Growing cannabis in a greenhouse

Why grow cannabis in a greenhouse?

The recent explosion in ‘growing your own’ cannabis includes the rapidly growing community of greenhouse growers. For many people, growing cannabis seeds in a greenhouse is simpler than creating an indoor grow room. Greenhouse growers find it easier than growing cannabis seeds outdoors.

Greenhouses offer protection from the elements, insect predators and grazing animals. Greenhouses also provide a longer growing season. You can plant your cannabis seeds a little earlier than if you were growing outdoors. The greenhouse also protects your plants form the worst of the autumn weather during harvest, allowing you a greater chance of a safe harvest.

The many benefits of growing cannabis seeds in a greenhouse allows growers in northern European regions the ability to grow high quality cannabis with either feminised seeds or autoflower seeds.

Cannabis seeds and greenhouses

Greenhouses can be heated if required. This allows you to optimise the greenhouse environment even if you have unexpected frosts or stormy weather. It also allows you to germinate your cannabis seeds earlier than you might do for an outdoor grow. Read all about how to germinate your seeds in our germination guide.

With a little bit of thought and planning, growing cannabis seeds in a greenhouse can be a safe, secure and pleasurable way for the home-grower to be self sufficient in recreational or medical cannabis. Greenhouse-grown cannabis is incredibly potent when grown from good cannabis seeds. It’s no surprise that more people than ever are growing in greenhouses and enjoying the benefits.

Autoflower seeds in greenhouses

Many greenhouse growers enjoy large harvests of over 100g (dried buds) from each autoflower plant. The protective environment of a greenhouse protects the autoflower plants from the worst of the weather and the extreme low temperatures which can occur especially in early spring and autumn.

Autoflower seeds usually grow from seed to harvest in around 100 days in a greenhouse. Compared to feminized cannabis seeds that is a short life cycle. The short life cycle of an autoflower means it is extra important to optimise growing conditions and environment in order to maximise harvest quality and quantity.

A greenhouse protects your plants from high winds, damaging rain and low temperatures. Insects and pests can be easier to deal with in a greenhouse and less of a nuisance. All these factors are reasons why greenhouses are a great asset for the cannabis grower.

Feminised cannabis seeds and greenhouses

Feminized seed varieties grow very well in greenhouses. Often, they will grow vegatively until they sense the shortening days, and will start to show first signs of bloom around August, depending on your variety. In the northern hemisphere feminized seeds are often grown from around April to October in a greenhouse.

In a heated greenhouse the seeds can be germinated and planted around March/April, depending on your local weather conditions. This allows an earlier start than is usually possible outdoors. That’s because the greenhouse offers extra protection from the worst of the early season weather.

As a result, a greenhouse allows you to grow larger plants which can start earlier and finish later than would be possible growing outdoors without a greenhouse. Many growers of photoperiod feminized seeds routinely harvest several hundred grams, or more, of dried buds from a single plant. Plants can easily reach 2-3 metres tall and just as wide. A greenhouse also protects your plants from the autumn/winter storms. This can allow the greenhouse grower to grow a broader range of later blooming cannabis seeds than is possible outdoors.

Greenhouse cultivation of cannabis seeds. Using pots vs planting directly into the ground

Greenhouse growers are split into two main groups; those that grow their cannabis plants in plant pots and those that grow them directly in the ground. Plant pots can be easily moved if they need to be, for example if visitors, extreme weather or other problems are present. But plant pots will need more frequent watering and occasional feeds. A large cannabis plant in a 10 litre container will need watering daily during very hot weather. That could be a problem if you are away and unable to visit your plants.

On the other hand you may experience less issues with humidity, especially if your greenhouse floor is covered with paving slabs. Growing in containers also allows the grower more control over nutrient addition.

Some greenhouse growers even feed their greenhouse cannabis with automatic systems (e.g. AutoPot, Blumat, dripper systems or similar). It’s possible to grow cannabis in soil, coco fibre or any other grow medium in a greenhouse.

