Sweltering conditions, overbearing heat, extreme humidity or arid dry air—these are the difficult outdoor climate challenges that many growers face for at least part of the year. While indoor gardening might seem like the perfect antidote to such outdoor extremes, the requirement to vent fresh air in and stale air out of a growroom means a little bit of the outdoors invades our protected growing haven.

When ambient temperatures are mild, drawing air in from outside poses little problem (in fact, it helps maintain an optimum environment for growth), but when it's 100°F outside and the air is dripping with humidity or dry as desert sand, drawing air in and over the crop doesn't quite provide those ideal growing conditions that most plants are after.

Fortunately, these types of problems are one of the reasons we have such highly developed greenhouse and indoor gardening technology—dealing with unfavorable outdoor conditions is what protected cropping is all about!

Plant physiology and excess heat

Temperature is a major controller of the rate of plant growth and affects many plant processes. Generally, as the temperature increases, chemical processes within the plant proceed at faster rates until a maximum is reached and chemical inhibition occurs. Chemical processes in plants are regulated by enzymes and cell membranes, which perform best within a certain temperature range.

Outside this range, chemical processes begin to slow down and can even completely stop. At this point the crop becomes stressed, growth is reduced and the plants might eventually die.

Even temperatures slightly above the ideal range can have significant effects on plants. As temperatures increase, so too does the rate of respiration. Respiration is a process that burns the sugars (A.K.A. assimilate) produced during photosynthesis. If respiration is burning a high proportion of these sugars, less is left for plant growth and development, so yields subsequently fall.

To make matters worse, high temperatures can accelerate water loss from the plants via the stomata on the leaf surface. If the plant senses the rate of water loss from the foliage is higher than it can sustain via root uptake, the stomata will close to protect the plant. Once stomata close, no CO2 can diffuse into the leaf for photosynthesis.

As such, the photosynthesis process will shut down until such time as the conditions improve, meaning no sugars for growth are being produced. Combined, these processes (or lack thereof) mean plant growth can completely stagnate.

Transpiration and root zones

Plants do have a natural process for cooling themselves: transpiration. Energy is lost as water evaporates into the air surrounding the leaf during this process, causing the temperature to fall (this is the same cooling we feel when sweat evaporates from wet skin). This process is one we can take advantage of to measure heat stress.

By using a simple tool, an infrared thermometer that non-destructively measures surface temperatures, we can determine if a leaf is actively transpiring and photosynthesizing, or whether it has shut the stomata to prevent excessive water loss. If the leaf surface temperature is a few degrees lower than the surrounding air temperature, then transpiration is occurring and cooling that surface.

If the leaf temperature is the same as the air temperature or even slightly higher, than the stomata have shut and no transpiration is happening. This is also a great way to monitor plants under heat stress conditions to determine at exactly what temperatures they start to shut down, as this can vary between different species and in different growing environments.

Note that humidity also plays a role in this process; if air humidity is very high, plants struggle to transpire enough water to help cool themselves, but if humidity is low the rate of water loss might be too high for the plant to sustain and wilting can rapidly occur.

Transpiration and temperature also have an indirect effect on the plant's root zone. Under hot and dry conditions, the rate of water loss via transpiration from the foliage can be high (particularly for those plants with large leaf areas). When this occurs, the plants draw water up faster from the root zone.

This higher proportion of water to nutrient uptake means salts can build up in growing media, and solution culture systems often see rapid increases in EC under these conditions. So, carefully monitoring the EC in the root zone becomes essential when plants are under this type of heat stress. Some plants are sensitive to this type of EC increase, so dropping the solution strength assists with water and calcium uptake under these conditions.

Dealing with high temperatures

Controlling the effects of high temperatures in an indoor garden when the outside air is no help with cooling requires an integrated approach. Firstly growers need to know the optimal temperature range at which their plants perform best. Many species such as tomatoes, cucumbers, melons and peppers are warmer season plants and require higher temperatures than cooler season crops like as lettuce and other salad greens.

