Authored By: John Harrison
Our resident allotment expert John has has written articles for Garden News, Grow Your Own, Sunday Post, Vegetarian Living and a number of trade publications on topics associated with ‘grow your own’.
If you’re wanting to extend the growing season of your garden or to start cultivating some more exotic species of plants, a greenhouse might be the perfect addition to your garden. WhatShed has produced this extensive greenhouse buying guide for folks who are looking to buy a greenhouse but have no idea where to start.
We begin by introducing the important considerations you need to make prior to you starting to shop, before moving onto common greenhouse construction materials and the different applications they’re most suited to. The guide then touches on different accessories that can improve the functionality of a greenhouse, before concluding with some basic maintenance tips.
From the different types of greenhouse available, to finding the perfect site for your new garden structure and getting the thing assembled, this greenhouse buying guide should leave you with all the knowledge you need to choose the perfect home for your plants, whether you’re growing tulips or tomatoes. Let’s begin!
- Things to Consider Before Buying
- Greenhouse Construction Materials
- Other Considerations
- Greenhouse Delivery and Assembly
- Greenhouse Maintenance
Things to Consider Before Buying Your Greenhouse
Costing between a few hundred and several thousand pounds, a greenhouse isn’t a purchase you should rush into. There are a few really important things to have clear in your mind before you start shopping.
Of course, you want to grow things in a greenhouse. That’s what they’re designed for after all. However, it’s important to be more specific. There are all kinds of greenhouses these days, so think about your goals in the garden before you part with any cash.
Some greenhouses are tailored towards starting seedlings and bringing on young plants in winter and spring before they eventually make it to the garden in the summer. Meanwhile, other models are better suited to cultivating more exotic species for their entire life.
Generally speaking, greenhouses can be classified in the following categories:
Hot Greenhouse: Also known as a hot house, these buildings maintain a constant temperature in the 18 to 20°C range. Hot greenhouses will typically house tropical plants and make use of heaters and/or grow lights to create a more exotic climate.
Warm Greenhouse: With a temperature ranging between 10°C and 13°C, a warm greenhouse will allow year round growing of plants native to the UK. Additional heating might be required in the very coldest months to maintain the temperature.
Cool Greenhouse: Those wanting to nurture seedlings will often favour a cool greenhouse. Usually the cheapest of the three categories, they typically do not require additional lighting or heating.
Before continuing reading this greenhouse buying guide, take a moment to consider what are your goals in the garden. Think about the type of plants you want to grow and tailor your search to products that are best able to produce their ideal habitat.
Where Will You Build Your Greenhouse?
When deciding where your greenhouse will go, it’s important to remember that it will need a level base and to be in a reasonable position to catch a decent amount of sun. The first step should be to survey your garden. Check at different times of the day to get a clear idea of when different areas get the sun and when. When you have a site in mind, you can get out the measuring tape.
WhatShed Tip: Several resources recommend positioning your greenhouse with the ridge pointing east to west. The rationale is that more of the building will be exposed to the sun, thus resulting in a more efficient build. This is certainly true. However, we’ve found that unless you’re working with an enormous greenhouse, you’re unlikely to notice any difference. As long as the sun is hitting a smaller greenhouse, it will be warming it.
Once you know the size of the area you are working with, you can consider the actual size of the greenhouse you will build. Remember, you will want to leave access space around the greenhouse to periodically clean and maintain it. You might want to leave space for a water butt too.
The most common complaint we hear from greenhouse owners is that they wish they had bought a bigger one to begin with! Therefore, you should always try to build the largest possible greenhouse your space will accommodate. That said, you really shouldn’t sacrifice necessary maintenance access space for a larger building!
The most important measurements to consider when sizing your greenhouse are the length and width of its footprint. Greenhouses typically lack roof overhangs or other design features that are common on sheds and other garden buildings that might compromise space higher up from the base.
You will usually find greenhouse manufacturers make the same model in a range of widths and lengths increasing in two foot increments. For example, it’s common to see manufacturers producing 6′ by 8′, 8′ by 8′, 10′ by 8′, and perhaps 12′ by 8′ models.
Although you’ll see sizes like 6′ by 8′ quoted, greenhouses will usually end up slightly larger than these dimensions. When planning how much space you will need, remember to take this into account. Manufacturers quote greenhouse sizes this way for simplicity. You can usually find the exact dimensions of the building if you look a bit deeper in the sales literature.
Unless you’re cultivating plants that love to grow tall, the height of you greenhouse will be less important than the width and length. That said, it’s worth remembering that you will be working inside the space and a taller roof will mean you can fit more plants inside. Most greenhouses will be around 7′ or 8′ tall.