Plants that are rooted directly in the ground obviously can’t be moved, but will survive well even when you are away for a week or two. There is less need to worry about daily watering. If your greenhouse contains good quality moist soil then your cannabis plants will establish an extensive root network which will support a large plant with heavy yield potential. However, if the ground under the greenhouse contains poor quality soil then remove it and replace with good quality soil before growing plants in it. Serious greenhouse growers will dig in plenty of manure and compost at the end of each growing season to ensure optimized soil quality for the following years crop. One advantage of rooting your plant directly into the ground is that you can grow very large plants with unrestricted root space. It’s one way to get XXL harvests. Some greenhouse growers, when using the best cannabis seeds, find that a single plant can occupy the entire greenhouse and occasionally even run out of space. If this happens you can try low stress training to tie down the taller branches. In extreme situations, some growers have even had to remove some roof panels from their greenhouse if the plant grows uncontrollably. Plants which are rooted directly into the ground can be grown using automated feeding systems such as dripper systems etc.

Greenhouse overheating. Protecting your plants from extreme summer heat

One problem with greenhouse growing is that the temperatures can get very hot in sunny mid-summer weather. High temperatures make it difficult for the plant to grow. Leaves may wilt and the roots can find it difficult to provide sufficient water. This can be a serious problem, especially in a heatwave when temperatures inside the greenhouse can be in excess of 30ºC or even 40ºC. At these temperatures the cannabis plant struggles since conditions are so far from optimum. But the determined greenhouse grower can find ways around these challenges.

Greenhouses can be fitted with windows which open in hot weather and close automatically when it cools down again. These allow the hot air to escape quickly. An open door in your greenhouse can allow a cooling breeze to flow through. Greenhouse doors made with panels of fine wire mesh keep insects out but allow air to flow through.

Sophisticated greenhouse growers have watering systems in place to simplify and even automate water delivery. If the greenhouse is not in an entirely private location then it might benefit from a coat of white shading paint. This is sold in garden centres and used to protect greenhouses from overheating on the hottest summer days. For the cannabis grower this shading paint also stops people seeing what is inside your greenhouse, but green plastic plant mesh and tomato plants have also provided excellent cover for many generations of greenhouse growers.

Anyone that has been in a greenhouse on an early spring day will note how quickly they warm up in sunshine. This allows growers to get their preferred cannabis seed varieties started earlier in the season than they could outdoors. This allows the growing season in a greenhouse to start earlier. Your greenhouse will therefore benefit from larger plants and subsequently superior harvests. Cold nights can be mitigated with the use of greenhouse heaters that are available to purchase at any garden centre.

Just as with outdoor grown cannabis, the shift towards shorter days during the summer is a signal to the cannabis plant to start flowering and produce the desired female buds.

How do I get rid of excessive greenhouse humidity?

If you have ever seen your greenhouse windows dripping with condensation then you need to give some attention to reducing greenhouse humidity. Excess humidity creates an unhealthy growing environment for your plants and can encourage bud rot and mildew. Wet greenhouse glass panels will soak any leaves, branches or buds that press against the glass. If you want to ensure optimized plant health then consider some ways to make sure humidity doesn’t damage your plants.

Ensuring air movement and regular air changes in your greenhouse is important. Some of the techniques used to keep your greenhouse cool will also reduce humidity. Having adjustable roof vents which can open to release humid air is a great help. Having a through-draft thanks to an open greenhouse door is also a good idea.

Many greenhouse cannabis growers have fans in their greenhouse to move air around, and even to push air out of a door or open roof vent. This may mean fitting an outdoor, weather proof electricity connection to your greenhouse. But its a small price to pay for improved cannabis quality. A fan will also strengthen the stems of your cannabis plant by mimicking the effects of an outdoor breeze.

Some people copy the large legal greenhouse growers in the USA and fit an extraction fan to pump humid stale air out of the greenhouse. This can be combined with a carbon filter to remove any cannabis aroma from the exhausted air.

How large do cannabis plants get inside a greenhouse?

