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Benefits of Controlled Environment Hybrid Greenhouses
Peter Cervieri

Sun grown cannabis is at the mercy of the environment while indoor growers can play God with plants, controlling every aspect of the grow to produce consistent, perfect looking flower. Greenhouse grows have historically been somewhere in the middle.

Greenhouse technology, however, is getting better and more competitive with indoor. At some point indoor will lose the battle with greenhouses as greenhouses become more high tech, offering growers unprecedented levels of control over the grow environment, without the costs associated with indoor lighting and climate control.

Indoor growers can control the grow environment and collect a lot of data that can be analyzed, which is a competitive advantage, especially for producing consistent pharmaceutical quality clean room type of medicine. Variables that can be analyzed and controlled include temperature, humidity, Vapor Pressure Deficit (VPD), Daily Light Integral (DLI), photoperiod, CO2, Dissolved Oxygen (DO) content of nutrient reservoir, among others.

Modern climate controlled hybrid greenhouses have all the environmental controls of an indoor environment, but with the added benefit of natural sunlight and lower cooling costs.

I sat down with Paul Golden, Stanford graduate (B.S. in applied physics and M.S. in Environmental Engineering), and engineer for Nexus Greenhouse focused on the cannabis industry, at the Cannabis World Congress & Business Expo in Los Angeles, to talk about modern greenhouse technology and growing best practices.

A modern greenhouse fully integrates a controller (the brains) with all the equipment in the structure. The main controller controls humidity, CO2, water, light levels, photoperiod (12-12) light cycles, temperature, cooling and can automate most of these functions.

greenhouse_controller

For example, a blackout system is built right into the greenhouse structure and can be automated through the environmental controls. When a grower moves to flowering and wants a 12-12 light cycle, the greenhouse controls can automatically close the light deprivation panels at a specific time and go into blackout. A smart system knows that the plants are still transpiring and the humidity levels will increase. When you go into blackout you close the shade system and reduce the volume of air in the greenhouse. The controller knows to raise the  air temperature so the plants, which are still radiating infrared radiation, to not reach their dew point, which might cause an outbreak of powdery mildew. The controller will pulse ventilate and heat fresh outside air, and keep the plant from reaching its dew point temperature as the shade system is closing. The goal is to ensure that the relative humidity stays stable.

light_deprivation_blackout_greenhouse

Dew-point temperature indicates the temperature at which water will begin to condense out of moist air. Condensation on plants occurs when the leaf surface temperature is below the dew point. This is when there is too much moisture in the air to remain in the vapor state. A properly set controller will ensure that the buds and leaves never reach the dew point temperature.  The controller will automatically heat and ventilate the greenhouses to reduce the amount of moisture in the air.

HVAC vs. Evaporative Pad System

A pad-wall system rarely costs more than a $1/ft2 per year to operate even in the hottest of climates, whereas HVAC systems in a warehouse environment can be multiple time more expensive to operate.  Every 3 watts of power used for lighting typically requires about 1 watt of HVAC to cool and dehumidify the warehouse environment. A warehouse cost about $10 per square foot in electricity for HVAC. A greenhouse costs about $1 per square foot to cool.

evaporative_pad_swamp_cooler

Swamp cooler / evaporative pad fan system: Air moves into the greenhouse through an insect screen. The temperature drops about 25 degrees and moves across from the pad wall to the exhaust fan side. Pad system is more energy efficient than an HVAC system.

evaporative_cooler_schematic

Lighting

A greenhouse controller will monitor and regulate the amount of light within the structure, as well as how much supplemental lighting is required. Nexus sees about 90% HPS installations, but LED installs are growing. Nexus has relationships with leading LED companies such as LumiGrow and Heliospectra.

LED lights give growers the ability to modulate the light output continuously from 0 micromoles up to 600 micromoles, instead of just being able to turn lights all on or all off like with HPS systems.

A warehouse environment costs about $15 – $20 in lighting, while a greenhouse environment costs about 1/3 to 1/4 of that, even in locations with poor sun conditions.

Light Levels
The plants ideal instantaneous light levels and daily accumulated light levels.

The plant can’t produce anymore growth once it receives 1,500 micromoles per meter squared per second of PAR (Photosynthetically Active Radiation). That’s the maximum PPFD (Photosynthetic Photon Flux Density). Photon flux is defined as the number of photons per second per unit area. In a greenhouse environment growers try to hit 1,000 micromoles per meter squared per second of PAR, which is light between the spectrum of 400 and 700 nanometers. Light outside that spectrum is generally wasted on the crops, as is PPFD of PAR – how many droplets of light are hitting a square meter right now – greater than 1,500 micromoles per meter squared per second.

DLI (Day Light Integral) is how much total light accumulates over the course of the day. 40 moles per day per meter squared seems to be the ideal DLI for cannabis. Use an Apogee light sensor to measure, at the canopy level, how much light is falling on the crop and supplement as needed to hit the target numbers of light hitting the plant per second and total accumulated light hitting the plant over the course of a day.

In a warehouse you typically get about 30 moles of PAR per day and in a greenhouse environment you’ll get about 40 moles per day of PAR.