How to Grow Cannabis in a Greenhouse: A Guide

Learn to grow cannabis in a greenhouse.

Growing cannabis in a greenhouse is ideal because it combines the benefits of outdoor growing with those of indoor cultivation. In this guide, we'll show you how to do it, offering tips and advice as we go along. If you don’t have a greenhouse yet, and want to build one, we can show you how to do that, too.

Why Grow Cannabis in a Greenhouse?

Maybe you’re wondering why you'd want to grow weed in a greenhouse, as opposed to growing indoors in a tent or having an ordinary outdoor grow in your garden. Let’s look at the advantages of greenhouse cultivation over other growing methods.

1. You Can Extend Your Growing Cycle

You can extend the cultivation period into the fall and winter seasons simply by adding more light during cloudy days, and progressively keeping them on longer to compensate for shorter daylight hours. Likewise, if you want to start a new grow before the end of winter, you can add a low-powered light to supplement natural daylight.

2. Provides the Ideal Microclimate for Growing Cannabis

A greenhouse environment can sustain higher average temperatures than chilly open fields or gardens. Plus, during cold or hot seasons, you can heat or cool a greenhouse if needed. Even without a heater or cooling system, a greenhouse will provide a more consistent range of maximum and minimum temperatures, without the extremely dry and hot conditions typical for indoor grow rooms. A more consistent microclimate means healthier plants and bigger harvests.

3. Allows for Year-round Cultivation

The climate within greenhouses can easily be controlled, for instance by opening windows or installing fans. And, since the walls of a greenhouse keep humidity and heat inside, it allows for year-round cultivation. This control over climate and exposure to light isn’t possible in regular outdoor cultivation.

4. More Consistent Harvests

Greenhouse harvests tend to be more consistent over time, both in quantity and quality. This is also why the majority of commercial medicinal cannabis produced today is grown in greenhouses.

5. More Energy-efficient Than Indoor Growing

Since cultivators can take advantage of natural sunlight, growing in a greenhouse is way more energy efficient. You will also have much lower electricity costs compared to growing in a room or tent.

6. Provides Protection From the Elements

Unlike outdoor cultivation, a greenhouse protects your plants from the elements. There is no threat of rainfall or storms harming your precious plants. In addition, a greenhouse prevents scavenger animals from having their way with your crops.

7. Keeps Plants Safe From Detection

A greenhouse allows for discreet growing with a greatly decreased risk of detection. Your plants will be safe and secure, so you can go to sleep without worrying about thieves and vandals.

8. It's Not Expensive

Growers don’t need to spend a lot of money to set up a greenhouse. A simple walk-in greenhouse can be purchased for little money, and will do a decent job of housing your plants. Building a DIY greenhouse is another inexpensive option. For a simple grow using natural light and hand-watering, there’s no need for expensive grow equipment.

How to Grow Weed in a Greenhouse?

Growing cannabis in general involves many variables that need to be kept in check. Growing in a greenhouse is no exception. However, differences in methodology from indoor and outdoor cultivation need to be taken into account. Here is a list of steps you can take to run a successful greenhouse grow.

• Pick a Suitable Strain

Cannabis strains differ in how they grow, and in regards to their requirements. The best type of cannabis for your greenhouse setup ultimately depends on a number of factors, including your local climate, available space, and whether you’re planning to use an artificial grow light. Let’s dig in a little deeper:

Autoflowering Seeds

Autoflowering cannabis is optimal if you’re growing using natural sunlight. As autoflowers don’t require a set light schedule to veg and flower, variations in sun exposure won’t harm your plants or diminish yields as much as photo-feminized plants. This also means you can plant and harvest autos in your greenhouse at any time of year—as long as you support ideal environmental conditions.

Lastly, the smaller size of most autoflowering varieties comes in handy if space is constrained, which is usually the case with small greenhouse setups. To make up for lower yields from smaller plants, you can simply grow more of them in your available space (e.g. the sea of green method).

Feminized Seeds

Growing photoperiod feminized cannabis is also possible in a greenhouse, but comes with some constraints, especially if you’re using exclusively natural light. As these strains flower based on the hours of light they receive, you rely on the seasons to support vegetation and stimulate flowering.

However, you could also set up an artificial grow light on a timer to control your plant’s development. But then you may also need to look into making your greenhouse light-proof. Otherwise, the natural daylight could interfere with your artificial light schedule, causing its own issues.

Another issue when growing photo-feminized plants in a greenhouse is that you may need to train and prune your strains to prevent them from growing out of contol. Especially if you’re using only sunlight, plants can get tall and spindly, and may require some topping, scrogging, and the like to remain at a reasonable height.

