Wholesale Seed Quotes Menu Weed Seeds USA

Weed Seeds USA

Souvenir Seeds

Grow Room Temperature

Temperature For Growing Weed

Written by Neal Brown . Updated: February 6, 2021

Best Climate for a Cannabis Grow Room

Though it is one of the most frequently overlooked parts of running an indoor grow room, creating the right climate for your plants makes the difference between a mediocre crop and a great yield. These days you can even control the gender of your plants, by using our popular feminized seeds, so why not control the environment the best you can as well? In this guide, you will learn about indoor grow room climate and how to control it, regardless of if you live in Florida or Maine.

Climate: What Is It?

Climate is an observable area’s prevailing weather conditions. However, when growing marijuana indoors, you are working on a much smaller scale. In such a context, climate control means working with some key components such as:

  • Humidity
  • Temperature
  • CO2 levels
  • Air movement

This article examines each of these crucial factors, lists ranges for every stage of plant development and explains how to provide cannabis plants with what they need to develop. And we will explain why proper climate control is so important in a cannabis grow room.


The temperature of a cannabis grow room has a significant impact on how the plants grow. The rate at which photosynthesis occurs is affected by temperature, so if your grow room is too cold or too hot, it will adversely affect the plants’ growth. At the same time, the amount of available light often changes the room’s ideal temperature. If you are working with a high square foot/watt ratio, optimal photosynthesis only occurs at the top end of the temperature scale.

  • Strong light and low temperatures yield short stems and slow growth.
  • Medium light and high temperatures result in stretched stems.

Setting the Right Temperature for Your Grow Room

As with other environmental factors, temperature needs to shift according to the plant’s developmental stage. For instance, the ideal temperature for seed germination is approximately 25°C (or 77°F). If the temperature is too low, weed seed germination may be delayed; if it is too high, seed chemistry might be affected, and emerging sprouts will not have much of a chance to grow. Once the seed leaves or cotyledons pop out, lower the temperature by a degree or two as high temps may cause the stems to stretch.

Night-Time Temps

There may be up to a 10° difference between daytime and nighttime temperatures without ill effects. However, if nighttime temperatures are more than 10° lower than daytime temperatures, it may cause:

  • Stunted growth
  • Stem stretching
  • Delayed ripening of flowers
  • Bud rot and mold

The warm environment created by HID (high intensity discharge) grow lights causes plants to retain water. When the lights are turned off and the temperature gets too low, the moisture turns into condensation that may lead to mold growth. Running a dehumidifier at night will help, but it is usually easier to prevent condensation from happening in the first place.

Grow Room Too Cold

Grow Room Temp Too High

Best Temp and Humidity For Growing Weed

Special Deals For You

Spend $100
and get $10 OFF


Spend $200
and get $40 OFF


Top 20 Marijuana Seeds For Sale

[products limit="15" columns="5" category="top-20-marijuana-seeds-for-sale" SEE ALL TOP 20 MARIJUANA SEEDS

Recent Posts

Node Spacing and Grow Room Temps

Temperature does not just affect the rate at which marijuana plants grow; it affects how they grow. Flowers start out as stigmas, which are fine hairs that emerge at the branch and stem nodes. Calyxes, or tear-shaped nodules with high numbers of trichomes, form at the base of the stigmas. As the calyxes grow, they combine to form colas (what we commonly call buds). If the internodal spaces are too wide, the calyxes cannot combine, and you will get a lower yield with smaller buds.

Temperature Affects Internodal Length

More specifically, the differences between daytime and nighttime temps affects how plants grow.

  • The closer your grow room’s daytime and nighttime temperatures are, the shorter the internodal length will be.
  • With a substantial temperature gap, the internodal length will be greater.

Managing grow room temps to control internodal spacing is effective mainly during the pre-flowering stage. If you raise the nighttime temperature almost to daytime levels during the growth spurt that comes during the 12/12 lighting schedule, you will have shorter internodal gaps, bigger buds, and a better yield. After the plants have been in the flowering stage for two or three weeks, it is okay to lower the nighttime temperature again, so the plants do not get stressed. Luckily, having this control at your fingertips means that even if you live in a windy state like Kansas, your plants will be kept perfectly warm, so as to maximize your yield.

ideal-humidity-for-cannabis-plants ideal-temperature-for-cannabis-plants

Cold-Air Dumping

Researchers have discovered that most temperature-related stem elongation occurs during the first few daylight hours. It is possible to yield short and compact plants with short internodal gaps by keeping the temperature lower than the nighttime average during the first two lights-on hours. Use a timer and a portable AC unit to create a cold-air dumping effect.

What to Do When Temperatures are Too Low

If the grow room’s temperature drops below 15°C or 60°F, the plants’ metabolism will slow, and growth will stop. At or below 4°C or 40°F, tissue damage will occur, and it may not be repairable. If this occurs during the flowering stage, the plants might not mature properly. Generally, with the use of HID lamps, daytime temperatures will remain at an appropriate level. During the night, or if you are using LEDs, the room may get cold. We have already explained what happens to cannabis plants if they get too cold, but thankfully, the problem is easy to resolve. Follow these tips:

  • Do not bring air in from the outside during winter.
  • Keep pots off the floor to fight low root temps.
  • Minimize exhaust fan usage. With a speed controller or timer, you can ensure that the fan works for air exchange, not heat extraction.
  • Use a submersible heater to keep the nutrient solution warm.

Next, you will learn what to do if it gets too hot in the grow room.

