Greenhouse growing is all about combining the best of both worlds of indoor growing and outdoor gardening, which can have some major benefits for all concerned. This is because greenhouse growing is considered among the very best methods for the cultivation of any plant, combining precise and detailed garden control with the powerful benefits of our sun. The Pros and Cons are as follows:

Pros

• Usually produces a higher quality product vs. outdoor and almost equal to indoor gardens
• Less expensive per square foot than indoor grows
• Building structure falls under both commercial and agricultural zoned areas
• Earlier start than outdoor growing
• Controllable environment
• Protection from the elements
• Less exposure to pests
• Easier to control smell
• Slightly more concealed and secure vs. an outdoor garden

Cons

• Higher cost than an outdoor grow
• Additional maintenance necessary
• More exposed than growing inside
• Extreme temperatures can make it difficult to work inside
• Requires a ventilation system
• Humidity control is a must
• Pests must be controlled (once introduced)
• Slightly reduced sunlight
• Limited space compared to outdoor gardens

So what is a greenhouse? A greenhouse is a building especially constructed for growing plants under controlled conditions, but with a crystalline ceiling and walls that allow sunlight to enter naturally, the result being that the best part of outdoor cultivation – the sun and its powerful spectrum of light – is brought into an indoor environment.

Inside the greenhouse, all conditions can be established and controlled by the grower. This can be an ideal setup for maximizing a garden’s output, but it also requires a great deal of labor and time management. So again, the first question to ask is how much time and labor is to be dedicated to the garden? Let’s take a look at the different types of greenhouses available before getting into the more technical details of building your own greenhouse grow.

Greenhouse Types

Greenhouses come in a multitude of sizes and types to accommodate almost any space and satisfy each grower’s specific circumstances and needs. The three basic types of greenhouses include: attached, detached and connected.

Attached greenhouses share a wall with a residence or an existing building, traditionally built off the back of the building, but can be built on the side depending on the orientation of the building. The most popular type of an attached greenhouse is the lean-to, in which the greenhouse roof is attached to the preexisting building’s wall and then slopes away, making it look as though the greenhouse is leaning into it.

A freestanding greenhouse is exactly what the name implies, a stand-alone structure with four discrete walls and a ceiling. The advantage here is that a freestanding greenhouse is not limited in its positioning by the need for a preexisting structure. It can be built to reap the benefits of a given location’s natural advantages, such as sunlight, wind or water. Freestanding greenhouses are most commonly referred to as A-frames or even-spans, and these are the most common greenhouse designs. A-frames with asymmetrical rooftops and designs are less common, but more adaptable to hillsides or other rugged terrain.

A multi-span or connected greenhouse are multiple greenhouses that are joined together. Connected greenhouses are ideal for large-scale commercial operations, where the greenhouse units are joined along the eaves to create a large, undivided space. This arrangement makes heating and cooling the gardens inside much more efficient and economical.

In order to grow plants on a smaller or temporary scale, there are a couple of easy set up, climate-controlled structures. Hotbeds and cold frames are two very popular types of greenhouses for the home grower. A cold frame is a transparent-roofed enclosure, usually composed with glass, built low to the ground, used to protect plants from adverse weather, primarily excessive cold or wet. A cold frame fitted with any sort of heating system is known as a hotbed. Cold frames and hotbeds can be as small as a large box or large enough for a human to get into.

Cold frames are only heated by sunlight and are subject to the whims of the environment, which keeps their operational costs low. Cold frames do not have the capacity to enable any sort of extensive environmental control equipment. They are also not very durable in extreme weather and not as stable as the more permanent greenhouse types discussed above.

Environmental Control

When addressing environmental control inside a greenhouse, we are referring specifically to humidity, temperature and air-quality control systems. Supplemental lighting could be another component of environmental control. The greenhouse that you choose is going to determine what options will be available for controlling the interior grow environment. A durable, stable greenhouse structure is required to incorporate industrial-sized equipment since the smaller, less durable greenhouses will have trouble fitting or holding the larger gear. In addition, larger greenhouses usually have some ventilation tools built directly into them like low-velocity intake fans and exhaust vents in the rooftop.

Soil temperature is not something that is often mentioned when it comes to cannabis cultivation. Indoor growers do not need to be as concerned as outdoor growers, who are at the mercy of the elements. Soil temperature can determine how quickly the nutrients are transferred and how slowly growth can occur, with no growth occurring below 45°. Root mass will determine growth and how big the harvest will be.

Temperature is perhaps the most important aspect to control in greenhouses, simply because of the purpose of the structure. Greenhouses are built to allow sunlight in, thereby trapping the sun’s radiant heat, which is known as the “greenhouse effect.” This can be a benefit during colder seasons, but quite a problem during hot ones. The range of ideal daytime temps in a greenhouse is usually between 65°F and 80°F, with a majority of strains preferring between 72°F and 78°F. Nighttime temps are generally cooler and range from 55°F to 65°F, depending on the geographic location of the greenhouse and the season.

