The most recent cannabis greenhouse in Colorado is a different variety than the hundreds of others built in the state since legalization in 2014.

Instead of being constantly ventilated, the automated cannabis greenhouse designed by Ceres Greenhouse Solutions is sealed off from outside air. Without ventilation, the greenhouse relies on a web of advanced climate control systems to control heat and humidity. The result is an indoor growing environment that combines natural light with precise environmental control and blurs the line between greenhouse and indoor grow room.

Open v. Closed Greenhouses

Traditional greenhouses are intensively ventilated to cool the structure.

In most North American climates, the air volume is exchanged two to three times per hour. That means every 20 minutes, the entire volume of air is flushed out and replaced with fresh, outdoor air. Even when fans are off, air exchange continues because greenhouses are notoriously leaky structures; gaps between the glazing (plastic) and frame leak air to the outside.

Airflow in a ventilated open greenhouse.

In short, the traditional greenhouse is largely an extension of the outdoor environment with some added climate control.

Closed greenhouses (sometimes called sealed greenhouses) aim to reduce, or nearly eliminate, air exchange in order to control indoor conditions.

The greenhouse usually has much “tighter” construction—air gaps between the greenhouse and glazing are sealed; construction methods are closer to commercial buildings. To better seal the structure, designers utilize hybrid greenhouse designs—blending vertical metal walls with a glazed (plastic) roof.

“Insulated metal panels are much more efficient and don’t shift under temperature swings like plastic does,” explained Josh Holleb, project manager for Ceres Greenhouses, which specializes in hybrid cannabis greenhouses. This creates a structure that is more energy-efficient and can be better sealed.

Instead of ventilation fans, a closed greenhouse requires automated cooling and dehumidification systems. That makes the design and engineering more complex, and usually expensive. Yet, due to their added benefits, particularly environmental control and odor control, more growers are turning to a closed greenhouse strategy. 

Airflow in a closed greenhouse.

Why Invest in a Closed Greenhouse?

Continual air exchange in standard greenhouses makes it difficult to prevent pests, molds and mildews from entering the structure. A closed greenhouse, in contrast, can be isolated from the outdoor air, reducing exposure to foreign contaminants. For cannabis producers, this reduces outbreaks of spider mites, powdery mildew and aphids.

Other growers are turning to a closed greenhouse not because of what the structure could let in, but what it may let out.

Odors from a grow operation are tightly regulated in urban areas. To accommodate, traditional greenhouse growers must install carbon filters in all exhaust fans. This reduces efficiency and creates a considerable operating expense, since filters must be replaced every few months. Plus, many growers struggle to create sufficient air exchange with filters and still abide by strict regulations.

A closed greenhouse resolves the odor issue by containing air inside. The greenhouse still has back-up ventilation fans with carbon filters, and of course, no structure is ever perfectly sealed. But in stark contrast to standard greenhouses, air exchange is minimized. This makes odor control practical for growers in regulated areas.

A final benefit of a closed greenhouse is preventing cross-pollination with hemp crops.

Hemp is normally grown in open fields, and the pollen can travel many miles, eventually entering a ventilated greenhouse and pollinating crops. The result is hermaphroditic, or “seeded,” cannabis. Though still technically cannabis, seeded plants cannot be sold directly to consumers. The crop loses most of its value, an immense financial hit for growers.

Sealed greenhouses eliminate the threat of cross-pollination, or at least reduce it to negligible. For growers in regions with hemp production, a closed greenhouse is essential to ensure the full value of the final product.

Environmental Control in a Closed Cannabis Greenhouse

When experienced growers hear of a closed greenhouse strategy, they typically fear an excessively hot and humid environment… a hotbed for pests and mildews.

Indeed, without ventilation, traditional greenhouses will drastically overheat during the day, like a car sitting in the sun. What growers may not realize, though, is that advances in climate control systems enable growers to regulate heat, humidity and carbon dioxide, while recycling the indoor air in a closed loop.

Many climate control systems mirror indoor grow rooms.

The traditional method for cooling is standard air conditioning. While effective, the high heat gain of a greenhouse makes air conditioning extremely expensive for most growers to rely on exclusively. The upside is that companies are quickly innovating to increase efficiency and lower operational costs.

Greenhouse designers are also borrowing from other industries cooling methods.

Cooling towers, commonly used in industrial applications, use an evaporative cooling system. The system is similar to a wet wall, but is housed in a separated unit, making it feasible for a closed greenhouse application. Cooling towers provide much more efficient cooling compared to air conditioning. The downside is a higher upfront cost, though it is paid back with lower operational costs.

A third strategy is storing excess heat from the greenhouse in a medium like water or soil via a heat exchanger.

The GAHT system from Ceres Greenhouse Solutions, for instance, is a ground-to-air heat exchanger specialized for closed cannabis growers. The system circulates hot air from the greenhouse through pipes buried underground, where heat is transferred to the soil. The air is then exhausted back into the greenhouse cooler and drier. Thus, the GAHT system creates a closed-loop airflow, which cools and dehumidifies the greenhouse.

The GAHT System

Humidity is another top challenge for closed greenhouses.

Again, the typical method for reducing humidity is ventilation, so closed greenhouse growers must become more creative. Dehumidifiers are the standard go-to. However, like air conditioners, they are energy-intensive to rely on full time.

Some growers are incorporating desiccant systems to supplement or handle the full dehumidification load.

These use a media, like a silicon gel, to adsorb water vapor from the air. A recent iteration of this is the Agam Ventilated Latent Heat Converter (VHLC), which combines a desiccant system with a compact cooling tower. The patented piece of equipment was originally designed for cold climate greenhouses—to reduce the need for intaking frigid external air—but it has found a new application in closed cannabis greenhouses.

Airflow in a closed greenhouse using a GAHT system for heat storage.

Is a Closed Greenhouse Right for You?

The closed greenhouse concept is the pinnacle of efficient and controlled indoor growing.

Theoretically, it combines the best of both worlds—the environmental control of a growroom with the natural light and energy savings of a greenhouse. While they offer greater control, they necessitate additional climate control systems, which generally increase expense. Thus, the design is well suited for growers who want to ensure a quality crop, often those growing a niche high-value product.

Additionally, for growers facing the recent challenges of odor control and cross-pollination, a closed greenhouse design is indispensable.

Though it requires some upfront engineering, creating a precise environmentally-controlled environment without ventilation is a surmountable challenge with modern growing technology. As climate control technologies continue to advance, closed cannabis greenhouses are likely to secure their place in the spectrum of cultivation options.

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