We already did an article about vapor pressure deficit (VPD) a while back–you should read it because it totally explains all the technical details of what vapor pressure deficit is. In today’s article, however, our goal is to emphasize that vapor pressure deficit is an increasing problem for indoor and outdoor marijuana growing, and one that’s very hard to fix.

To make it as simple as possible, look at the chart at the top of this article. Here’s a stark fact: if your marijuana growing space’s temperature and relative humidity aren’t within the parameters shown in the chart’s green zone, and also in the range of temperature that marijuana plants like, your plant performance significantly decreases.

Deficits in growth rate, physical development, nutrients and moisture utilization, bud maturation, bud potency, and overall metabolism happen every second your marijuana garden is out of the green range.

Vapor pressure deficit problems delay crop maturation, foul up feeding and watering, create leaf issues that lead to pests, molds, and mildews, and cost you money and time. In some cases, they’re fatal to plants.

As climate change heats the planet, vapor pressure deficit problems increase whether you grow indoors or outdoors.

Climate change creates more evaporation, leading to increased humidity, accompanied by hotter temperatures.

When you look at the VPD chart and start applying it to your cannabis growing situation, you see the huge challenges.

For example, grow rooms lit by LEDs and/or with added carbon dioxide (C02) should operate at 78-84°F. To avoid vapor pressure deficit problems, relative humidity at those temperatures must be kept in a very narrow range, over 60-65% and under 85%.

But humidity that high creates growth conditions for botrytis (gray mold), which flourishes in dense buds when humidity is above 50-55%. You see from the VPD chart that temperatures that work coincide with humidity below 50-55% are too cold for growing marijuana.

Adding humidity to grow room air isn’t as anywhere near easy as removing it. Sure, using a Quest professional dehumidifier, you can easily remove excess humidity.

Unfortunately, we’ve tested 15 humidifiers, including so-called professional models that cost as much as $700, and none work well or at all.Their main problems are inaccurate humidistats, hard to clean/fill/drain, leaks, insufficient mist volume, faulty engineering and workmanship, insufficient warranty and tech support.

Models that cost less than $100 on Amazon work as well as more expensive humidifiers you can only get from so-called grow stores and ag supply shops, but none of them work well enough.

Out of 13 low-cost humidifiers we tested, the one that had the most accurate humidistat and was about 65% effective at keeping humidity at set target levels was the “Fabuletta 6 liter top-fill” model that at present costs $70. The first of this model we tested was totally useless, but the second one was accurate within +- 12 relative humidity points, which is better than not accurate at all, lol.

I recommend you always buy the extended warranty/replacement plans with these kinds of units, because the unit will likely fail within a year or two, but if you have the plan, you get a new one free, or you get your money back.

Another issue is that many of us remove air from our grow room by using exhaust fans, which removes the humidity you’re trying to add.

Ancillary issues include the fact that many hygrometers that measure humidity are woefully inaccurate and unreliable, whether they be mechanical or digital. The same goes for most humidity controllers and climate controllers used for automated triggering and shut-off of fans, dehumidifiers, humidifiers, and other gear.

Part of the problem is humidity isn’t uniform throughout a grow room. Levels in the plant canopy may be quite different than levels where your humidistat and hygrometers are.

For growers in the American West (especially deserts) but also increasingly in other parts of America, humidity is often ridiculously low.

We heard from a Florida cannabis grower whose only humidity problems used to be there was way too much of it. But this year (2023) so far, climate change drought has made Florida’s humidity as low as a desert, which plagues marijuana growers indoors and outdoors.

At the same time that air is super dry, temperatures are very high, so air conditioning is used more. Problem is, air conditioning dries out air.

We’ve tried all kinds of hoped-for fixes so we could have a grow room at the right temperature and relative humidity for ideal VPD. These include leaving open vats of water in the grow op, trying to have the lights on when the humidity outdoors is highest and sucking that air into the grow room, using a mist system and/or foliar spraying.

The mist system and foliar spraying are somewhat effective, but messy, hard to manage, water-wasting, and can damage leaves.

The best fix we’ve come up with is labor intensive. Using humidifiers that have automatic-off function and a digitally-set humidistat, we monitor grow rooms every hour, sometimes manually turning on and off the humidifiers and adjusting their mist amount and flow rate. It’s a hassle, but it works better than relying on faulty humidistats and humidifier controls—we’ve never tested a humidifier that was reliable enough to automatically, always achieve pre-set target humidity.

The most important thing for you to know is that maintaining ideal VPD helps ensure optimum marijuana plant growth, but for every second your lights are on when VPD is out of range, your plants stop growing and experience stress.

This lowers harvest weight and potency, extends crop times, makes plants susceptible to insects and pathogens, and in extreme cases, kills plants.

In outdoor settings and greenhouses, mist systems and foliar are more practical than in indoor grow ops, and  can also be used to deliver nutrients and protectants (such as potassium silicate).

The other thing to know is that if vapor pressure deficit is caused by too-low humidity, growers have to carefully monitor root zone moisture and nutrients feeding.

That’s because low humidity air sucks water directly out of root zones, leading to excess nutrients salts, at the same time plants are trying to intake more water. This can cause overfeeding and root zone toxicity.

I’m sorry that I can’t give you a total fix for the VPD problem. Marijuana plants prefer a narrow temperature range of 74-84°F, but it’s the relative humidity accompanying that temperature that determines VPD compliance and plant performance.

The good news is that in situations when plants had stopped growing because VPD was out of range, as soon as we restored correct VPD, the plants started vigorously growing.

We’ve seen situations that defy logic and science, such as when Lethal Paul, played by Javier Bardem in No County for Old Men, and who runs New420Guy Seeds, grew large plants outdoors in bone-dry, Venus-hot Nevada in summer.

Paul’s outdoor marijuana garden was so far out of VPD range we were surprised his plants grew at all, and more surprised they successfully matured to produce gooey, huge outdoor marijuana buds.

If you want to get rich as an inventor, create and patent an affordable, reliable grow room and greenhouse humidifier with a reliable humidistat so growers can set target humidity and be sure the device will maintain it. Many marijuana growers need a reliable humidifier, but can’t find one. If you make one that works well, you will have plenty of buyers.

If you want the fastest growth, highest potency,  and heaviest harvests from your plants, do whatever it takes to ensure your marijuana garden temperature and humidity are in the green zone on this article’s VPD chart.