An indoor garden is the ultimate retreat for those who love warmth, bright light and green, leafy surroundings in areas with cold, dreary winters. But creating that perfect, productive environment for an indoor hydroponic system is not always a simple task when the world outside is experiencing extreme temperature fluctuations.

While indoor gardens are protected against outdoor temperature extremes, fresh air is still required to remove humidity and replenish CO2 for photosynthesis. Air intake and general outdoor temperatures affect climate control within the indoor growing environment, and growers need to monitor and adjust these conditions to maximize plant growth and yields.

In the past, the main concern with insulated indoor gardens was heat removal, as HID lamps raise temperatures significantly, often above optimal in the summer. Night temperatures have always been a concern in areas with cold winters, as once HID lamps are switched off, mid-winter temperatures in these climates can plummet.

With more growers using efficient, cool-running lamps such as LEDs, cold winter conditions have become more of an issue both day and night, and additional heat sources are often required to maintain temperatures.

The Dangers of Temperature Swings in an Indoor Garden

Hydroponic systems are thriving worldwide in incredibly diverse climates, from ice-bound Antarctica, to the hottest desert locations, and everywhere in between. While outdoor climates may differ significantly, temperature requirements don’t, meaning some form of environmental modification is required in most indoor gardens.

Exposing plants to temperature extremes can be fatal. Plants, like humans, have little tolerance for conditions outside the 32-113oF range, while the optimal range for many of the plants we grow is between 61 and 82oF.

Cold damage is not always easy to recognize, and many growers may initially misdiagnose it as a disease, nutrient deficiency or toxicity issue instead. The most common scenario is temperatures crashing overnight, inflicting mild cold injuries on sensitive plants. Symptoms include slight wilting, leaf curling, discoloration or a water-soaked appearance with spots of damaged and discolored tissues.

More severe damage may not show up for several days, making it even more difficult to diagnose the cause. A severe freeze usually causes far more damaging symptoms on warm-season plants. As the water inside plant cells turns to ice, it expands, destroying the cellular structure and resulting in widespread foliage browning and even plant death.

High temperature extremes pose a different set of problems, which can be just as damaging as cold conditions. Plants can cool themselves to a certain degree via the process of transpiration.

As water vapor is lost from the stomata, it reduces the temperature on the leaf surface. However, 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.

Once this happens, no CO2 can diffuse into the leaf for photosynthesis and this process shuts down until conditions improve, meaning no sugars for growth are being produced. Coupled with a high rate of respiration, a lack of photosynthesis for some, or much of the day, means plant growth can completely stagnate. Early signs that heat stress is occurring include mid-day wilting, although this can also be caused by other problems such as root rot.

A simple way to determine if plants are under heat stress is to use an infrared thermometer, which non-invasively measures leaf surface temperature. 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, then the stomata have shut and no transpiration, with its associated surface cooling, is occurring. This is a great way to monitor plants under heat stress conditions to determine at what temperatures they start to shut down, as this can vary between species and in different environments. At even higher temperature extremes, plant tissues can literally cook, and plants rarely survive.

Growroom Design for the Seasons: Different Weather Conditions

Indoor gardens can be designed with the local climate in mind to help control and maintain temperature levels. For example, insulation helps slow the rate of temperature increases or decreases over time and helps maintain the carefully controlled conditions within.

Second, the space the indoor garden takes up is important: floor space may be limited, but higher ceilings allow for more air to be maintained, which slows the rate of temperature changes.

Where cold outdoor air temperatures are known to be a significant problem, designing a system that allows the air to be warmed (and humidified, if necessary) before it is introduced into the growing area is one way to obtain a more even and uniform temperature control.

Finally, where heating is required, consideration of the type, running cost and efficiently of heaters is important. Indoor gardens can use household floor heating or central heating, or in more isolated structures, electrical fan heaters, oil-filled radiators and gas heaters have all been used.

Under extreme cold conditions, more than one heating system is often required to get even temperature control, particularly at night, and air mixer fans are essential to distribute the warm air evenly through the canopy.