With temperature comes humidity, the two work together in any greenhouse. Humidity is the relative amount of moisture in the air. Most plants, including marijuana, do well with a level of 60%. Lower levels of humidity can stress the plants resulting in lower quality products and reduced yields. The changing seasons create variations of humidity, with winter months being dryer due to the use of heaters to maintain temperatures.

Technology has enabled modern greenhouse climate control to be highly automated. Sensors are used to monitor all environmental conditions, especially temperature, and trigger mechanical operations to bring all conditions to optimal levels. For example, when a greenhouse gets too warm, motorized windows or vents that are usually located at the top, or ridge of the greenhouse, can be opened to exhaust hot air from the garden area. Large fans are used to circulate air within the greenhouse or to aid in venting hot air out. In some greenhouses, particularly those in warmer regions, large industrial air conditioners are used to lower the garden temperature.

Greenhouses in colder climates use heating systems within the greenhouse to supplement the heat collected from the sun. To prevent the stressing of plants, hot-water and steam systems are two popular ways for growers to heat the greenhouse.

Supplemental Lighting

Greenhouse lighting usually refers to supplemental artificial lighting, just as one might find in a typical indoor grow. Supplemental lighting in a greenhouse is used for two basic functions. The first is the need to provide longer photoperiods for a seedling or clone nursery and to provide longer light cycles for transplanted or vegetating plants not yet ready to flower. The second is the need to accumulate extra annual flowering cycles once the Sun’s natural photoperiod becomes too short, which means higher yields for a greenhouse grow.

In the first case, an additional wall within the greenhouse is used to cordon off a vegetative area with extended light periods from areas that have flowering cannabis plants requiring no more than 12 hours of light. These areas function just as a veg room or indoor nursery would and may use less intense forms of artificial lighting, such as fluorescent lights or low-wattage metal halide bulbs (to be covered in ‘Lighting’). During daytime hours, the artificial lighting is either dimmed or turned off completely. This cuts down on power costs and makes a greenhouse that much more efficient.

When the goal is to add additional flowering cycles to a greenhouse’s annual run, the supplemental lighting follows along the same lines as extended lighting for a veg room or nursery – but in this case, the additional light extends the grow season beyond what the Sun naturally provides. In most areas away from the equator, greenhouse growing is only viable from late spring through mid-fall. In some less fortunate areas of the planet, the growing months can be cut even shorter.

Growers know that an extra cycle or two equals more yield, so the savvy greenhouse grower will take advantage by hanging high-intensity discharge (HID) lamps over their gardens. When the Sun’s daylight begins to wane to 10, or eight, or even six hours a day, growers can switch to their HID lighting to complete the 12-hour photoperiod for marijuana flowers. This supplemental lighting not only adds annual crop yield, but it is also more economical than going with a standard indoor grow, because the greenhouse will still get a few good hours of sunlight, providing the grower an incredible savings on the power bill.

Light Deprivation Techniques

With the legalization of cannabis becoming more popular during the past decade, the use of light deprivation techniques for marijuana greenhouses has become widespread in the US, though such practices have been in use in other forms of agriculture for centuries. Light deprivation, or “light dep” for short, is a method of reducing the light cycle of flowering plants in which the greenhouse is actually covered to block light, thereby depriving gardens of a prolonged photoperiod.

For cannabis cultivation, this process is necessary to ensure that plants stay in the flowering stage – a growth phase that requires 12 hours or less of light. Once the photoperiod begins to exceed 12 hours, flowering marijuana plants are in danger of returning back to their vegetative stage of growth. This makes light dep techniques vital for cannabis farmers who rely on timely crops.

An added benefit of using light-dep techniques lies in the ability to squeeze in an additional harvest during the summer, when the sun naturally provides a prolonged light cycle. Limiting the daylight that plants receive to 12 hours by covering the greenhouse, greenhouse growers can force a crop to flower earlier in the summer and get that extra harvest in before the traditional fall harvest.

There are several methods that may be used to cover a greenhouse and block out the sunlight. For smaller greenhouses growers can simply pull heavy tarps over the structure after 12 hours of sunlight. Automated shading is usually installed on larger commercial greenhouses and operated using programmable timers. The latter option is also preferable for smaller greenhouses because it helps to eliminate any human error during the sensitive flowering stage. The last thing marijuana farmers need is light hitting their plants after the 12-hour photoperiod is complete which will trigger them to revert back to a vegetative state. Such light stress confuses plants and inhibits their growth and vigor, and it may even cause them to go hermaphrodite and produce smaller seeds. Missing a crucial blackout time could result in a total loss for the crop.

There are many options for greenhouse growers who want to employ light-deprivation techniques for an extra harvest. Along with automated shades, there are certain types of glass that can automatically darken to block out light. There are also greenhouses of varying sizes equipped with retractable roofs to ensure a complete light seal and a proper light-deprivation crop.