The High Times Grow Guide to Beating the Summer Heat

(Photo by Erik Biksa)

What should you do when the thermometer climbs to 85°F in your growroom or greenhouse and continues to rise? Put down that cool drink and start thinking about how you can beat the heat so you can still harvest dense and tasty buds.

A Hot Mess

Even plants grown outdoors under the open sky can suffer in the intense heat of summer. Apart from the need for copious amounts of water, here’s some of what you could be facing if you grow your crop in the temperature red zone:

Insect damage: Spider mites can devastate your cannabis plants fast under hot, dry conditions. In fact, they love it so much that they reproduce up to twice as quickly during such periods.

Hermaphrodites: Less-stable genetics can be triggered to produce a few male flowers (or even lots) in otherwise female plants under hot and dry conditions, thus seeding your crop (highly undesirable in any circumstances, especially since the seeds will be very likely to go “hermie,” too).

Foliar blights and diseases: Excessively hot conditions, whether humid or dry, invite blights, molds, fungi and bacteria that cause diseases in your crop, because the plants are stressed and the conditions favor the incubation and reproduction of spores.

Root and stem rot: Anaerobic bacteria and fungi that cause rot in the lower portions of cannabis plants often prefer warm, wet conditions around the roots because, at warmer temperatures, water holds less oxygen and the roots are weakened as a result.

Poor quality and low yields: Even if your crop makes it to harvest, it’s very likely the quality will suffer. Loose and airy buds lacking flavor and aroma are a telltale sign that a crop has experienced stress under excessively hot growing conditions.

What to Do When It Gets Hot

Water-cooled air conditioning, while effective, is very powerful and water intensive. (Photo by Erik Biksa)

If you have an industrial air conditioner already keeping things cool, chances are you’re resting easy (although you’re still not immune from every issue to be aware of). However, for most growers, there are some things you’re going to want—or need—to do in order to ensure healthy plants and high-quality harvests. Naturally, the first step is to try and lower temperatures through the following methods.

Air Conditioners

For every 1,000 watts of high-intensity discharge (HID) lighting in your growroom, you’ll want around 3,500 to 5,000 BTUs of cooling power. The range is wide because not all air conditioners perform up to their claims, coupled with the fact that different types of lamps and ballasts produce different levels of heat.

Be aware that a good-quality AC unit—i.e., a “split” style—doesn’t come cheap. In fact, it can be the most expensive piece of growing equipment you’ll own, making it a serious investment for those who can afford to purchase one.

Split ACs have a unit that runs outdoors to scrub off the heat and a separate air handler that is installed indoors, in the growroom. These are perfect for growers because there is no exchange of air from the growroom to the outside or vice versa—it’s essentially “central air” for growrooms.

Carbon dioxide can help buds grow but the heat created by CO2 generators must be scrubbed away. (Photo by Erik Biksa)

Pro-tip: Common window ACs and portable room ACs from your local building center typically exchange inside air with outside air to provide cooling. This means you’ll potentially be blasting some pretty pungent odors outdoors during the time of year when there are more people outside doing stuff (and therefore more likely to notice). Inline carbon filters or odor neutralizers can be added to the discharge, although these will usually not eliminate the problem of strong, dank odors entirely.

Water-Cooled Heat Exchangers

These are essentially growroom radiators, although they’re quite a bit different in construction from an actual car radiator. The hydronic fins in a good-quality heat exchanger are very fine, giving them a large surface area in a small-dimensioned cube to scrub away heat from the fluid that’s circulated through them, originating from the growroom.

If you have access to a free stream of water that stays cool and plentiful, a water-cooled heat exchanger can be a great solution for keeping a growroom or even a greenhouse cool during the hot summer months—or even year-round in a sealed growing environment.

The water that circulates through a heat exchanger remains clean and very usable afterwards. It’s just much warmer than when it started out—and plenty of water gets circulated during the course of a hot day. Liquid cooling can be very efficient, and the heat can be directed as far away from the growroom as your circulation lines allow. Water chillers, tanks and geothermal principles can also be used to cool the water in a heat-exchanger system.

