How To Build the Best Greenhouse for Growing Marijuana

Growing weed in a greenhouse combines the best of both indoor and outdoor growing, allowing you to exploit the powers of natural sunlight without sacrificing control over your growing environment.

If you’re lucky enough to have the space to set up a greenhouse, keep reading for an in-depth guide on how to build the perfect greenhouse for growing cannabis.

Why Build a Greenhouse for Cannabis?

The main benefits of growing weed in a greenhouse include:

• Less energy consumption: Growing in a greenhouse allows you to utilise sunlight, meaning you won’t need to face the electricity costs of running a grow light for 18 hours per day.
• Environmental control: Unlike growing outdoors, where your plants are completely exposed to the elements, growing in a greenhouse gives you added control over your plants’ environment, allowing you to optimise temperature, air circulation, humidity, and more.
• Better potency, aroma, and flavor: The strength of your weed and how good it smells/tastes will vary greatly depending on the genetics you’re growing and your skills as a grower. However, greenhouse-grown bud has earned itself a notable reputation for its potency, aroma, and flavor, likely because of the control this environment gives growers over the quality of their crop.
• Greater pest control: When growing cannabis outdoors, it's impossible to entirely eliminate the risk of running into pest problems. Growing in a greenhouse, however, you’ll be able to significantly minimise the risk of pests, like you would in an indoor grow room.

How To Build a Greenhouse for Growing Cannabis

Building a greenhouse is no easy task and involves addressing a ton of different variables that’ll be unique to your situation (such as the amount of space you have to work with, the materials you have access to, your budget, your DIY skills, etc.).

For the sake of this article, we won’t address all of these variables in detail. However, we will walk you through the main aspects of building a cannabis greenhouse as best we can.

1. Picking the Right Type of Greenhouse

Greenhouses come in countless shapes, sizes, and designs. And, while they can all be used to grow weed, choosing one over another will come down to your budget, the number of plants you're planning to grow, and the space you have to work with.

Below are some of the different types of greenhouses on the market and their pros/cons.

• Small pop-up greenhouses: These are the most budget-friendly greenhouses on the market and are usually designed for hobby gardeners working with limited space. They typically feature a basic frame, a plastic cover, a few shelves to hold your plants and gardening tools, and a zip-up door for easy access. The cheapest option on the market, these types of greenhouses sell for as little as £30 on Amazon.

• Walk-in greenhouses: These greenhouses are a step up from those we mentioned earlier. While they are usually still made with basic metal frames and plastic covers, they offer a lot more room, bigger shelves, and the ability to step inside and immerse yourself in your gardening. Walk-in greenhouses of this type come in a variety of different sizes and styles, and are usually designed for hobby gardeners with a little more space (such as an outdoor patio, private terrace, garden, or basement). While they come at very different price points, basic walk-in greenhouses sell for around £50.

• Polytunnel greenhouses: Polytunnel greenhouses are tunnel-shaped and one of the most popular greenhouse structures used in commercial production. Due to their size, polytunnel greenhouses are typically designed for outdoor use, and therefore also tend to feature better build quality. A solid polytunnel greenhouse may set you back up to £200.

• Professional outdoor greenhouses: Again, greenhouses can come in countless shapes and sizes. But, you’ll easily be able to tell apart professional-grade greenhouses by their size, build quality, and price. They are typically made with heavy-duty, rust-resistant aluminum frames and extra-durable polycarbonate panels, offering maximum light transmission and protection from the elements. One of these greenhouses may easily set you back around £1,000 on Amazon.

Note: Greenhouses typically come in a few standard shapes: simple racks/shelves, gabled greenhouses (square or rectangular houses with triangular roofs, designed to support heavier coverings like glass or fiberglass), Quonset greenhouses (tunnel-shaped), and lean-to greenhouses (similar to Quonsets but with a gradual decline on one end).

Remember, choosing one type of greenhouse over another will come down to how much space you have to work with, how many plants you plan to grow, your budget, and how sturdy you need your greenhouse to be.

Consider Your Location

Whatever greenhouse you choose to buy or build, make sure its dimensions match those of the grow space you have available, bearing in mind that you’ll want to keep some free space to comfortably move around inside. Also, remember to set up your greenhouse in an area that receives the most sunlight.

Security

If you’re growing cannabis, security is important. Always make sure you invest in a greenhouse that keeps your plants out of sight from prying eyes, as well as security locks to make sure you’re the only one with access to your plants.

2. Setting Up Ventilation and Watering

One of the main issues we see people run into when growing weed in a greenhouse is high humidity. While humid conditions are great for seedlings, they quickly become problematic for mature plants, creating a breeding ground for bacteria, fungi, and pests.

Make sure to use a thermometer and hygrometer to keep track of the temperature and relative humidity inside your greenhouse, just like you would in an indoor grow room. More importantly, make sure to invest in intake, outtake, and oscillating fans to provide your plants with plenty of fresh air and to keep the humidity and temperature down.

