Oct 12, 2018

Temperature & Your Grow 101

Do you ever worry that your AC will break down while you are away, and you will come home to a lost cause in your grow room? If you are like most growers, this concern has crossed your mind more than once. Temperature is incredibly important to the health and vitality of your plants and ultimately your yield. Yes, many other elements need to be considered, but for now, let’s look at temperature’s influence on the health of your grow.

What is the ideal temperature for my grow?

Hydroponic garden growing plants using mineral nutrient solution

Although some plants may vary, the typical range that most flowering plants thrive in is between 72 and 78 degrees Fahrenheit. Plants grown in a natural outdoor environment are more resilient to hot and cold temperatures due to the high rate of air circulation. To mimic the outdoors in a closed system, make sure you replace the air within the room up to five times per minute. With constant circulating air, you will also need fans, air conditioners, and/or heaters to regulate temperatures within your grow room.

How does temperature affect the plant?

The temperature in your grow space affects the rate at which all metabolic processes take place. The porous openings of a plant (the stomata) govern the intake of CO2, heat, and light depending on the availability of these elements. Stomata are the plant's personal built-in AC unit. As stomata open and release water vapor, or transpire, they pull cool water from the roots up and through the plant.

When the environment is at a lower than ideal temperature, processes slow down at times to the point of collapse due to inefficiency. When the environment is warmer than ideal, systems speed up and the stomata close. When the stomata are closed, there is no water, nutrient delivery, or air exchange and the plant cannot cool itself and will typically overheat and die.

Effects of Warmer Temperatures in a Nutshell

At warmer temperatures (above 90 degrees), plants increase their water consumption and rate of transpiration. Thus, higher temperatures raise the humidity in your grow room. When increasing the temperature and decreasing the humidity (by implementing a dehumidifier), the water levels needed for consumption increase.

Unfortunately, warmer temperatures create more hospitable environments for unwanted pests, bacteria, and fungi. In the heat, plants become more susceptible to white powdery mildew, root rot, nutrient burn, spider mites, and possible rhizosphere asphyxiation (lack of oxygen at the base of the plants near the root system).

Effects of Colder Temperatures in a Nutshell

Colder temperatures constrict water molecules and slow the movement of nutrients and metabolic processes. In turn, colder temperatures are less apt to raise humidity levels. Due to a slowed rate of transpiration, plants will experience stress and slow their speed of growth. When it is cool (below 65 degrees), plants become susceptible to certain types of flower mold, especially when humidity levels are high.

Seeds germinate in cooler temperatures (think springtime) and require moist soil to come to life. In situations where your soil becomes overwatered in cool conditions, an environment is created for fungal spores to produce at rapid rates. When fungal spores are given this chance, they attack your plants before they even crest the soil. Effects of this attack will show later in the growth cycle.

Molds (such as black sooty mold) and mildew (such as powdery mildew) thrive in standing water on soil and will become prolific if you do not take strict sanitary precautions. Other molds can appear during the flowering phase that thrive in cooler temperatures. Particularly, Botrytis Cinerea (AKA gray mold), which attacks at times of high humidity and can spread quickly if not immediately caught.

Temperature and Your Grow


Gavita Pro E Series High Pressure Sodium hydroponic grow light

Heat Generation

Grow lights emit heat, no surprise here. When lights are on for 12-18 hours daily depending on which cycle you are in, the heat generated will often raise the temperature in your grow above the ideal range. This is when you will need to consider installing an air conditioning unit.

If residual light heat is still negatively affecting your plants after installing an AC, there are a couple more options to consider. The first is the simplest. Raise your lights higher, further away from the canopy. If your grow does not allow for you to mitigate your heat issues this simply, you may want to consider making the switch to ceramic metal halide fixtures to lower your heat output.

Typical signs of heat stress caused by proximity to the lights include yellowing and bleaching of the plants, burnt leaf tips, and curling leaves.

To mimic the outdoors, an oscillating fan such as a Hurricane 16” or 18” can be used to circulate warm air residing in and near the canopy. Alternatively, you can ping-pong your low cool air, with Hurricane Pro 20 floor fans. As your exhaust pushes warm air out at the top of your grow, that cool air ping-ponging below the canopy will be pulled up and through your canopy.

When the lights are not on, you should allow temperatures to drop to 66-70 degrees Fahrenheit, or 10 degrees less than that of “daylight” temperatures. These lower temperatures are more conducive to a prime flowering environment and allow the plants to grow to their full potential size and density.

Lighting Placement

Depending on the type of lighting system that you choose for your grow, there are recommended distances from the plants based on the heat emitted by each lighting technology. You should keep your double-ended HPS lights at least three feet above your plants. Air-cooled lights can be between 18 and 24 inches above the plants.

Your lighting system is a very useful tool when used correctly. Balance is key here. You must find the appropriate distance to give your plants the light source it needs while not allowing the light’s radiant heat to cause harm to the canopy. You may consider using a larger number of lower wattage bulbs if heat is an issue. Alternatively, you may just be able to find the sweet spot by just raising your lamps by a few inches. Choosing a lighting source may be restricted by the size of your grow space, and will need to be chosen and placed accordingly.


When augmenting your grow system with CO2, allow your temperatures to be slightly higher. Temperatures should reside between 82-86 degrees. The higher temperatures also allow the stomata of the plant to open wider. This allows for the further intake of elevated levels of CO2.