Each of these air exchangers for the AC system is paired with an air handler inside the growroom, which is stealthily located beneath the ground. (Photo by Erik Biksa)

Pro-tip: Just because your water supply is cold and plentiful during winter and spring doesn’t mean it will be the same in the summer months. A water-cooled heat exchanger is very dependent on the temperature of the water coming in to work properly, making sure that the air blowing out is sufficiently cooled to manage the temperatures involved in growing your valued cannabis crop.

Fan Cooling

Exhaust fans for indoor growrooms, tents and greenhouses can be effective for cooling even in summer months—but this largely depends on where you live and whether you’re having luck with the weather. Even the best fan-cooled set-ups (i.e., in/out exhaust) usually need about a 15°F difference between the outdoor air temps and the desired indoor temperature to work near the optimal range for growing cannabis.

Pro-tip: For every 700 cfm (cubic feet per minute) of exhaust output that your shutter-style fans push, you need 1 square foot of opening to ensure that static pressure doesn’t create excessive resistance to air movement. Typically, this is accomplished with a motorized or thermal shutter opening, available in various sizes.

The inline fans above the canopy remove heat directly from the bulbs. (Photo by Erik Biksa)

Hydroponic Crop Cooling

Water-culture hydroponic methods like nutrient-film technique (NFT), deep-water culture (DWC), recirculating deep-water culture (RDWC), and aeroponics require little to no growing medium. Bare cannabis roots drink freely from a recirculated mineral-rich solution that is kept well aerated. Temperatures in the hydroponic reservoir over 75°F can create problems like poor nutrient uptake or even root disease in some instances.

The cool thing about hydroponics in warm weather is that savvy growers chill the nutrient solution instead of the air. This is much more efficient from an equipment-cost and power-usage perspective. Remember, it’s the plants that need to be kept cool, not so much the air. Think of sitting with your feet in a cool stream on a very hot day … thermodynamics is fun!

What About Some Less-Expensive Solutions?

If you’re on a budget and can’t spend a lot on growroom tech, here are some other suggestions: 

Indoor or Greenhouse Lighting

While they’re not exactly cheap, air-cooled HID light reflectors are a great way to remove heat from grow lamps before it ever enters the growing environment—and since most growers end up buying a lamp reflector anyway, it’s often not much more of an investment as a startup cost. Good-quality air-cooled lamp reflectors spread light evenly and can be sealed off from the environment, which means you won’t suck out carbon dioxide or telltale odors along with the heat you discharge from the growroom to send outside.

Alternative crop-lighting sources like LED (light-emitting diodes) or LEC (light-emitting ceramic, such as a ceramic metal halide) can cost more to buy for the same-size area versus HID lights, but they often produce considerably less heat for the level of bud production they offer. Of further bonus to offset the initial cost is the fact that you use less electricity to power them or any other equipment you may use to cool your growroom—meaning that your cooling needs and power draw decrease proportionately.

Pro-tip: An easy and inexpensive way to reduce heat in the growroom by as much as 30% is to operate your HID lighting fixtures via remote ballasts. These are lighting systems that allow the “driver” (what makes the lamp light up) to be put in an area outside the growroom, at the other end of the cord, thus keeping the heat generated by the electronics away from your plants.

Fans cool the lights, maintain room temperature and keep fresh air moving through the plant canopy. (Photo by Erik Biksa)

Plant Genetics

Some cannabis strains deal with the heat better than others—there are even strains that crave it. Exotic sativas (for example, Cambodian varieties) may actually prefer higher heat coupled with high humidity levels versus the more common domestic hybrids. Desert strains like Afghanis may also take the heat, but they won’t do well with high humidity.

Strains with sensitive roots or that are prone to insect infestations are also not advisable when growing in less-controlled conditions during the hotter months. Because frequent watering is necessary in hot climates, there’s a much better chance of getting root rot in hot, wet soil conditions.

Cultural Practices

1. Watering

It’s easy to see why warmer temperatures lead to a higher demand for water when growing plants. With this in mind, cut back on your fertilizer levels. Remember, your plants will use proportionately more water than nutrients in hot climates versus cooler ones (kind of like a summer beverage-to-food intake). Otherwise, nutrients can build up, eventually reaching levels toxic to your plants.