If the greenhouse grower can get good soil for his plants and (most importantly of all) start with good cannabis seed genetics then the plants can grow into true monsters by the end of the grow season. The greenhouse grower can see cannabis plants reach sizes rarely seen indoors. Many greenhouse growers have grown single plants that have filled small greenhouses and produced several hundred grams of top quality weed per plant. Good quality cannabis seeds may cost more but they deliver far superior results in the long run.

As previously mentioned, some greenhouses have windows that can be opened to let any monster plants grow out. But a safer option is to use cord to try to ‘tie down’ the plants if they do start growing massively. The photos above show what can happen when a prolific strain starts to turn into a large bush that wants to grow as high as a small tree. The plant simply becomes too tall for the greenhouse. The soil conditions, temperatures and good care resulted in a plant that was thriving as if in the jungle rather than a garden in northern Europe.

Of course this type of extravagant growth rarely occurs when cheap cannabis seeds are grown in poor quality conditions with badly prepared soil. Growing a monster plant requires properly dug, well prepared nutritious soil at the start of the season. The pictures show large cannabis plants simply outgrowing the greenhouse, delivering heavy harvests for the grower. The Dutch Passion Shaman variety was bred specifically for outdoor/greenhouse conditions. Shaman combines some incredibly potent skunk/purple skunk parents with vigorous growth. Shaman eventually grew into a bush 3 metres tall and two meters wide, winning the Dutch Passion photo contest award in 2006. In this case the grower was forced to remove a pane of glass from the greenhouse as his resin soaked Shaman outgrew the tall greenhouse. The plant was too strong to be tied down, and security was not a problem on this particular grow so the grower went on to harvest an abundance of top quality greenhouse grown cannabis, despite the obvious size problems of the Shaman.

Cannabis genetics and greenhouse growing

When growing with good cannabis seeds in a greenhouse remember that some plants will grow as wide as they are tall especially if they have plenty of sun, good soil and ample root space. Remember too that a few seeds could provide a years worth of medication or recreational weed, so regard the cannabis seed as an investment and get a good variety. Cannabis seeds are one area of your life where you really don’t need to compromise. Look at the Dutch Passion website for some cannabis cup winning options. Proven varieties for greenhouse growing, especially at northern latitudes, includes Shaman, Durban Poison, Passion #1, Frisian Dew and Frisian Duck. These varieties have been used by numerous repeat growers over many years. They have passed stringent standards for potency, quality, pest/mold resistance and vigour. Growing these feminized seeds in a greenhouse will deliver excellent results.

Early flowering using a ‘darkening system or extending your growing season

Some growers equip their greenhouses with a ‘darkening’ system. Essentially this involves using dark blinds/curtains to create blackout conditions inside even when it is sunny outside. This can be used to simulate short summer days and ‘force’ the plants to start flowering and producing buds much sooner than normal. Although this involves extra expense it is one way of accelerating greenhouse weed production. The grower simply uses the blinds to reduce daily light exposure to 12 hours (or less) and the plants in the greenhouse respond by flowering.

Greenhouses also have the benefit of allowing the grower to extend the end of the growing season. Cold sunny autumn days on the outside become warm and comfortable in a greenhouse and allow plants to ripen properly. Pests are normally easy to control in a greenhouse, and so long as the greenhouse is secure it is a great and simple way of growing.

Many also grow cannabis seeds in polytunnels these days, the benefits of the poly tunnels being much the same as greenhouses. Polytunnels protect against the worst of the elements, keeping the plants safe and giving a long growing season. Polytunnels come in a wide range of sizes and they also offer privacy to the grower.

Greenhouse growing of cannabis has lower ongoing costs compared to indoor growing, where large amounts of cash are spent on grow lights and ongoing electricity use. By contrast, growing cannabis in a greenhouse only requires a seed and some substrate to grow in. The sun provides free energy for your plants to grow.

