How To Build A Greenhouse For Outdoor Cannabis

You may be just starting out your outdoor adventure and need some ideas about the materials and equipment needed to construct a fully functioning greenhouse. Growing cannabis outdoors may not be for everyone but for the keen gardener a greenhouse set up might be of interest. Growing in a greenhouse takes some work to set up but with the right planning and use it can really pay off. We've put together this guide to help you understand the basics behind constructing your own greenhouse, in addition to its maintenance and environmental control.

Growing Cannabis Outdoors

The outdoor environment provides all the necessary resources to grow huge cannabis plants. Weed only needs oxygen, water and light (as a minimum) to thrive and produce delicious flowers. However, the unpredictability of the outside brings a set of challenges that can mean the difference between a successful or failed harvest.

Fortunately, we have the ability to 'take control' of  the environment to provide added protection and care to our beloved cannabis plants by building a greenhouse.

If you've grown outside before then you will already be a step ahead in terms of knowing the requirements of your cannabis plants. Moving them into a structured space requires investment but it can be done very cheaply. Setting up a greenhouse does not have to be expensive but it can sometimes be necessary to install equipment if the plants are still not in a comfortable environment. 

Why Build A Greenhouse?

Building a greenhouse is a fun project that can expand the possibilities of outdoor growing for anyone. Not only can you grow cannabis for longer during the year, but you also create an ecosystem for growing all sorts of other plants. The experience can be very rewarding but is also practical in many situations.

Before You Get Started

Building a greenhouse isn't an easy task but is worth the effort if you are into outdoor gardening. Most of the effort and expensive depends on whether you plan to buy one or build it yourself from scratch.

Things to consider before you get started:

Planning

Plan your greenhouse build beforehand and gather the exact materials you need. It's best to overshoot a bit on the materials so you have extra just in case. The best thing about building your own greenhouse is it can be designed to suit your goals.

Study the weather patterns in your local area so you know your greenhouse will be strong enough to stay up. Having your construction destroyed by a storm is usually the result of bad planning.

Location

Finding the right location for your greenhouse is critical to the success of your cannabis plants. Consider how the light cycles throughout the year so you can pick a sunny spot facing a southern direction (if you live in the northern hemisphere). Essentially, the long side of the greenhouse should be parallel to the equator for the best exposure.

You also want to choose a place with plenty of drainage, as well as place with possible access to a source of power, i.e. An electricity plug. Electrics are not entirely necessary and are only really recommended for permanent, solid structures otherwise they become a big danger.

Safety

It gets boring having to say it, but safety is of utmost importance when growing outdoors. You're already taking a positive steps towards keeping your outdoor garden safe by planting it in a greenhouse. Just keep in mind that the smell can be a massive giveaway once you have more than a few plants flowering.

Marijuana laws are gradually being relaxed in many places across the world and more growers are now able to experience the joys of growing outdoors. If you do live in an area where cannabis cultivation is still prohibited and choose to do it but can't grow indoors, greenhouses could be the perfect solution.

Strains

The genetics of your chosen seeds can dictate how suited they are to an outdoor environment. Consider aspects like the size of your greenhouse, the climate and time of year. Strains that handle lots of stress training and are described as mold resistant tend to be good choices for outdoor environments. Both Indicas, Sativas, and Autoflowers can all be grown in a greenhouse when the conditions are right.

The Great Outdoors

Types of Greenhouse

Polytunnels - Also known as hoop houses, polytunnels are the cheapest kind of greenhouse and are very easy to build. They have a curved frame, usually made from aluminium or PVC, which is covered using a plastic sheeting made from polyethylene. Polytunnels are commonly found on large scale farms where it would just be too expensive to set up permanent structures with glass or panels.

Different plastics for greenhouse sheeting:

• Polyethylene - The standard plastic used to cover most small scale greenhouses. Available in nearly every gardening store and at an affordable price. You'll need 6mm thickness.
• Polycarbonate - Essentially a stronger version of polyethylene. Made with layers of plastic to create a lightweight, durable material. Ideal for all year round gardening due to its ability to retain heat.
• Polyvinyl (PVC) - The more expensive option but lasts for much longer than polyethylene or polycarbonate if it is well looked after. Expect it to last between 3-5 years.

If you're looking for more of a solid structure, you may also want to look into standing greenhouses (A-Frames). These tend to be made of aluminium or wood and are shaped like a small house with a slanted roof. They are detached and not fixed structures like other greenhouses sometimes are, meaning they can be placed almost anywhere. Coverings are usually either plastic panels or glass, making them more expensive than the polytunnel-style greenhouse.