• Plan Your Grow Window

With natural light as the main source of energy in your greenhouse, you need to plan your grow window accordingly. Ideally, you want to be ready as soon as daylight hours (and temperatures) start increasing in spring.

Even though your greenhouse will maintain higher temperatures than those outdoors, you should wait until there is no risk of frost before bringing your plants into the structure. Depending on your location and local climate, this may be early or quite late in spring.

• Germinate Indoors

Building on the above, you can always germinate your seeds indoors and start nurturing your plants with low-powered CFLs to give them a head start. You won’t need much space for this, and CFLs tend to run cheap. This way, even if the rest of your grow is conducted using sunlight, you can still take full advantage of the outdoor growing season.

• Move Seedlings to the Greenhouse Gradually

As soon as your plants get a little bigger and stronger, after about a week or two, you can gradually get them used to their new growing environment. Go easy on them, and don’t move them abruptly from the cosy indoors to the greenhouse. Set them in the structure for just a couple of hours each day and increase over the first few weeks.

This way, stress is reduced and plants can continue to develop without a hitch. Your plants are sensitive, especially at this stage, so don’t use your greenhouse as an excuse to be negligent.

• Supplement Lighting According to Weather

One of the great things about greenhouses is that they allow growers to supplement natural lighting with an extra grow light. Stretches of poor/cloudy weather can be overcome without fear of a diminished harvest, and your electricity bill will be vastly lower than if you grow indoors.

Then again, if one is growing in a suitable climate with plenty of sun, a supplemental light may not be necessary. Still, having one at the ready is never a bad thing for a cannabis grower.

• Use Blinds or Covers to Force Flowering

A greenhouse makes it possible to force-flower cannabis using light deprivation techniques. Elaborate timed shutters could be installed to obscure plants for this purpose, but simple blackout covers or blinds can also do the trick—at a fraction of the price and with much less effort.

In addition to allowing you to flower your weed at any time, blackout covers also serve to prevent cannabis light pollution (e.g. nearby street lighting), which could otherwise interfere with flowering for photo-feminized plants.

Pro Tips for Growing Cannabis in a Greenhouse

You want your cannabis to be happy, comfortable, and productive in your greenhouse grow-op. So, what else do you need to know to achieve a great harvest? Check out our pro tips below.

🌡️ Watch Your Temperatures

Unlike growing in a tent or room, temperatures in a greenhouse can fluctuate greatly. For example, a greenhouse in direct sun could easily spike temperatures above what your plants can handle. Installing a simple thermometer will give you a constant reading so you can decide whether your setup needs additional heating or cooling.

💨 Install a Fan

The enclosed environment in a greenhouse can promote high humidity levels, which could lead to mould threatening your harvest. Ensure your greenhouse has openings for adequate airflow to reduce the risk. A standing fan blowing a gentle breeze across your plants can go a long way in keeping humidity levels in check and preventing hot and/or humid spots.

📐 Plan for Your Space

Space in your greenhouse will be limited, so plan beforehand how many plants you can feasibly grow. Check strain descriptions to learn the expected final height of your specimens, and avoid obviously lanky strains like pure Hazes. Using a SOG (sea of green) setup—where you grow a large number of small plants in close proximity (as opposed to a few large ones)—is a great method for small greenhouses.

💧 How Will You Water?

A greenhouse allows you to be flexible in how you grow. You could use an old-school setup with a bunch of pots that you water manually, or you could choose from a variety of watering systems like drip irrigation or misting to fully automate your grow-op.

🔒 Secure Your Greenhouse

Your greenhouse may attract curiosity, and possibly some unwanted attention. If your structure isn’t placed out of sight of nosey neighbours, opaque walls allow sunlight through but won’t reveal your precious “tomatoes”. A burglary alarm and cameras, or at the very least keeping your greenhouse doors locked, will provide a baseline level of security for your crops. In any case, a locked greenhouse is much safer from would-be thieves than growing weed out in the open!

🧹 Keep Your Greenhouse Clean

Dirt will attract pests and diseases, which is why you should keep your greenhouse clean at all times. Make it a habit to collect debris like fallen leaves, etc., as these can harbour fungus, mould, and pests. Use sterile soil and containers to prevent diseases and pests from gaining a foothold in your greenhouse. Likewise, it’s a good idea to give your entire greenhouse, including all pots and tools, a thorough clean once a year, preferably before you start a new growing season.

Growing Weed in a Greenhouse - Conclusion

Growing cannabis in a greenhouse offers the best of both worlds. Massive harvests powered by sunlight, control over your environment, protection from wind and weather, and lower energy costs are convincing arguments that speak for themselves. Start your greenhouse grow today!