Combining the best of both worlds – which is to say, an indoor growroom and an outdoor garden – can have major benefits for cannabis farmers and their crops. This is because greenhouse growing is among the very best methods for the cultivation of any plant, combining precise and detailed garden control with the powerful benefits of our sun.
So what exactly is a greenhouse? Simply put, a greenhouse is a building specially constructed for growing plants under controlled conditions, but with a translucent ceiling and walls that allow natural sunlight to enter. In fact, in Europe, greenhouses are most commonly referred to as glasshouses, because glass is generally the material of choice used to build greenhouses there. Hence, the best part of outdoor cultivation – the sun and its powerful spectrum of light – is brought into an indoor environment.
Inside the greenhouse, all other conditions are established and maintained by the grower. While this is an ideal setup for optimizing a garden’s output, it also requires a great deal of work and management. Let’s start by taking a look at the different types of greenhouses available before getting into the more technical details of setting up your own personal greenhouse grow.
Greenhouses come in a variety of sizes and types to accommodate almost any space and suit each grower’s specific circumstances and needs. The three basic types of greenhouses are attached, detached and connected.
Attached greenhouses are simple and less expensive to design and construct since they’re always connected to another building, which cuts down on materials and costs (i.e., because one side of the future greenhouse already exists). The most common type of 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 detached or 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 detached greenhouse is not limited in its positioning by the need for a preexisting structure and can be built to reap the benefits of a given location’s natural advantages, such as sunlight, wind or water. Detached greenhouses are most commonly referred to as A-frames or even-spans, and these are the most common greenhouse designs. Other A-frames with asymmetrical rooftops and designs are less common, but more adaptable to hillsides or other rugged terrain.
An example of a less expensive, detached greenhouse with translucent sides and ceilings.
When multiple greenhouses are joined together, they create a multi-span or connected greenhouse. Connected greenhouses are ideal for large-scale commercial ops, where the greenhouse units are joined along the eaves to create a large, undivided space. This connected arrangement makes heating and cooling the gardens inside much more efficient and economical.
In addition to these larger, permanent greenhouses, there are a couple of easily set up, climate-controlled structures for growing plants on a smaller or temporary scale. Cold frames and hotbeds are two very popular types of greenhouse for the home grower or hobbyist. A cold frame is a structure of any size (usually made of inexpensive wood or plastic) that is covered with a translucent, heat-trapping material such as clear, thick plastic or thin plate glass. A hotbed is simply a cold frame fitted with any sort of heating system. Cold frames and hotbeds can both be as small as a large box or big enough for a human to walk into.
Cold frames are heated by sunlight, which keeps their operational costs low. However, they’re also less stable and not very durable in extreme weather; nor do they lend themselves to any sort of extensive environmental control equipment, as do the larger, more permanent greenhouse types listed above.
When we talk about environmental control inside a greenhouse, we’re referring specifically to temperature, humidity and air-quality control systems. Supplemental lighting is another possible component of environmental control. Depending on the type of greenhouse you choose, the options available for controlling the interior grow environment will vary greatly. Needless to say, a durable, stable greenhouse structure is required to incorporate industrial-sized equipment since smaller, less durable greenhouses will have trouble fitting or holding the larger gear. At the same time, larger greenhouses usually have some ventilation tools built directly into them – for example, low-velocity intake fans and exhaust vents in the rooftop.
This greenhouse utilizes an industrial-sized air conditioner (entire back wall) with huge industrial fans to keep GH temps cool.
Temperature is perhaps the most important aspect of environmental control in greenhouses, simply because of the nature of the structure. Greenhouses are built to allow sunlight in, thereby trapping the sun’s radiant heat (known as the “greenhouse effect”). This can be a good thing during colder seasons, but quite a nuisance during hot ones. Ideal daytime temps in a greenhouse usually run between 65°F and 80°F, depending on the strains (most strains prefer between 72°F and 78°F). Nighttime temps will generally be cooler and range from 55°F to 65°F, depending on the latitude of the greenhouse and the season.
Good ventilation helps prevent rising humidity. Smaller structures can roll up plastic siding to help vent moisture.