Greenhouses, particularly glass ones, can be quite fragile. If there are spots in your garden that get particularly windy during storms, you should consider finding another location. Heavy gusts can damage a greenhouse themselves or carry items that will break panes. It’s best to minimise the risk of such occurrences by positioning the building in a sheltered spot.
If your garden lacks features to use as a suitable windbreak, you can always create something. Consider planting bushes or constructing some fencing to shield the structure from the elements.
Similarly, you don’t want to put your greenhouse directly under a tree. Falling limbs will almost certainly cause major damage to both glazing and the framework itself. Even if you get lucky never have to replace a pane due to a branch breaking overhead, greenhouses under trees usually require a more regular cleaning schedule. Leaf buildup in the autumn and bird droppings will both require removal to keep your plants’ home in its best condition.
Electricity and water
All greenhouses need a water supply. Some owners will use an outside tap and hose to irrigate, whereas others might prefer to use recycled rainwater collected in a water butt.
Not as essential as water, not every greenhouse owner wants electricity running to the building. If your greenhouse’s role is to bring on seedlings during the winter months before adding to your garden in the spring, you might not see the value of powering the space. However, some growers will still want to use electrical devices in these cool, starter greenhouses. For example, you could add non-horticultural lighting so you can work in the space later on short winter days, or automatic irrigation systems to reduce the your overall workload.
If your plans involve tropical plants, you’re probably going to need heaters, possibly grow lights and propagators too. All these devices need powering. Of course, you should consult a qualified electrician before tinkering with any wiring to a greenhouse. You should also consider sites closer to your property to make the process of powering the grow space as easy as possible.
Something that a lot of people overlook is access to a greenhouse. They figure that the walk from their house to the opposite end of the garden really isn’t that far and forget that there is a high probability that they will be doing said walk on dark, wet, winter nights, or carrying heavy items, like bags of substrate, for example.
When you’ve considered all of the above, you’re finally ready to start thinking about the actual base of the greenhouse. There are a few different options here, each with pros and cons:
Paving Slabs are the most common form of greenhouse flooring. They provide good drainage and a solid, level grounding for the structure. You can anchor a metal base directly to paving slabs to provide additional support.
Concrete is another popular choice. Like slabs, concrete will create a solid, level grounding for the greenhouse and anchorage points can be included too. That said, concrete does not drain anywhere near as well as paving slabs will. You’ll be using a lot of water in the greenhouse so ensuring excess liquid can escape is very important.
Gravel is another great base option for a greenhouse. It drains fantastically and is easy to lay. It’s perhaps not as permanent as either paving slabs or concrete though. That said, you can incorporate gravel areas into either a slab or concrete base for the additional structural support of anchored walls, whilst enjoying the superior drainage qualities of gravel.
Some greenhouse owners will build directly on a lawn. Whereas this is undoubtedly the cheapest and quickest way to construct a greenhouse, the resulting structure will be nowhere near as strong. With only grass or earth flooring and regular watering, the space will become very messy quickly too. For these reasons, we recommend using a designated base.
Whatever material you choose to use, you should create a base that is slightly larger than the floor dimensions of the greenhouse. About six inches around all sides will be plenty.
In days gone by, many greenhouse manufacturers expected people to build a small brick plinth as a base for the structure. These designs are robust, permanent, and can look very attractive. They do, of course, require more work too.
Today, a lot of manufacturers have designed special metal bases for their products to sit directly on top of. Many customers prefer these because they require a lot less site preparation. They’re also less permanent whilst still being incredibly durable. This gives the gardener the flexibility to move the greenhouse much more quickly at a later date.
WhatShed Tip: Think about the colour of material you will use for your base. With clear, non-diffusing glazing, your plants will benefit from a light base. Pale paving flags coupled with white gravel would increase the overall light in your greenhouse making for happier plants!
Greenhouse Construction Materials
Greenhouses might just be a framework and glazing but you’d be surprised at how many different types of each are available. Different glazing and framing options offer different qualities. Some glazing products will provide superior levels of heat retention and light penetration. Meanwhile, some are inherently safer than others. Both glazing and framing is also important when it comes to how sturdy the building will be. The next section of this WhatShed Greenhouse Buying Guide will tell you all you need to know about the various construction materials on the market today.
Horticultural Glass: The cheapest of the glazing options available, panes usually come in 2′ squares, which fill the greenhouse’s framework with overlaps where they join. Being clear, horticultural glass lets around 90 percent of light through. It also won’t degrade over time like plastic options will. Replacing a piece is also very simple if one does break.
Horticultural glass suffers from a few main drawbacks. Firstly, it’s not very strong. When it breaks, it forms dangerous, long shards. This makes it a poor choice for those with children or pets. It’s relative weakness makes the overlap design the optimum way to use the product. However, this makes the resulting greenhouse much more difficult to clean. Algae will eventually grow between the overlaps. A thorough cleaning to remove all algae will be needed every couple of years and will likely involve dismantling the greenhouse.