The Cold Season

Winter brings some unexpected risks for many indoor gardeners. While it may seem like a simple task to provide some form of heating to keep the hydroponic system up and running, inexperienced gardeners often make the mistake of trying to economize heating costs by closing up the indoor garden and preventing or severely restricting air exchange with the outside.

This quickly creates a humid, stale atmosphere in which CO2 can become depleted within a few hours, and once this occurs, growth stops, irrespective of how warm it is. Fresh air is a requirement, even if that air needs to be continually warmed.

A second issue with winter temperature extremes is humidity. In many climates, winter is not only the coldest, but also the driest time of year, so humidity may be far lower than is optimal for a hydroponic garden. While growing plants transpire and increase the humidity in the air, when they are still small and the outside air is dry, humidity levels may need regular adjustments.

This problem is further compounded when cold, dry air being drawn into an indoor garden is then warmed, as warmer air holds more moisture than cooler air, dropping the relative humidity even further once the temperature has increased to optimal levels. It is not unusual for this air-warming effect to drop the humidity significantly, down to levels as low as 20-30%, well beneath the optimal levels of 60-65%. When heating cold, incoming air, checking the humidity and adjusting it with a humidifier if necessary is important.

The Warm Season

In a well-insulated indoor garden using HID lights, temperature buildup is a common problem when the lights are on. Heaters with air mixer fans are relatively simple tools to warm the air, but heat removal, particularly in warm climates and during hot summers, requires additional equipment. Simply venting out the warm and drawing in cooler air is not an option when it’s sweltering outside, especially if that heat is combined with high humidity. In this situation, growers have two options.

If the outside air is not humid, evaporative cooling can effectively lower temperatures. As water evaporates into the air, energy is lost, which causes the air temperature to fall. Common evaporative cooling methods include fan and pad systems, or evaporative cooling walls in greenhouses, where air is drawn through or over wet, absorbent pads. The cooled air is then circulated through plants, lowering temperatures until it is vented out the other side. In an indoor garden, for fan and pad cooling to work at maximum efficiency, the grow area needs to be as airtight as possible so that air drawn in from outside is forced over the wet pads.

In humid climates, evaporative cooling cannot be used and air-conditioning units are a good alternative, which also provide a higher degree of temperature reduction. Air conditioning produces relatively dry air, so the air may need to be re-humidified with either humidifier units or wet pads and open pans of water before being circulated over plants.

Indoor garden design in warm climate areas should factor in features such as air conditioning, but also equipment that doesn’t add to the unwanted heat loading of the hydroponic system. LEDs and other cool-running lamps, or water- or air-cooled lighting systems, can significantly reduce temperature levels in a small, enclosed area, putting less pressure on hot air removal or chilling via air conditioning.

Temperature Extremes in the Root Zone

Temperature not only affects the rate of transpiration, photosynthesis and respiration, but it also has an indirect effect on the root zone. When it is hot and dry, the rate of water loss via transpiration from the foliage may be high, especially for plants with large leaf areas, so plants draw water up faster from the root zone, which can concentrate the EC.

A higher proportion of water-to-nutrient uptake results in salt buildup in the grow media, and solution culture systems often see rapid increases in EC under these conditions. When plants are under this type of heat stress, dropping the EC to account for the higher rate of water uptake, and carefully monitoring EC in the root zone, becomes essential. Dropping the solution strength also helps with water and calcium uptake under these conditions.

Apart from keeping a close eye on root zone temperatures to make sure they are not cooking the roots, there is a well-proven technique that can fool many plants into handling higher-than-optimal air temperatures. Many commercial growers in warm or tropical climates chill the nutrient solution, most often for cool-season crops such as lettuce. Chilling the nutrient solution down to as low as 61-65oF allows cool-season vegetables to thrive in air temperatures that are well above optimal.

Dealing with outdoor temperature extremes in an indoor garden involves designing a well-insulated, correctly equipped environment to provide the right level of heat generation, air removal, cooling systems and monitoring systems to ensure plants are protected and encouraged to perform at optimal levels throughout the seasons.