How To Build A Greenhouse for Cannabis

The style of greenhouse you build is up to you but for this example we will be focusing on a low-cost, polytunnel-style option that doesn't require a lot of technical knowledge to set up, but is reliable and won't fall down in the event of a storm! You can make this greenhouse for between 100-200$ depending on where you source the materials from.

Size and Space

Think about how many plants you want to grow in your greenhouse and go slightly larger if possible. It always helps to have the extra space to move around and access the plants when you need to. Although they might not be necessary for you, the addition of equipment such as dehumidifiers, lights or fans should also be taken into the equation.

For our greenhouse calculation we are going to need a floor space of 8' x 12'. The height will measure roughly 8'.

Some growers may just use the greenhouse to start their plants earlier in the season so they are ready to put outside once the days are warmer. If you're planting in the greenhouse and plan to keep the plants there for the whole season, then of course each plant will likely need more space. Try not to overcrowd the plants as this can cause some real difficulties with humidity and mold.

• Give each plant a minimum of 3x3 feet. We will be building a greenhouse that could comfortably house 4-8 plants.
• Make sure you provide enough head room. The greenhouse should be slightly taller than it is wide for structural reasons.

You might even decide to grow other herbs or vegetables in there to compliment your cannabis crops or provide some natural repellents against pests. Lavender, thyme and rosemary are all good choices.

The size you decide on will ultimately determine how much of each material you need. Remember, the containers and strains make a big difference so choose pots to suit the size of the plants you wish to grow.

Materials Required

Diffused Plastic Coverings - When it's a cloudy day outside sunlight is diffused, causing the rays to be scattered more evenly. This is essentially what a diffused covering does for a greenhouse. Diffused light creates a better spread, allowing more of each plant's surface to take a share of the resources.

Glass and other clear plastics that do not have diffusing properties can create hotspots and also do not protect the plants from view, which is why we recommend diffused coverings or panels. The plastic should also have hydrophilic properties, which prevents condensation from dropping off the sheeting onto plants. Rather, it stays on the plastic to be evaporated.

There are other qualities you can look out for, such as UV protection however most greenhouse product manufacturers should be able to provide this information. Generally, greenhouse plastic covers are labelled for garden use.

The base of the greenhouse can be made using two raised planting beds frames filled with soil. The box frames can be built to different heights depending on how deep you want the planting bed to be. Around 1.2 feet in height should be enough, which would be the width of the boards. These measurements have been calculated so there would be two boxes of 12 'x 3' with a walkway down the middle with a width of 2 feet.

• 4 x wooden board 12' x 1.2'
• 4 x wooden board 3' x 1.2'
• Polyethylene greenhouse sheeting 20' x 24'
• 3 x PVC tubing 12' long, 1.25 in. diameter.
• 4 x PVC tubing 20' long, 1.25 in. diameter.
• 8 x rebar pins 2.5' long, 0.5-1 in. diameter.
• Garden wire
• Wood screws
• Staples
• Duct tape
• Nylon cable ties
• Utility knife
• Tape measure
• Mallet
• Drill
• Spare plywood or palette pieces for doorways and extra support (optional)

Pine or cedar works well for the base structures, but if you're on a tight budget you can also use something like palettes, which can be collected from all kinds of stores for free. The idea is to creates two frames placed parallel to each other with a walkway through the middle.

Construction Steps

Now we have all the materials together, we can start to put together our DIY greenhouse. To start, you'll need to clear the ground to create an even base for supporting the weight of your greenhouse. Clear out any weeds and stones and rake the area to soften the ground a bit. Get it as flat as you can.

Next, measure out a 8' x 12' space and mark the corners so you have the positioning organised. Don't forget to have the long side (12') facing the sun's path.

Steps for constructing basic polytunnel greenhouse:

1. Screw the boards together to make two rectangular frames measuring 12' x 3'
2. Place the frames into position, leaving a 2ft wide walkway between them.
3. Connect the two raised beds across the bottom front and back using screws and plywood boards.
4. (optional) Build 2 simple door frames using 3 pieces of timber. These can then be fixed to the middle corners to provide support at each end and give the greenhouse easier accessibility.
5. (optional) Tall fence posts or long stakes (roughly 8' high) can be driven into the ground to provides more places to fix the coverings, doorways, etc.
6. Using a mallet, drive the rebar pins roughly 1' into the ground, one at each of the 4 main corners. Insert the remaining pins oppositely along the inside edges. Once the boxes have been filled with soil you want the pins to stick out by around 4-5 inches.
7. Fit one end of a 20' PVC tube over the rebar pin, and do the same on the opposite side with the other end of the tubing to make an arc. Use duct tape or screws to hold them in place, if necessary.
8. Repeat for the other three pieces of 20' PVC tube so you end up with four along the 12' length of your greenhouse, one at each end and two in between.
9. Using cable ties and duct tape, fix a 12' piece of PVC tube along the inside centre at the highest point to secure the arching tubes at equal distances.
10. Do the same for the other two pieces of 12' PVC pipe, but halfway down the insides of the greenhouse on the long sides.
11. Now you have the main structure completed, proceed to fit the polyethylene sheeting over the greenhouse. Stretch it tight over the greenhouse and staple it down to the raised beds and wooden frames. Double up or roll the edges before stapling to avoid tearing easily. You may need someone to help you here. You don't want there to be flappy, loose parts.
12. Using spare plywood pieces and screws, secure the plastic sheeting along the edges by sandwiching it between the outside of the raised bed and the plywood pieces. Use a long 12' piece on both sides if you have it available.
13. The plastic on the ends can then be folded or rolled and stapled into position. It helps if you have the ability to open both ends to allow air to pass through. This is where the wooden frames at each end can really be useful, so we recommend making the extra effort to build them, especially if you live somewhere where it gets particularly windy or snows heavily.
14. Place soil or mulch, plus stones or bricks, around the bottom outside edges of the greenhouse to seal it further.

This should give you a basic enough structure to start working in a greenhouse. Obviously, these instructions can be changed to suit your needs or the materials you have available but just make sure you get the plastic very tight and thoroughly secure everything. It is worth keeping in mind that you will likely need to change the covering every year or two.

PVC piping emits a chlorine gas which reacts with the polyethylene, so you may want to cover the PVC tubing with duct tape before adding the cover to make it last longer.

Tip: Polyethylene plastic stretches more easily when it's warm, so preferably choose a sunny day.

The Environment

Once you have your greenhouse built, it will need proper maintenance in order to keep it fully functioning. Keep it as clean as you can and regularly clean out any weeds or dust that might accumulate.

If you do decide to install electrical equipment into the greenhouse you must make sure it's safe from water. Consider putting in gravel, a ground sheet, or some kind of flooring so nothing ends up on the damp ground.

Pots

Although you will have built planting beds for this type of greenhouse you may still want to plant in pots so you can move the plants around. To save on height space or if temperatures as dropping too much, holes can be dug in the planting beds for the pots to sit inside.

Temperature

Try to maintain the temperature in your greenhouse between 20-26°C. If the nights are getting too cold you may want to add a small heater. On the other hand, if it is too hot you can open up the ends of your greenhouse to allow more air to pass through.

Humidity

Get a hygrometer installed so you can always measure temperature and humidity. You should be able to control it by opening the vents but if that's not enough it might be a good idea to prune your plants to help reduce transpiration. Try to maintain it between 40-65% RH.

Ventilation

In your design plan, you should consider where you can place ventilation holes to allow hot, humid air to drift out. Normally it's best if you can have at least one vent high up in the greenhouse, and one low down. Naturally then, hot air will move out of the top so fresh air can come in through the lower vent. Being able to fully open both ends of the greenhouse gives you more control.

Tutankhamon (Pyramid Seeds) greenhouse grow by alchemist from GrowDiaries.

Conclusion

Good luck building your first greenhouse! If you've grown outdoors in a greenhouse before we would love to hear from you. Share your tips and tricks with fellow growers down in the comments section!

External References

Biostatistics of Greenhouse whitefly on different greenhouse cucumber cultivars. - S., Mirzamohammadzadeh & Iranipour, Shahzad & Lotfalizadeh, Hossein & Jafarlou, Mohammad. (2020). 

Effects of Diffuse Light on Microclimate of Solar Greenhouse, and Photosynthesis and Yield of Greenhouse-grown Tomatoes. HortScience. - Zheng, Liang & Zhang, Qi & Zheng, Kexin & Zhao, Shumei & Wang, Pingzhi & Cheng, Jieyu & Zhang, Xuesong & Chen, Xiaowen. (2020).

Analysis and Design of Low-density Polyethylene Greenhouse Films. Biosystems Engineering. - Briassoulis, Demetres & Schettini, Evelia. (2003).

Integrating Hoop House Construction and Operation into an Undergraduate General Education Horticulture Class. HortTechnology. - St. Hilaire, Rolston & Sammis, Ted & Mexal, John. (2009).

Reducing Heat Losses in Polyethylene Greenhouses. - Simpkins, J.C. & Mears, D. & Roberts, W.J.. (1975). 

This article was updated November 2020.