Modern greenhouse climate control is highly automated. Sensors can monitor all environmental conditions, especially temperature, and trigger mechanical operations to bring them to optimal levels. For example, when a greenhouse gets too warm, motorized windows or vents (usually located at the top, or ridge, of the greenhouse) may open to exhaust hot air from the garden’s environs. Large fans, either hanging or placed at ground level, may be used to circulate air within the greenhouse or to aid in venting hot air out. In some greenhouses, particularly those in warmer regions, sensors may be used to turn on large industrial air conditioners to lower the garden temperature.
Conversely, some greenhouses are constructed to protect against colder climates. In these situations, heating systems within the greenhouse may be needed to supplement the heat collected from the sun. Hot-water and steam systems are two popular ways for growers to heat the interior of a greenhouse.
Greenhouse lighting (i.e., light from sources excluding the Sun) usually refers to supplemental artificial lighting, just as one might find in a typical indoor growroom. Supplemental lighting in a greenhouse can be used for a variety of reasons, though it boils down to two basic functions. The first is the need to provide longer photoperiods for a seedling or clone nursery (or related mother plants), or to provide longer light cycles for transplanted or vegetating plants not yet ready to flower. The other is the need to garner extra annual flowering cycles once the Sun’s natural photoperiod becomes too short (primarily in the winter months or in extreme northern or southern locations).
The first case usually warrants an additional wall within the greenhouse 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 an indoor nursery or veg room would and may use less intense forms of artificial lighting, such as fluorescent lights or low-wattage metal halide bulbs. During daytime hours, when the Sun provides adequate light, the artificial lighting is either dimmed or turned off completely. This helps cut down on power costs and makes a greenhouse that much more efficient.
In the second case, where the goal is to add an additional flowering cycle (or two) 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 can naturally provide. In most areas (with the exception of those close to the equator), greenhouse growing is only viable from late spring through mid-fall – and in some less fortunate areas of the planet, these growing months can be cut even shorter.
Growers know that an extra cycle or two equals more harvests, so savvy greenhouse growers take advantage of their structures to hang 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 cannabis flowers. This supplemental lighting not only adds an additional crop to their yearly total, but it’s also more economical than going with a standard indoor grow, because the greenhouse will still get a few good hours of sunlight, saving the grower a chunk of money on additional kilowatt hours.
With the increasing popularity of medical marijuana during the past decade, the use of light deprivation techniques for cannabis greenhouses has become widespread in the US, though such practices have been in use in other forms of agriculture for centuries. For those new to the concept, 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 integral to ensuring 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 cannabis plants are in danger of reverting back to their vegetative stage of growth. This makes light deprivation techniques crucial for cannabis farmers who rely on timely crops.
One added benefit of using light-dep techniques lies in the ability to squeeze in an extra harvest during the summer months, when the Sun naturally provides a prolonged light cycle. By covering the greenhouse and limiting the daylight that plants receive to 12 hours, greenhouse growers can force a crop to flower earlier in the summer and get an extra harvest in before the traditional fall harvest.
Huge, black, opaque tarps are rolled up during peak daylight, then unfurled to cover the greenhouse after 12 hours of sunlight to force flowering.
To cover a greenhouse and block out the sunlight, several methods may be used. For smaller greenhouses, such as lean-tos and cold frames, growers can simply pull heavy tarps over the structure after 12 hours of sunlight. For larger commercial greenhouses, automated shading is usually installed and operated using programmable timers. The latter option is preferable even for smaller greenhouses because it helps to eliminate any human error during the sensitive flowering stage. After all, the last thing cannabis farmers need is light hitting their plants after the 12-hour photoperiod is complete, thus triggering them to revert back to veg. Such light stress confuses plants and inhibits their growth and vigor, and it may even cause them to go hermaphrodite and produce smaller seeds. If a grower misses the time to manually cover the greenhouse – even by a few minutes – it can have disastrous effects on his or her cannabis crop.
Today, there are many options for greenhouse farmers who want to utilize light-dep techniques for an extra harvest. Aside from automated shades, there are certain types of glass that can automatically tint and become opaque to block out light. There are also entire greenhouses of varying sizes available with retractable roofs to ensure a complete light seal and a proper light-dep crop.