In the last article, we gave an overview of the broad range of temperatures for growing cannabis indoors. In this article, we will specifically dive into how to optimize temperature and humidity during the flowering stage.

We will first give basic guidelines for beginners. Then we will move on to a more nuanced and technical approach for those looking for deeper optimization and even better results.

Guidelines for Temperature and Humidity during the Flowering Stage for beginner growers

Flowering your plant is an exciting time. This is where you finally get to watch the plant bloom, and get those buds that you have been waiting for. It goes without saying that the flowering stage is the most important part of the process.

During veg, you can deal with problems, since there is no deadline or end date in veg. You can continue to train your plant, top your plant, deal with an infect, etc, and you can just drag out the vegetative stage until all problems are resolved.

But with the flowering stage, you have a definite end date where the plant is finished. Therefore, it’s critical that things go right, because there’s no turning back! But that is okay.

Don’t worry about screwing this up, as cannabis is a durable and hardy plant, and can deal with a lot of stress, including suboptimal temperatures. But the more consistent you can be, day to day (when the lights are on), and night to night (when the lights are off), the better your outcomes will be at harvest time.

Keep your temperature in the mid-70’s and humidity in the mid 50’s

While there is actually a broad range of temperatures that will work, we suggest you stay in the mid-70’s (fahrenheit) during the flowering stage. We have a simple formula that is easy to remember for beginners that we call 75/57.

If you can remember 75 degrees, then flip the numbers to get 57% Relative Humidity. Your humidity can be even less, but you don’t want it higher than that. And the deeper you get into flowering, (and the bigger your buds get), you will want to keep humidity on the lower side, closer to 50%.

When dealing with equipment, you will always have fluctuations. But try to hover around that 75 degree F (23.8 C) mark. A little more is totally fine, and even a bit less is okay too.

When the lights are off, you don’t want it dropping down too much. Try to keep it in the upper 60’s during the dark cycle.

If you follow this advice, and tweak your grow room so that you hover around this mark for the entire duration of flower, you will be all set to pull down some great weed. As long as you are feeding your plants, keeping them healthy, keeping an eye at them, then you can coast at this temperatures/humidity setting and have a great outcome.

A more technical approach to temperature and humidity during the flowering stage

The above prescription of 75 degrees, 57% RH is perfectly fine and dandy, and will yield a good outcome, both in terms of weight and quality. But if you don’t mind a little more adjusting of the knobs here and there, we have a more technical approach for those desiring to eek out just a bit more optimization. If you have the time and the wherewithal, here is a more nuanced approach.

Weeks 1-2: Mimic the temperature and humidity of the vegetative stage

During the vegetative stage, we typically employ higher temperatures with higher humidity. The goal is to mimic a more tropical environment. Therefore, it just makes sense to employ similar temperature and humidity during the early weeks of flower.

After switching your plants to the 12/12 light cycle, go ahead and keep your temperature and humidity exactly the same as you had it during veg. About 78 to 80 degrees F., with humidity between 60% to 65%.

This approach is great for keeping plants happy. Giving the plants a familiar temperature eases the stress of the transition from veg to flower.

We take a similar approach when it comes to light intensity. We will mimic the low intensity of our veg lights during the first week of flowering, and then incrementally increase the wattage each week.

Week 3: Lower the temperature and humidity and hold it for 3 to 4 weeks

At this point, you can now set your thermostat for what you intend to use for the duration of flowering. If you are hungry to be as technical as possible, you can follow a VPD chart, which will give your a corresponding RH% for every temperature setting.

We believe it is sufficient to say that you want your temps in the mid-70’s. If you are growing with LED, you will get more out of even warmer temps, with higher RH percentages, but you will have to keep an eye on mold/mildew/rot further into flowering.

Another more technical approach involves taking leaf temperatures, as opposed to room temperatures, and adjusting room temperature to meet the desired leaf temperature. If you lack the equipment for this, that’s okay. You don’t need to get this technical to be successful.

Keeping your eye on the big picture, you started out by mimicking veg during the first two weeks of flowering. All you’re doing now is lowering those temps a bit, and decreasing the humidity so that it stays below 60%, and hovers in the mid to low 50% range.

After coasting for three to four weeks in the mid-70’s on temperatures, you can keep the temperature the same all the way through harvest. We do recommend that you decrease your humidity incrementally for those last couple weeks. You want to be around 50% or less by the end of flowering.

Lowering the Temperature in the last two weeks of the flowering stage

If you are looking to bring out the most vibrant colors possible out of your plants, then this section is for you. Cool temps can go a long way in encouraging color expression. Strains that are normally bright green can even darken in hue and turn out some atypical color schemes. And strains with a propensity to turn purple can get crazy colorful in colder temps before harvest.

Lowering the temperature and humidity near the end of the flowering stage also mimics the natural life cycle of cannabis in the natural world. As an annual plant, cannabis seeds sprout in the early summer and spring, when the earth becomes warm and the humidity is higher. They bloom during the transition from summer to fall. And cannabis is harvested later usually in the month of October, when the temperature (and humidity) has dropped.

The more you can imitate the outdoor seasons inside your grow room (or grow tent), the better.

Control the Fluctuations in temperature when the lights turn off

This article would not be complete if we didn’t discuss the importance of control the differential between the “day cycle” and the “night cycle.” And by that, we are referring to when the lights are on and when the lights are off.

The temperature and humidity combinations that we have been discussing so far apply to the “day cycle,” when the lights are on. However, once those lights go off, there will be a change in the environment.

By turning off the lights, a significant source of heat is removed from the atmosphere. This will cause the temperature in your grow room to drop. This may be mitigated by the fact that your A/C will shut off, since it won’t have to work as hard with the lights off. However, by shutting off the A/C, you lose a significant remover of humidity, so your dehumidifier will have to kick on to compensate for the lack of HVAC.

You do not need to maintain the exact same numbers during the dark cycle as you do when the lights are on. However, you do not want the swings to be excessively big, either.

Ideally, you don’t want the temperature to drop more than ten degrees or so when the lights shut off. It’s okay if it drops down twelve degrees, but just know that it is not ideal.

You don’t want your plants getting too cold. The best yields come from tropical environments. And you don’t want leaf temperature dropping too much, even during the dark cycle. That being said, don’t get too wrapped up in this. Aim for a drop in temperature of about 10 degrees or so, and try not to let it get out of hand.

Beware of your Watering Schedule during the Dark Cycle

One thing to be keenly aware of is how saturated your pots are going into the dark cycle. Ideally, you should water right when the lights come on, so they have “all day” to bask in the “sun” (i.e. grow lights) and allow that water to evaporate, contributing to the humidity.

When the lights go off, you do not want your pots to be saturated, for several reasons. One, you do not want to add humidity to the room when it is colder. Since the lights are off, the temps will be dropping, and you don’t the air to be cold and wet.

Second, if the water does not evaporate out of the pot, then the plant will absorb it. With the uptake of this additional fluid, the plant will become colder in the cooler temps of the dark cycle. This will cause your plant to be more susceptible to various forms of mold and mildew, especially botrytis (black mold).

The takeaway lesson is to water you plants early in the day cycle, not later in the day, and definitely not when the lights are off.


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