Stay hydrated, stay beautiful—that goes for plants and people! (Photo by Erik Biksa)

Pro-tip: Increase the drainage in your growing medium or system. While seemingly counterintuitive, the principle is that you’ll need to water a lot anyway—so your plants should be left with some air to breathe at the roots, since they’ll need to be watered more frequently and the medium will be kept saturated more often.

Don’t think you can keep up with increased watering demands? There’s an easy fix for that: Install an inexpensive drip-irrigation system (no electricity is required with gravity). For the backyard or growroom, it’s easy and cost-efficient to piece together a small-scale drip-irrigation system using a pump and timer. Be sure to use a good-quality drip-irrigation filter, though, because if your lines plug up, your crop of buds could be denied water when it’s needed most.

2. Feeding

The guard cells that control the openings on cannabis leaves to let moisture out and breathe gases in are largely regulated by the plant essential potassium (K). Naturally, if these cells get worked harder, the plant is going to need more potassium than usual.

Silicate (the element in sand) is considered a plant essential element. It has benefits to crops, including cannabis, because it helps to create a “harder” plant cell that is less susceptible to damage or moisture loss.

This irrigation system allows water and nutrients to be delivered to different sections of the growroom. (Photo by Erik Biksa)

Pro-tip: Potassium silicate is a common nutrient additive that gives plants the building blocks for protecting against stress and excessive moisture loss in hot climates. It’s relatively inexpensive to purchase and apply—but note that many potassium-silicate products will increase the pH of your nutrient solution, making it less acidic and more alkaline. Be careful not to over-apply, or simply monitor and adjust the pH during mixing.

3. Building Biology

You don’t have to be a biochemist to benefit from natural processes that you can enlist to help protect your plants in hot growing situations.

Trichoderma are a form of beneficial fungi that eat root-disease-causing organisms that occur as soils heat up while remaining moist. They’re just one type of many specialized organisms that are usually part of beneficial-microbial blends.

Digestive enzymes are available in a variety of formulations for applications to benefit crops. Basically, these living enzymes dissolve dead root tissue (much like you shed skin, plants shed root membrane), removing a preferred food source for root-disease-causing organisms while converting nutrients from fertilizers into forms that are easier for plants to digest.

Aerobic microbial teas—the type that growers brew with lots of aeration—can also be of benefit. If the plant foliage and root zone are alive with beneficial microorganisms, there is little to no room left for the “bad guys” to come and stay, causing problems. Living brewed teas may be sprayed onto foliage and applied to roots with periodic waterings to “recharge” their population levels.

Plants that experience big swings in temperature are prone to problems with insects and diseases. (Photo by Erik Biksa)

4. Kill ’Em All

On the flip side of building healthy microbiology, keeping the foliage and root zone hygienic and sterile is another approach to preventing plant-disease outbreaks in hot growing climates.

Hydrogen peroxide and ozone are management tools available to skilled growers that allow microorganisms to be “zapped” while not damaging plant tissue. These need to be applied with some level of skill and care, however, as too much can hurt your plants, and too little will be ineffective.

Hypochlorous acid is a nutrient additive that acts like a “friendly bleach” to roots, keeping them sterile and hygienic with regular applications as recommended by the manufacturer.

Stay cool out there!

  1. I grow outside, in California. Depending on the size of your pot, you will definitely be soaking your soil at least once a day when temperatures are in the 90’s, and when the temps start nearing triple digits, I soak in the morning, and I make sure the soil is moist (but not soaked) around sundown. The variables involved are plant size, pot size, and pot material. Clay pots drink about as much water as my plants! If you have a 5 gallon clay pot in high summer heat, you will probably soak twice a day. Porcelain pots do a little better, fabric pots a little worse at maintaining moisture in high heat. In my experience, a large plant in a large fabric pot is basically impossible to overwater in high summer heat.

  2. Skunks do a little better than say blueberry and chem d hybrids with the heat rise in summer indoors. But the best remedy is temporarily cutting back on the grows energy . The 102 f summer late days we had this year, 2020 kept June, July and August refrigeration of all sorts in peril. So nice to feel the fall now and cranking the wattage back up.. good growing, world.

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