However, the exact ideal temperature of a plant is also dependent on other factors such as light level, stage of development, health, humidity and any other stresses (such as root problems) that could be present. Many warm season plants have an approximate ideal temperature range of 68 to 82°F and cooler season plants a range of 53 to 75°F, with humidity in the range 70 to 75%.

If the incoming air is being heated outside and so is much higher than these optimal levels, the plants will often come under thermal stress.

Within an indoor garden there are a number of methods that can be used to cool the air and assist the plants cope with temperature stress conditions. First, many indoor gardeners take advantage of having a small, insulated space and use A/C units to cool the incoming air as it enters. This approach works fine, so long as a sufficient flow of air in and out of the garden still occurs to keep the environment healthy.

Another approach in areas where the outside air is not continuously humid is to use evaporative coolers. Evaporative cooling works by pulling air into a greenhouse or indoor garden through wet pads or even entire walls of running water (this is termed fan and pad cooling).

Water evaporates as air is pulled through the wetted pads, removing energy from the air and lowering the temperature in the process.

The cooled air is then circulated through and over the crop, cooling the growing environment until it is vented out the other side. For fan and pad cooling to work at maximum efficiency, it is important to keep the growing area as airtight as possible so that air being drawn in from outside is forced over the wet pads and does not come in through other openings where it won't be cooled.

Some smaller growers have made their own fan and pad system using capillary matting and similar materials; however, these pads can rot over time if they are used continuously (and therefore wet for most of the day).

Commercially designed pads for evaporative cooling are manufactured from material that often incorporates wetting agents and other compounds to resist rot. Algaecides can also be used in the water circulating through evaporative pads to prevent the buildup of algae on surfaces, which lowers the efficiency of the cooling pad.

Another method of evaporative cooling is using misters or high-pressure fog systems, which are more suited to larger growing environments where uniform cooling of a wider area is required.

Fog or mist systems use high-pressure nozzles to form fine water droplets, which absorb heat from the environment as they vaporize. Fog contains droplets of 0.05 to 50 microns and mist, larger droplets of 50 to 100 microns. Fog is a better option for most situations because it tends to fully evaporate before any droplets can fall on the plants and wet the foliage surface.

Wet plant surfaces encourage the development of many fungal and bacterial pathogens, and so should be avoided with any evaporative cooling system in use. That combined with the fact that humidity levels increase with evaporative cooling and plant transpiration is why (like with fan and pad systems) it is vital to have sufficient air ventilation and movement to circulate the cooled air and draw it out.

While evaporative cooling can be useful to cool the air temperature, it can also be combined with another technique that focuses on the root zone when the heat outside is really sweltering.

Root zone chilling with hydroponic nutrient solution is a technique used by many commercial growers in warm or tropical climates, and most often with cool-season crops like butter head lettuce, herbs and other vegetables.

Chilling the nutrient solution down to as low as 60 to 64°F allows the cool-season vegetables to crop well at ambient air temperatures that are often well above optimal for these crops (82 to 98°F). Without nutrient chilling, the root zone usually warms to the level of the air and this gives numerous growth problems, including slow growth, lack of heart formation, bolting, tip burn and low marketable yields.

Other researchers have reported that nutrient chilling of lettuce also reduces the occurrence of the fungal root disease Pythium aphanidermatum. However, trials have also shown that root zone cooling must be applied soon after early crop establishment and maintained for the life of the crop for maximum effect.

The first line of defense for any grower battling climatic extremes is to know the environment, both inside and out. While the indoor garden can seem insulated from the outside world, ambient temperatures will play a role in the requirement for cooling, while factors such as humidity often determine which methods of temperature control are best suited in each situation.

Letting a crop cook and fry under extremes of heat is stressful for both grower and plants, so planning for that mid-summer heat is well worth the time and effort