The type of fans you choose will come down to the size of your greenhouse. Just make sure that both your intake and outtake fans have a CFM rating equal to or greater than the volume (ft³) of your greenhouse.

When setting up your oscillating fans, make sure to place them both under and above your canopy to prevent old air from getting trapped inside your greenhouse and driving up the temperature/humidity. You should aim to create a gentle yet consistent breeze flowing through the entire space.

To further prevent humidity issues from harming the quality of your harvest, make sure to adjust your watering schedule. If you’re accustomed to growing outdoors, be careful not to follow your old watering habits, as you’ll most likely overwater your plants in a greenhouse.

Outdoor plants are exposed to direct sunlight, wind, and plenty of fresh air, all of which cause the water from their soil to evaporate quickly. In a greenhouse, your plants won’t endure the same conditions, meaning they’ll likely need to be watered less frequently.

Automated Watering Systems

Investing in an automated system is a great way to take some of the guesswork out of watering your plants in a greenhouse. At RQS, we are particularly fond of drip irrigation systems, as they constantly provide your plants with the perfect amount of water.

Using drip irrigation helps to prevent overwatering (which simultaneously minimises your risk of running into issues with bad bacteria or fungi), promotes the growth of healthy roots, and much more.

3. Adding Grow Lights to a Cannabis Greenhouse

The idea of growing in a greenhouse is obviously to use sunlight rather than artificial lighting. And, while that might work fine for growers living in areas with consistently sunny conditions, those living in areas with more temperamental conditions might benefit from installing a set of grow lights that you can switch on as needed.

Obviously, there are many different grow lights on the market, and you’ll want to do some shopping around to find the type of light that best suits your setup. A great starting point, however, would be to use LED grow panels (remember to provide your plants with around 400W of light per square metre) for best results. We recommend LED lights because they are far cheaper to run than HID lights and also produce less heat.

4. Greenhouse Coverings (Force Flowering in a Greenhouse)

Cannabis plants naturally flower after the summer solstice, as the days get shorter and fall/winter approaches. When growing in a greenhouse, you have two options for dealing with the flowering phase.

First, you can opt to let your plants grow as they would in nature, and wait for them to flower based on the subtle light cycle changes that occur after the summer solstice. This method is obviously very easy and requires little more than patience.

Alternatively, you can choose to instal blinds or covers in your greenhouse to block out light and force your plants to flower (much like you would in an indoor grow room). This is obviously a much more hands-on approach, but it has the benefit of giving you more control over your plants and how they grow.

Some ways to force your cannabis plants to flower in a greenhouse include:

• Using a tarp: You could get a custom black tarp made to fit over the outside of your greenhouse, and manually use that to reduce the amount of light your plants receive every day. This is likely the easiest and cheapest solution if you're working with a small greenhouse.
• Using blinds: You could instal blinds along the inside walls of your greenhouse, and use a tarp to cover the roof.
• Installing shutters: This is by far the easiest solution, but also the most expensive. Installing automatic shutters in your greenhouse will allow you to block out the sun from your greenhouse at the push of a button.

5. Greenhouse Maintenance

Installing a greenhouse might come at a reasonable initial investment. However, once installed and properly set up, a greenhouse can house multiple harvests per year and last for an extremely long time, given you maintain it properly.

Make sure to take some time after each grow to go over all aspects of your greenhouse, such as your watering and ventilation systems, light covers, grow lights, etc. Use this time to clean your gear and make any necessary repairs, and you should enjoy your greenhouse for years to come.

Top Tips for Building a Cannabis Greenhouse

• Know your strains: It is just as important to know what strains you're working with when growing in a greenhouse as it is when growing indoors or outdoors. Make sure to store around for strains with the properties you're looking for, and strains that are proven to grow well in greenhouses.
• Know your space: Like in an indoor grow room, it is super important to carefully consider the amount of space you have to work with in your greenhouse, and how you plan to divide that space up to provide your plants with the best possible environment.
• Use the right size pots: For average-sized plants, we recommend growing roughly 2–4 plants per m² in 12–15l pots. Not only does this give each plant a decent amount of space to grow, but it’ll also make it easier for you to maintain them comfortably.
• Train your plants: As always, you can use a combination of low-stress and high-stress techniques to control how your plants grow and ensure they make the most of the light and space available.
• Keep water and drainage nearby: Make sure you have access to fresh water inside or nearby your greenhouse, as well as a place for runoff to drain to.
• Plan, plan, and plan some more: Whether you're buying a ready-made greenhouse or plan to build one yourself, make sure you have a solid plan to work from. This plan should consider the size and shape of your greenhouse, the number of plants you plan to grow, the materials you want to use to build your greenhouse (if you’re going the DIY route), and the everyday logistics of growing cannabis (watering, feeding, training/trimming, etc.).
• Remember to use carbon filters: If you’re growing more than a handful of plants, you’ll want to make sure to use carbon filters in your ventilation system to mask the smell of your operation.

Greenhouses offer a lot of benefits to cannabis growers of all experience levels. If you’re ready to make the move, check out our in-depth guide to growing cannabis in a greenhouse.