How to Handle Grow Room Temperature Spikes

At or above 30°C or 86°F, there is a noticeable decline in plant performance, and at about 32°C or 90°F, they really struggle. At that point, the plants will focus on transpiration instead of photosynthesis, and water will move upward more quickly to cool the leaves via evaporation. This occurs out of proportion to growth-related nutrient usage, leading to soil acidity and other issues. At a temperature above 35°C or 95°F, the leaves’ stomata close and the plants stop taking in CO2. Photosynthesis stops and the plants’ stored carbon is depleted. Even if your grow room does not reach these high temperatures, it may suffer detrimental effects such as:

  • Spindly, long plant stems
  • Loosely packed buds
  • Diminished trichome production
  • Greater pest susceptibility
  • Wilting
  • Risk of mildew
  • Decreased smell and flavor

HID grow lights are the main cause of excessive grow room heat. With a dimmable ballast, you will be able to control light production. With young plants, using a low setting will reduce power consumption and heat production.

Cool-tube reflectors can be used to draw heat from the grow area. For sudden temperature increases, try raising the lights above the canopy for a time to lower the heat level near the plants. Though there will be effects from the light decrease, they will be less serious than those found with excess heat.

Some growers recommend setting the lights to come on after 10pm, as ambient temperatures start to drop. It is best to keep oscillating fans around, which prevents hot spots from occurring. With a high-capacity exhaust fan, you will be able to remove more heat from the room. Couple it with a thermostatic speed controller that turns on the fan when temperatures get too high.

An inlet fan will draw in cooler air so you won’t have to count on passive intake. Do not forget to pull in ground-level air and push out hot air at the top. Ducting should be straight so the ventilation system can work at peak efficiency. However, if it must be bent, be sure to use metal couplings.

Bulbs are not the only heat source to consider. To keep things cool, move ballasts and other equipment outside the grow room. If heat is still a problem, it may be time to invest in an AC unit. Keep in mind that it is easier to cool a small room than it is to cool a large one.

monitor-the-health-of-your-plants-frequently this-is-what-moldy-cannabis-looks-like automated-marijuana-plant-watering-system

Preventing Mildew

To prevent the growth of mildew and mold, cannabis must be grown in a very well-ventilated space. Most successful growers use fans to decrease temperatures and provide their plants with the air circulation they need.

However, it is just as important to remove hot air from the room. Ventilation cuts heat and controls humidity, which prevents plant stress and reduces the risk of disease. Providing ventilation may be as simple as cutting an opening into a cabinet or closet door. If you cannot make cuts, though, try leaving the door slightly open or using an outward-facing fan that is directed toward the ceiling. Some growers use carbon filtration and ducting to minimize scent and control airflow.

Odor Control

Many refer to marijuana as “skunk weed,” and for good reason. The plant has a very noticeable odor, particularly during its flowering stage. Though a good exhaust system will take care of the smell, that’s not the end of the problem.

To really get rid of the smell use a carbon filter. These filters are typically put inside exhaust fans so the air leaving the grow room is clean. The carbon inside the filter sucks up the scent of cannabis as the air passes over it. Cotton will help the filter last longer: wrap it around the filter cylinder to trap additional dust particles. Even with this step, you should still replace the filter media every six months or so.

The Dos and Don’ts of Grow Room Climate Control

Now that you are running a fully functional, professional grade grow room, it is time to focus on your next steps. From this point on, maintaining proper climate and humidity will help you maximize crop yield and profit potential. We have put together a list of dos and don’ts to help you achieve crop success.

  • Do: use equipment that is customized to your needs. For maximum output, it is best to use custom-built equipment. Many growers believe that home dehumidifiers will work, but that is simply not the case. These units were not made to work at the low temps required for cannabis maturation.

Furthermore, cannabis plants add about eight pounds of moisture to the air every day. If you are growing a sizable crop, that is a lot of water (and a home dehumidifier simply will not cut it.) Get the custom gear you need, and you are more likely to succeed.

  • Do balance the environment. It is crucial to maintain a balanced grow room environment, so bud rot and mildew do not damage your crops. The goal is to balance the crop’s health, the room’s airflow, and the light needed to illuminate the space.
  • Do measure twice and cut once. It is crucial to double-check your work before deciding. Knowing your grow’s specs and defining your goals will help you maximize yield and profit potential.

And now, a few don’ts:

  • Do not waste too much energy on portable equipment. As mentioned before, portable, home-use equipment will not work in a cannabis grow room. These units are typically inefficient, and they must work harder to do half the job done by a custom-made dehumidifier. When you are wasting energy, you are cutting into the bottom line.
  • Do not permit temperature fluctuations. Cannabis is sensitive to changes in humidity and temperature, which will affect your yield. Referencing the need for balance, you will need to exert significant control over the indoor growing environment. It is a good idea to log indoor conditions every day, throughout the day, to protect your investment.
  • Do not get impatient. Let your crops mature before harvesting them. After all, you have spent a few months tending to the crop, and you want to maximize your time investment and your yield. There are a few ways for even novice growers to tell when it is time to harvest the crop. The easiest way is to look at the pistils covering the buds. As flowering begins, they are stringy and white; when that period ends, they begin to turn brown or dark red. Though these color changes signify bud maturation, time frames and actual colors may vary from one cannabis strain to another. It can be very tempting to harvest your buds as quickly as possible, but it is crucial to wait until they are mature. If you have followed our tips this long, why not wait a few more days? With patience and perseverance, you will reap the reward of a full, lush, and fragrant crop from your authentic American cannabis seeds.

In Conclusion

Growing cannabis is simpler than it seems, but there are still many factors to consider when growing it indoors. By following the tips and guidelines provided in this article and others, you can control the climate inside your grow room and produce quality pot plants from seed. If you have not found seeds yet, please visit weed-seeds.com to begin your collection. Expert growers from Massachusetts to Connecticut boast about our premium seeds. Happy gardening!

How To Lower Grow Room Temperature

Ideal Grow Room Temp With CO2

Other Blog Posts