosynthesis), as well as production of secondary metabolites. The relationship between leaf surface temperature and photosynthesis is shown in the figure below. The figure consists of data from a variety of plant species.

But leaf temperature is only part of the story. The critical factor is the interplay between leaf temperature and the relative humidity in the grow room. Those two factors (temperature and humidity) determine the vapor pressure deficit (VPD), which, in turn, determines transpiration efficacy and ultimately photosynthetic rates. An example of the relationship between temperature, humidity and VPD is illustrated in the chart below. To optimize production yield, the VPD must remain in the “sweet spot” identified in the green boxes.

High pressure sodium (HPS) lighting, has long been the workhorse in many indoor grow facilities. HPS emits in a broad portion of the electromagnetic spectrum that includes infrared (IR) energy – otherwise known as heat. IR energy from HPS heats the canopy and increases the leaf surface temperature. LED grow lights typically have only a small fraction of their emission in the IR portion of the spectrum, so they do not increase leaf surface temperature like HPS. In fact, it is typical to see a 5°-10° decrease in leaf surface temperature by changing the lighting from HPS to LED. If no other action is taken, the decrease in leaf temperature may throw the VPD out of its sweet spot – thereby decreasing transpiration and photosynthesis. This will most certainly not make you a hero in the grow room.  

So how do you ensure you are still in the proper VPD range after installing LED lights? Follow the steps below:

1. Understand your baseline. Measure the leaf surface temperature and relative humidity while you’re still using HPS. Although humidity is easily measured, measuring leaf surface temperature requires specialized equipment such as a forward-looking infrared camera. Here’s one IR camera that will do the job: tequipment.net/fliri7.html. Don’t assume the leaf surface temperature is the same as the ambient air; this is rarely the case. Once you’ve taken the measurements, the VPD can be determined.
2. Repeat step #1 after switching to LED.
3. Determine if your VPD is still in the optimal range. If it isn’t, you should:
   1. Increase the ambient air temperature to raise the leaf temperature to the target temperature that satisfies the VPD requirement.
   2. Modify the relative humidity in the room to bring the VPD into the ideal range. 

One reason LED grow lights are so efficient is that they don’t produce excess heat in the light beam like older technologies (including HPS). However, to fully achieve all the benefits of LED technology, growers must understand how the lower heat content will affect their plants and take the proper steps to achieve optimal production.