Knowledge: The most high-tech equipment won’t matter unless you have a thorough foundation in how to grow cannabis. This means understanding when seeds should be sown (generally April), when flowering occurs (eight to 12 weeks typically), and which cannabis strains do best in greenhouses (autoflowering strains such as Purple Punch Auto, Jack Herer, and 8-Ball Kush enjoy a great reputation among cannabis growers).
If DIY is not your style, think about your budget and how many plants you’ll want to grow then decide how much space you’ll need for plants, climate control, and the rest of your greenhouse setup. Armed with that information, consider the options below.
When planning to build your own marijuana greenhouse, consider which material will create the ideal environment for happy plants. While glass is the traditional material, you can also use fiberglass or plastic. The latter two will be equally effective but not as fragile or expensive. Whichever material you choose, make sure you install proper ventilation to release trapped heat and let your plants breathe.
Whether you want to grow recreational weed or medical marijuana, the best greenhouse may be one that you build yourself. If you have the time and skills (or helpers) to construct your own greenhouse, this route will be the most cost-effective.
Whether it’s a simple walk-in greenhouse or something more elaborate, check local laws to see if building permits or other formalities need attention before you start building.
Free-standing greenhouses: From a simple walk-in greenhouse with room for a couple of plants to the massive connected behemoths set up by commercial growers, free-standing greenhouses are what most people picture when they think of a greenhouse. They are apex shaped, like a house, and consist of metal or wood frames and glass, fiberglass, or plastic panels. Many have roof and/or side panels that open to allow for ventilation. This may be the most expensive option, but it also has the most room for plants while still allowing for location flexibility so growers can move it to best capture the sunlight. Inexpensive free-standing models start at less than $100 but they may not be very durable. Sturdier models cost closer to $1,000 and prices go up from there.
Polytunnel: These elongated, dome-topped greenhouses have an aluminum frame and a covering of polythene. They are usually big enough to walk into but not as sturdy as actual greenhouses. Accordingly, polytunnels are on the lower end of the price range, starting at around $160. These can be durable but do need some maintenance, especially replacement of the cover every three to five years.
Building a cannabis greenhouse
Climate: In addition to protection from rain and wind, marijuana greenhouses can contain climate control equipment like dehumidifiers, exhaust fans, and heating and cooling systems. With the help of these tools, greenhouses can extend the growing season and allow for year-round cultivation.
Lighting: A greenhouse gives cannabis plants the benefit of natural light without exposure to the harmful environmental stressors of outdoor grows. Using natural sunlight is energy efficient, so you can save money on electricity costs by not using artificial lighting.
Greenhouses also provide cover for your plants, as heavy rains will damaged them and cause them to rot if too much moisture is trapped inside the buds. Some can also have dehumidifiers, heaters, air conditioners, and fans, all of which will also regulate the climate.
During the winter solstice in Seattle, for example, there’s less than nine hours of daylight, and the light that is available is low in the sky and poor for growing. This light and energy will still help heat the greenhouse, but you would need to use supplemental lighting to extend the hours of light in the day in order to get a quality product.
If you want to continue growing cannabis through the fall and winter, you will have to lean more heavily on supplemental lighting and heaters.
Once cannabis starts getting 12 hours of light or less, it’ll start flowering and producing buds. This happens outdoors at the end of summer, when days start to get shorter.
Weather and Climate Control
Solar radiation (energy from the sun) passes through the transparent walls of a greenhouse and heats up soil and plants, keeping a greenhouse warm even when the outside air temperature is cold.
Advanced greenhouses can allow you to grow year-round by controlling light. They can provide supplemental lighting when it’s too dark outside and they can block out all incoming light if it’s too light outside.
On a hot day, you experience the science behind a greenhouse when getting into a parked car that has been left in the sun.
Manipulating Life Cycles
A common practice among greenhouse farmers is to run cycles of plants known as “light deps”—short for “light deprivation”—during the summer season. By cutting off the amount of light a cannabis plant gets before the end of the season, you can trick it into flowering early. This will allow you to pull a crop early, which is key if you live in a climate that gets cold and wet early in the fall—you’ll want to harvest before the rain sets in and causes your buds to get moldy.
They also allow for year-round cultivation, climate control, and a controlled exposure to sunlight. What’s more, they’re cheaper than growing indoors and produce a more consistent product than fully outdoor grows.
Humidity and temperature affect each other. When it is warmer, warm air hold more water than cold air. For cannabis to grow nicely the temperature and humidity levels need to be in an ideal range for the plants to thrive. When the plants are in the seedling stage the humidity can be between 70% and 80%. Some growers will tell you that seedling and clones prefer the level to be from 65-70% humidity. The high humidity is necessary because the roots are not yet developed, and the plant will take in water through its leaves.
Considerations When Growing Cannabis in a Greenhouse
It’s tempting as a new cannabis grower to overwater your plants. Plants will droop and can drown and die in water. One simple way to know if the plants are dry is to stick your finder in the top inch of the soil and see if the soil sticks to your finger. If it does not stick, then it’s time to water.
pH Level for your cannabis in your greenhouse
Once the plant is in the vegetative stage, the humidity should be between 40% and 70%. The humidity needs to be lowered by 5% each week. The temperature can also be increased a bit because the roots are now absorbing more water, and evaporation is occurring through the leaves which cools the plant.