The chill of winter is in the air, which means it is time for indoor horticulturists to make the appropriate cold weather adjustments to their gardens. Growers who live in a climate with severe cold weather will have to make some substantial changes to ensure their indoor gardens continue to provide the proper conditions for accelerated plant growth.

Many factors determine how much the cooler weather outside will affect the climate of an indoor garden or greenhouse. For example, the way a ventilation system is set up can make a huge difference to an indoor garden’s environment during the winter months. If the fresh air intake for a garden’s ventilation system is drawing air directly from the outside, significant changes will need to be made when the outdoor temperature plummets. Even relatively sealed growrooms that use air conditioning to cool and maintain the temperature of a garden will need slight adjustments to operate at optimal conditions during the cooler winter months. Cold temperatures outside also present some unique opportunities to indoor growers and greenhouse horticulturists.

Geographical Location

The geographical location of a garden will be the ultimate factor determining how much the winter climate will affect an indoor garden or greenhouse operation. Growers who live in warm climates will have a few minor changes to make in preparation for the winter. However, growers who live in cold climates may need to make some major changes to their growrooms before the cold sets in. As an example, a grower in southern California will probably see a welcomed reduction in the garden’s cooling requirements during the winter months but not much else. On the other hand, a grower in Colorado or Michigan will need to adjust the way the growroom operates during the cold weather.

Configuration of the Ventilation System

The configuration of the ventilation system plays a considerable part in how much the cold winter weather will affect an indoor garden. Many growers rely on motorized fans to bring fresh air (containing fresh CO2) into the growing space. As the temperature outside drops, the way the ventilation system operates will need to be adjusted. Depending on a grower’s geographical location, there may be a large variance between the temperature of the air brought into the growroom during the summer months and the air brought into the growroom during the winter months. Growers with intake fans connected to an outside source should pay close attention to the garden’s operating temperature as the winter months approach. As the temperature outside falls, the amount of time it takes for the fans to effectively cool the garden’s environment will decrease. Growers who use cycle timers to control the ventilation system will need to reduce the operation time of the fans or the growroom temperature could become too cold. Even growrooms equipped with atmospheric controllers may need to have the parameters of the ventilation system adjusted so the operating temperature stays uniform. If a large amount of frigid air enters a growroom in a relatively short period of time, it may be difficult for the plants and the atmospheric controller to process. A lot of growers swear by using variable speed fans equipped with a speed controller. This allows the grower to control the speed of the fan and, therefore, the rate at which the outside air is brought in. Put another way, the grower can reduce the amount of air the fan intakes during cooler weather and increase the amount of air during warmer weather.

Air-cooled Reflectors

Ventilation systems with air-cooled reflectors may need to be altered for the winter months. Many growers can reduce or eliminate the amount of time the lights are actively air cooled. Some growers take advantage of the cooler air being drawn through the reflectors and install additional reflectors during the winter months. Increasing the number of light fixtures is a great way for a grower to capitalize on the winter air coming through a ventilation system. More light fixtures equate to more heat, but they also equate to higher yields. When the opportunity arises to add light energy without exceeding the desired operating temperature, a grower can, and should, seize that opportunity.

Grow-Room Heaters

Growers with ventilation systems that do not include air conditioning units with heat pumps may need to add a heat source to the indoor garden in winter. This is especially true during the dark cycle when the heat emitted from artificial lighting is no longer warming the garden environment. Propane or electric heaters are often used by indoor growers. Keep in mind that propane or any fuel-burning heater will emit carbon monoxide and other by-products that can become a concern in a small, enclosed area. Generally speaking, fuel-burning heaters are best used in large indoor gardens or greenhouses. Many hobbyist growers instead rely on electric heaters to maintain warmer temperatures during the dark cycle in the winter months.

Mini Split Air Conditioners and Heat Pumps

More sophisticated growrooms may be equipped with mini split air conditioners with heat pumps. These systems are rated according to the outdoor temperature range in which they can effectively operate. A grower in a colder climate should shop for a mini split air conditioner with a heat pump that is rated for their particular location. These systems can cool the garden while the lights are on and heat the garden when it is necessary, such as when the lights are off.

The placement of a mini split system or, more specifically, the placement of the condenser unit (the part of the system that is outdoors) is an important part of winter preparedness. Proper spacing and positioning will influence how well the condenser unit is able to circulate and draw air. Leaving enough space in the right areas will help prevent snow and/or ice from building up on the unit. It is best to elevate the unit, so it will stay above the anticipated snowfall level. Placing the unit under an overhang or building some sort of roof-like covering is a great way to shelter the unit from snow and ice. In extremely cold climates, it may be necessary to install a drain pan heater that rests on the drain pan and plugs directly into the control board on the condenser unit. This prevents water from freezing in the drain pan and protects the condenser unit from damage. If the condenser unit is located in a high wind area, a grower can install wind baffles to prevent the cold air from moving too quickly over the coils, which could cause the system to fail.

Two Flowering Rooms

One way a grower can increase efficiency during the winter is to set up two flowering rooms that exchange heat with each other. The basic concept is to have two flowering rooms that operate on opposite 12-hour light cycles. When the lights are on in Room A, the heat created is ducted into Room B, where the lights are off. When the light cycle switches, the ventilation system switches also, so the room with its lights on is always providing heat to the room with its lights off. Although electric heaters may still be needed as a backup source of heat, the heat produced by the lights will be sufficient to heat the other indoor garden in most cases.

The most effective way an indoor horticulturist can set up two flowering rooms in the winter is to have both gardens equipped with closed air-cooled reflector ventilation systems. Each closed air-cooled ventilation system is then ducted directly into the adjoining flowering room. As mentioned, the flowering room with its lights on will provide the heat for the garden room with its lights off. When the light cycle switches, the direction of heat transfer switches simultaneously. It is possible that too much heat will be pumped into the lights-off flowering room. As a fail-safe, an air conditioning system can be used or an exhaust fan (separate from the one used in the closed ventilation system) can be connected to a thermostat to evacuate the air if the temperature exceeds the desired range.

Flip-boxes are invaluable tools for growers operating two flowering rooms. A flip-box allows two lamps to be powered by one ballast (not simultaneously). The ballasts connected to a flip-box operate continuously. The ballasts’s power is then diverted by a relay contained within the flip-box. This relay is triggered by a 12-hour cycle timer. Every 12 hours, the flip-box engages and the light cycles of the two flowering rooms flip-flop. Aside from recycling heat from one growroom to another, flip-boxes allow a grower to have half as many ballasts as lamps. For example, a grower with an eight-light flip-box could operate 16 lamps off eight ballasts. This would allow the grower to set up two separate flowering rooms with eight lamps each.

Final Thoughts

Indoor growers should consider excess heat waste. In some northern climates, cold winter temperatures offer some relief to growers who struggle with unwanted heat. Depending on the garden’s ventilation system, growers may need to make some adjustments to the system when the outdoor temperatures take a dive. Some growers look at the cooler temperatures as an opportunity to add more lights to the growroom and capitalize on the additional light energy. Others see it as an opportunity to reduce cooling loads and save a little money on electricity. Perhaps the ultimate way for indoor horticulturists to create heightened efficiency in winter weather is to set up two flowering rooms and recycle the heat from one room to the other, eliminating the need for additional heat sources like electric or fuel-burning heaters. Depending on growroom’s geographical location, the winter season may mean significant changes for an indoor garden. Growers who take the proper precautions when setting up their ventilation systems or those who take advantage of cooler outdoor temperatures can hunker down, let it snow, and enjoy watching their plants continue to grow.