Greenhouse grown cannabis versus outdoor cannabis

Many outdoor cannabis growers eventually upgrade to greenhouse growing. The greenhouse allows more potent crops, higher THC levels and a better terpene profile due to the protection offered by a greenhouse. Whether you are growing with a large commercial budget, or a small  greenhouse with just one or two plants, the quality improvements are always important and appreciated. Because greenhouse grown cannabis plants grow in a more sheltered environment the plants are healthier, enjoying more optimized conditions. The plants can grow without damage from extreme cold, storm/hail damage and wind damage. Pests and disease are easy to prevent and deal with in a greenhouse. A greenhouse protects your plants from the worst of the weather, hail and torrential rain. Greenhouses protect your plants from cold weather, creating a warmer environment where your buds can ripen and mature under conditions which are often far superior to outdoor weather.

Growing cannabis seeds in a greenhouse with artificial lighting

Even in warm climates, many greenhouse growers supplement the natural light with additional light on cloudy days. These days LED grow lights can be used to provide additional illumination which can be automatically switched on when ambient daylight (PPFD) levels drop too low. Growers in cool climates, such as Scandinavia may prefer to use supplemental HPS lighting which offers both light and heat to the plants. Supplemental lights also allow you to extend the natural sunlight hours. This would allow you to grow autoflower seeds in a greenhouse and have 18-20 hours of daily light. Artificial light would be used to give the autoflower plants their preferred 20 hours of daily light with optimized yields and quality.

The experienced grower can use the benefits of a greenhouse to grow larger plants, with improved quality and a longer life cycle. Plants can be started earlier and harvested later thanks to the extra protection offered by the greenhouse. During the cultivation process the conditions inside a greenhouse are better than those outside, promoting improved plant health, superior yields and better potency. Outdoor grown plants, without these benefits, usually offer lower yields and quality.

Where to locate your greenhouse?

If you get a greenhouse it pays to think carefully about the location. When it comes to security, always think about a location which makes it difficult for neighbours or the public to see/smell your crop. Also consider your local climate. If you are living in a cool climate it pays to locate the greenhouse where it will receive maximum sunlight for as much of the day as possible. If you live in a hot, desert climate you may want to consider locating the greenhouse under some trees which will shade the greenhouse during the hottest hours of the day in mid-summer. Mediterranean greenhouse growers may wish to place their greenhouse in a cooler, northern part of the garden where some shade may be present. Remember that when temperatures reach 40-45ºC the temperatures become damaging.

Many greenhouse growers also arrange a hosepipe/water connection near their greenhouse. This simplifies the process of getting water to your plants. The last thing you need on a hot day is numerous long walks from your house to the greenhouse with a watering can.

Greenhouse options for cannabis growers

Greenhouses are available in a wide range of sizes and budgets. The lowest cost ones use a flexible transparent plastic mesh sheet which fits over a small plastic/metal pole frame. These can cost as little as €20 but can only fit a single plant inside. Some greenhouses are designed to be leant against an outside wall of the house. Heat from the house helps keep the greenhouse warm. Greenhouses are also available for people that live in an apartment and have a small balcony. Polycarbonate greenhouses use toughened ‘unbreakable’ transparent plastic panels instead of glass, these are useful if you are worried about safety or have small children. Whatever your budget and space, somewhere there is a greenhouse for you.

Growing cannabis in a greenhouse. Easy and effective cannabis growing

Growing cannabis in a greenhouse is a simple and cost effective way to be self sufficient in your cannabis needs. Greenhouse growing seems to be a rapidly expanding pastime amongst cannabis growers of all backgrounds in a wide range of countries.

The more professional greenhouse grower will take care to introduce new, good quality soil/fertilizer each year and keep the greenhouse clean and tidy to minimise potential for pests and disease. Some greenhouse growers use strategically placed fencing and shrubs to make the greenhouse inaccessible to unwanted visitors and invisible to prying eyes. Greenhouses are cheap to buy. You can buy small low cost plastic ones for not much more than a hundred euros. Even cheaper ones are available with plastic covers. They are available everywhere and come in any shape/size required for all budgets. Some of the modern polycarbonate greenhouses are also opaque giving instant privacy and have lockable doors. What more could you need in your back garden? Remember that poor quality cannabis seeds can’t produce connoisseur quality cannabis, no matter how well you grow them. Invest in the best cannabis seeds you can get from a company with a proven reputation, and enjoying growing your own cannabis in a greenhouse.