cannabis lighting in a Prospiant greenhouse

7 Considerations for Optimizing Cannabis Cultivation in a Greenhouse By Mark Reich

Designing and building your cannabis cultivation facility is more than just the structure itself. Making sure the growing environment you choose is ideal for your crop is equally important; it takes careful, consistent effort in your day-to-day operations to ensure that you provide the right environment at each stage of the plant’s lifecycle.

The temperature and humidity of your greenhouse or indoor grow space, also needs to be constantly kept within the desired ranges through ventilation, lighting, insulation, irrigation, heating, and cooling systems.

Other than choosing the right growing environment, efficiencies can be improved by integrating and automating all these internal systems and reducing energy costs while producing the high returns from quality crops year-round.

Some of the critical growing equipment choices you’ll need to make will be integral to your actual greenhouse or indoor grow structure, while others will need to be able to move around easily to ensure that your growing space is flexible enough to meet the demands of your business. Here are seven areas to consider when planning your cannabis facility:

1. Cooling

Greenhouse ventilation is one of the biggest influences in maintaining an ideal climate for cannabis cultivation. A reliable ventilation method in many areas is a traditional fan and pad mechanical system—still common in areas that have temperatures and humidity conducive to evaporative cooling.

These systems will exhaust the warm air through fans and provide a cooler environment by pulling air through a wet wall and lowering the outside air temperatures. The wet wall is covered by a motorized vent or shutters.

The ventilation process involves horizontal air flow moving throughout the greenhouse from the evaporative pads to the exhaust fans. Fan cooling produces air flow through and around the plants, which reduces disease and assists in the growing process.

Natural ventilation allows for an inexpensive method of cooling. With the use of roof vents, the clear opening to the sky quickly allows the hot air to escape, similar to a chimney.

Side inlet vents and shade curtains lead to temperatures lower than outside temperatures. Natural ventilation may not maintain the required temperatures for good plant growth during the hottest times of the year.

Positive pressure cooling (an alternative to fan and pad cooling) pushes air through an evaporative cooling system, which forces the air into the greenhouse.

Exhaust vents must be designed to allow air pressure to stabilize while still creating higher air pressure inside the greenhouse than outside. Due to this internal air pressure, insect infiltration through greenhouse openings is further restricted and must be taken into account.

 2. Heating

 One traditional method to heat greenhouses is with overhead gas-fired heaters, which blow warm air into the greenhouse environment. The heated air is then mixed throughout the greenhouse by use of horizontal air flow (HAF) fans to circulate heat to the needed areas within the greenhouse.  These heaters are highly efficient, considering the inexpensive capital cost, ease of installation, and long product life.

Hot water radiant heat can be used in combination with an overhead heating system or as a standalone system. This efficient hot water system places the heat closer to the plant through piping under the benches or embedded in the greenhouse floor.

The proximity of this system’s heat distribution to the plant helps in plant growth and uniformity and decreases humidity at plant levels, which reduces disease.

These systems will have higher capital costs yet provide a return with money saved through operational costs. If the greenhouse area is broken up into multiple zones, the system can be designed to accommodate different environments for various crops for greater efficiency.

3. Environmental Control

Effective climate control is a crucial step toward producing any greenhouse crop, particularly cannabis. The environmental climate control system’s design needs to consider outside conditions to create the best possible inside environment for plant growth. Greenhouse ventilation is the factor with the most influence on climate control.

Controlling the internal greenhouse climate is usually highly automated. Sensors will monitor various conditions, such as temperature and humidity, which trigger mechanical systems to bring those conditions to a set point. For example, if a greenhouse gets too warm, motorized windows or vents will open in combination with exhaust fans to expel the hot air. Environmental control systems can be simple thermostats or sophisticated computer controls.

By setting parameters, efficient cycling of equipment through various stages can achieve the desired environment. Specific controls run ventilation equipment, watering systems, or alarm warnings to correct problems in single or multiple zones. Owners should contact local utility companies regarding rebate programs to help pay for control systems to reduce energy usage.

4. Supplemental Lighting

One of the major benefits of cannabis production in a greenhouse versus indoor growing is the collective ability to capture and utilize Mother Nature’s gift of sunlight. Greenhouse cannabis production maximizes the available natural light for plant production and reduces capital as well as operating costs of lights required for indoor growing.

