Greenhouse Canada

VPD: A game changer

The advantages of using vapour pressure deficit measurements

September 20, 2022  By Chris Knezetic

VRP, or vapour pressure deficit, measures how much room there is for humidity based on the current temperature.

In my time as a grower, I’ve had to learn how to make a plant grow in time for either planting or shipping, starting with understanding what the plant wants and needs and how to best manipulate that in a greenhouse setting. Trial and error seemed to be the go-to so long as I maintained my foundation of what I thought I knew to best proceed. 

In my less experienced days, I thought to myself, “Plants like lots of heat and lots of sun, so more the better, right? Plants don’t burn, they’re plants! Stress? What stress?” Silly things like that. So, once I realized that plants don’t exactly care for being stressed, I had to find ways to help them. One of the tools I found that has been a saving grace in more situations than one, was the use of our HPF (high-pressure fog) machines and controlling the output via Argus Controls VPD.

What is VPD? Vapour pressure deficit! I know that makes a whole lot of sense to everyone reading this, but to make things clearer, VPD measures how much room there is for humidity based on the current temperature. There is a lot of information on the science of how it is measured, but I will not be doing that here. Instead, I will share with you from where I started, to how growing is continuing in the greenhouses using these tools.


It all started when the summer hit and we couldn’t keep temperatures down in the greenhouse. Whitewashing the roof helped, but it wasn’t enough. We would hit 37C and up without any form of assisted cooling, other than using the irrigation booms to cool off the bays. Before I took control of the greenhouse, we tried using VPD solely for propagation, and we failed miserably on our first set of crops, losing near 50 per cent. After that, we swore off the HPF and decided to revert to traditional methods using plastic and cloth. While we figured that portion out, we never figured out how to properly cool the greenhouse without soaking the plants. So, one day I decided to just turn on the HPF in all the zones and observe what happened. Near immediately, the air inside the greenhouse became easy to breathe. Within a day, the plants appeared much less stressed, and within four days there was substantial growth across the crops. I had a hard time keeping their growth habits under control at this point, but I figured I’d have an easier time manipulating a plant that wants to grow versus plants that struggled to take off after rooting.

This was all a major turning point in greenhouse management and how using VPD measurements can help us understand removing plant pressures and keeping those stomata open, even in the dead heat. Now my eyes gazed upon returning to VPD and HPF for propagating unrooted cuttings (URCs). It took a few cycles, but I had to take some risks and see what would happen if I propagated in the open. I cannot deny there have been a few moments where my chest about caved in when I noticed some damage across the crops, though it was from something different. After diligently watching each crop type during the growing season and off-season, I was able to finely tune things to where I wanted them. Now it is a sought-after standard for propagation and finishing.

Using VPD measurements can help us understand removing plant pressures and keeping those stomata open, even in the dead heat.

I am not suggesting that every situation I have is ideal for this, but we are looking at the next greenhouse structure and redesigning it a bit to accommodate the fog unit system further. The biggest and best reasons I can give for preferring this system are my own, but they may appeal to those who are interested in this method. Starting with overall crop care, observations are made much easier when crops aren’t covered in cloth and plastic. It is much harder to miss something when you can see the entire crop, from drying sections to insect and disease pressures, and overall crop uniformity. It is also much easier to act for spray applications or weeding tasks. This is especially important since removing plastic on URCs can easily pull cuttings out of the cells, and it can beg to question if removing the plastic was better or worse for the crop. Furthermore, transitioning your VPD setting helps find a balance for the cuttings that need more time to root versus the ones that have roots which benefits both. In many cases, it may not be feasible to start this method right from propagation, but it is helpful having greater control over the environment in later growth stages.

The crops are specific to their needs in terms of VPD settings, as is your structure and setup. Designing your greenhouse around this method is important to mitigate unforeseen losses, which brings me to the drawbacks of using this system. Some less than ideal situations were made upon discovery, such as doors being left open by employees, vehicle movement causing doors to shift, and unsealed sections in the corners of the greenhouse chambers. These sudden shifts in the environment can rapidly change the outcome of the success of your crop. Having reliable sensors is also paramount. If your sensors are reading something different, whether it be from placement, malfunction, physical disturbance, etc., this can cause serious crop failure if you are unaware of the incorrect readings. It is also wise to have a backup system and a good maintenance team to jump into action should a mechanical failure be imminent.

As is with growing, there are more ways than one to get your crop to where you need it to be. There tends to not be only one effective method that is the best, but more preferred. I am always looking to learn and understand better ways to grow things efficiently. There is a lot yet to learn and time to put in, but given the experiences I have gone through thus far, VPD and HPF will be what I choose to get better with. Whatever you choose, do what’s right for your operation.  

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