When the plants are processing higher CO2 levels, they emit more heat due to the metabolic processes that are taking place. When using a CO2 burner system, for every pound of fuel that is burned, 22,000 BTU of heat is generated- as well as 1.5 pounds of water vapor.

For more about implementing CO2 in your grow, click here.


Your plants will require more frequent watering cycles at higher volumes when exposed to higher temperatures. At warm temperatures and during the light cycle, the stomata of the plant are open, releasing water vapor. As the plants release water, they must be receiving an adequate amount to subsidize the output of energy. If adequate water is not provided the plant will experience salt buildup which may result in “salt burn.”

Salt burn can also be confused with symptoms of nutrient deficiencies that also break down the cell wall. Think of salt burn as kidney stones for your plants—salt levels are too high and water is inadequate.

To get the most out of your watering cycle, set your schedule to water just before and directly after the hottest parts of the day. It will prevent heat stress and provide another mechanism for the plant to naturally cool itself by using the cool air off the residual moisture. Above all, keep your watering schedule consistent. Some growers may have heard a myth that slight water stress can create a more resilient plant but it often will cause more harm than good to apply this theory.

Nutrient Uptake

You may think that with an increased watering schedule that nutrients may need to be increased as well. This is not the case. A higher nutrient base will not influence the growth of your plants and may put them at risk by elevating levels to the point of toxicity in your plants. Maintain a base nutrient level in your water, but increase the watering frequency at higher temperatures to accommodate for higher rates of evaporation in the plant’s soil and root system.


Ventilation plays a large part in regulating the temperature within your grow room. When using an exhaust fan, it is best to place ducting near the top of the grow space. We all know that heat rises. So to conserve energy, place the exhaust in the warmest part of the room (near the ceiling). This allows for the most rapid exit of the warmest air. This will also pull the warm air up and out of the canopy, while simultaneously pulling the coolest air up from the ground and through the warm canopy.

Proper ventilation is also important to distribute the stagnant air that sits closest to the plants. On a micro level, the area directly surrounding the leaves of your plants becomes a microclimate. Imagine your plants sweating and emitting heat the way we do in the summer. The area closest to our skin is moist, humid, and warm in comparison to the still air surrounding our bodies.

These microclimates surrounding the plants have varying humidity levels, temperature readings, and atmospheric elemental concentrations that differ from the grow room as a whole. This is caused by the plant’s metabolic processes, and the byproducts of these processes being emitted. The need for air movement is necessary to reduce the size and length of time that plants are exposed to these microclimates. This is why oscillating fans are a necessity in an indoor grow environment. Just as a fan would help to cool our skin and dry the sweat, air circulation will do the same in your grow by circulating excess CO2, water vapor, and heat for your plants.


It is valuable to understand that temperature and humidity levels have a very intimate relationship. The humidity within your indoor grow spaces is referred to as relative humidity. The relative humidity is the comparison of maximum water vapor pressure to partial pressure when measured at the same temperature. Warm air holds more water vapor than cold air. Exhausting warm air and in-taking fresh cool air balances humidity. This prevents you from essentially suffocating your plants with excess water vapor.

In addition to creating a hospitable environment for your plants, a constant temperature reading of low to mid 70 degrees is also a prime environment for the incubation of spores and microorganisms. Regulating humidity levels helps to keep unwanted mildew and pests at bay.

Growers should strive to see humidity levels of 60-70% during the vegetative phase and 50-60% during the flowering phase. With higher levels of humidity, the plant is unable to cool itself as efficiently as it should, and this will result in the plant shutting down on itself. Lower humidity levels constrict water molecules and do not allow for the water supply to circulate at a fast enough pace. This results in proteins breaking down the cell wall.

Water Temperature

If using a soil medium for your grow, water your plants with room temperature water. Applying water that is below or over the 72-78 degree range will potentially shock the plant and lower its potential to provide the best quality yield. In hydroponic systems, if you allow your water to drop below 68 degrees, it will likely shock the root systems and place unintentional stress on the plant.

When growing in a basement or on a cement floor, it may be necessary to integrate a heating pad placed under the plants so that they are not cooled by the flooring. If the water is significantly cooler, a water heater such as a Titanium Reservoir Heater or Elemental H20 Water Heater can be integrated into your circulating water system to adjust the temperature to optimum levels

How to Alleviate Heat Stress

Heavy 16 Foliar spray

At some point in your growing career, you will most likely experience a failure in your AC system and your plants may become vulnerable to heat stress. Adding seaweed kelp extract to your nutrient base will assist your plant’s recovery. Seaweed kelp has also been shown to act as a preventative measure against potential heat stress. Heavy 16 Foliar is another great option for added natural stress protection.

A slightly warmer environment will not destroy your crop but in warmer temperatures (from 80-90 degrees), you will see longer stems on your plants, as well as less dense flowers. This elongation and loss of density is the result of your plant growing at an unsustainable rate. The stems continue to grow due to elevated temperatures, but nutrient intake can’t keep up. It is like feeding a growing child sugar and white bread without any nutrient-rich foods like veggies and whole grains; it enables them to continue to grow but is not providing the quality of nutrients to do it healthily.

By managing the temperature in your grow system you are providing the best environment for your plants to thrive. When plants are not struggling to deal with the effects of heat stress or cold temperatures, they can conduct life’s biological processes optimally and yield a dense, quality product.