Throughout the year, supplemental lighting is needed in a greenhouse. A few of these reasons include:

  • Supplement cloudy and lower natural light days (winter, northern latitudes)
  • Separate growing environments require varying degrees of light
  • Guarantee consistent light levels throughout the year

Typical lights used in a greenhouse are high-pressure sodium and metal halide fixtures. LED lighting technology is currently used and tested in a variety of ways. This lighting is presently most effective on a smaller scale, where a few fixtures enhance growth on a large quantity of smaller concentrated plant areas.

For large-scale use, the experimentation continues. Fixture costs are still too high to be offset by plant benefit and operating costs. Future advances in technology may make this method of lighting comparable to more traditional lighting methods with the potential to outperform those methods.

5. Light Deprivation

 With the emerging popularity of cannabis growing, the use of light-deprivation techniques for cannabis greenhouses has become commonplace. These methods have been an important part of other agricultural practices for hundreds of years. Light deprivation decreases the lifecycle of plants and induces flowering.

For cannabis growers, this process is vital to keeping plants in the flowering stage. When the photo period exceeds 12 hours, cannabis plants will not flower and will remain in their vegetative stage. Inducing flowering through light deprivation is a vital process in the cultivation of cannabis, and growers rely upon this process and equipment to ensure timely crops.

A light-deprivation system features retractable black-out curtains, metal side and end walls, and light traps/breathable walls over motorized inlet shutters and exhaust fans or evaporative coolers. This system produces adequate horizontal air flow while creating a dark environment for the plant. To cover a greenhouse and block out the sunlight, several light-deprivation methods may be implemented.

For smaller greenhouses, including lean-tos and cold frames, growers can pull heavy tarps over the structure. With larger commercial greenhouses, automation with programmable timers is the best option. Automation also reduces human error in any size greenhouse.

6. Benching

The use of greenhouse benches has several advantages over planting directly into greenhouse soil or setting pots on the ground:

  1. Increases growing space by eliminating aisles
  2. Convenient heights to work comfortably
  3. Permits a more effective display of plants
  4. Improves air circulation and  environmental controls
  5. Contributes to better disease and growth management

Benches use a steel or aluminum perimeter rail along with steel cross tubing for a strong top and leg system. These benches are either stationary or use a rolling bench top.   The rolling bench contains a single floating aisle to allow for more bench square footage inside the greenhouse. These benches are strong and durable enough for heavy potted crops. Bench tops can be covered with different coverings, depending on owner preferences.

Palletized rolling benches (often called Dutch trays) are useful in large growing operations when benches can take the place of a material handling system. Individual bench tops move along supports in dual directions (perpendicular and parallel) to the legs from the head house to the greenhouse and back to the head house for shipping. These systems increase efficiency and decrease labor costs.

7. Irrigation & Fertigation

Caring for hemp and cannabis plants requires considerable (manual) effort daily. However, manual watering systems are unable to provide consistency and homogeneity, which in turn produces inconsistent product and yield. Automating irrigation cycles enables growers to more efficiently achieve the correct levels of root saturation and provide properly balanced nutrients for the cannabis plants’ continuous growth, all while reducing water and labor costs.

Benches are often outfitted with drip irrigation tubing and stakes that provide precision irrigation to cannabis plants. Drip irrigation is used for its efficiency, accuracy, and low cost.

Overhead watering is not recommended for cannabis as it increases ambient humidity and wets the leaf, increasing the likelihood of disease forming on water droplets.

Drip irrigation delivers ‘pulses’ of water where cannabis plants are provided low flow streams of water until they reach a desired media moisture content. When fertilized water flows from the drip stake it immediately enters the plant’s media and does not wet the leaf or stem. Pressure compensated emitters ensure all plants receive the same amount of water in each zone.

Efficient drip irrigation systems use 75% less water than hand watering or overhead spray. Because the water is targeted directly at the roots the amount of runoff can more easily be controlled. This reduces costs for water management and drain systems.

Water treatment is commonly used on the front end where water enters the irrigation systems from well, surface, or municipal supply. Carbon and sediment filters are a necessity and reverse osmosis is often used to provide a clean slate, which is helpful for standardization across facilities in different locations. A water analysis can determine what treatment options are needed and can impact what fertilizer recipe one uses.

Fertigation units are used to automate one of the most laborious and technical aspects of growing cannabis. These units pull from 2-4 fertilizer concentrates along with a pH adjustment and sanitizer. Fertigation units measure the irrigation solution’s pH, EC, and flow rate.

Mark Reich

Mark Reich is a systems engineer at Prospiant. He takes conceptual cultivation ideas for customers and develops high level design for all environmental equipment. He enjoys working with customers to solve complex issues related to environmental controls and water treatment.

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