WhatShed Tip: Pay attention to how thick glazing products are. For single glazed products, we don’t recommend going for anything below around 3mm. This should give a sufficient level of both insulation and structural integrity.
Toughened glass: With the price of tempered glass coming down considerably in recent years, many greenhouse manufacturers now offer it as standard or an upgrade.
Toughened glass is normal glass that has been heat treated. This makes it a lot more durable than standard horticultural glass. Being supplied in larger panes means that there is no need to overlap pieces. This makes cleaning much easier.
Of course, it is also much more resistant to damage too. A moderately sized branch falling from a tree or a football hitting the greenhouse shouldn’t smash toughened panes.
It’s not indestructible though and if it takes a serious blow, it will break. However, unlike horticultural glass, which shatters into dangerous, dagger-like shards, toughened glass usually cracks into thousands of tiny pieces, which is much less likely to cause serious injury.
Care should be taken when handling any glass product. With toughened glazing, it’s easy to falsely believe that you can’t damage it at all. Pay attention to the edges of the glass, which are much weaker than the surface of the panes. We recommend using a sheet or similar cushion when resting the product on concrete prior to installation.
Like horticultural glass, toughened glass lets around 90 percent of light through. It’s much more efficient in this respect than non-glass options.
It’s common to see the toughened glass products come in three or four millimetre thicknesses. The thicker it is, the more insulating and durable it will be.
Fibreglass: Unlike glass, fibreglass is not clear. It, therefore, does not let the same amount of light through. However, the light it does let in will be more diffused, which is better for growing mature plants.
It’s typical to see fibreglass greenhouses constructed using corrugated sheets. The sheets are translucent and will be much better insulators than their glass counterparts. They also need replacing more frequently since panels will eventually yellow over time.
Fibreglass greenhouses are not very common in a domestic application. You’re much more likely to see them on commercial growing operations.
Polyethylene film: Polyethylene film is another material usually reserved for large commercial operations. It’s cheap and easy to work with. Typically, a wooden hoop framework will be finished with polyethylene film to create a poly-tunnel.
Polyethylene film can be double- or triple-walled. These products comprise of two or three layers of film with an insulating air gap between. Additional layers considerably increase structure’s ability to resist the elements.
Like fibreglass, polyethylene film is translucent. This means the light inside the greenhouse will be diffused or semi-diffused. Softer to the human eye, the quality of light is actually better for older plants, which use other wavelengths outside of the visible spectrum.
Polycarbonate safety glazing: Another non-glass alternative for greenhouse construction is polycarbonate safety glazing. As the name implies, these products are designed with safety in mind. Polycarbonate glazing is much more lightweight and flexible than glass, which is both a blessing and a curse. It might not break in the same way that glass will but it’s much more vulnerable to high winds. A particularly stormy night could see one or more panels pop out of their frames and end up wherever the wind wants to deposit them!
With polycarbonate glazing options sometimes costing the same or more than toughened safety glass, it’s difficult to make a case for the non-glass product for greenhouse construction. It is a better insulator than glass and, depending on the product, can be used to provide diffused light in a greenhouse. It is also available in double and triple layered options, which will increase its performance as an insulator and the light diffusion in the greenhouse. A drawback of these “double glazed” polycarbonate solutions is that they can be tricky to clean since algae can grow between the layers.
Aluminium Frame: Aluminium is without a doubt the most popular frame material. It’s cheap, effective, and requires little maintenance. It has a very long lifespan and won’t rust. Your chosen glazing option will simply sit between channels on the frame. You can even powder coat an aluminium frame to add a bit of colour to the product and give your greenhouse a touch of style.
One drawback of aluminium is that it provides no additional insulation. No matter which glazing solution you use in conjunction with an aluminium frame, you will lose heat through the metal.
Steel Frame: Steel is even stronger and lasts longer than aluminium. Given its superior strength, steel frames can often afford to be more minimalist in design.
That said, steel is not used often in domestic greenhouses. It is much heavier than aluminium, making it more difficult to both transport and assemble. As such, you will often see steel used for commercial greenhouses or poly-tunnels.
Plastic Greenhouse: Plastic greenhouses are becoming more popular than ever. They are usually cheaper than aluminium options and offer far better heat retention.
That said, plastic greenhouses are usually much weaker than those made with a wooden or metal frame. You’ll see plastic frames commonly used with polycarbonate panels or on smaller products.
Wooden Greenhouse: Wooden greenhouse frames are usually the best looking option but you will pay a heavy premium for all that style. Not only do they cost more upfront, they are more difficult to assemble and require a lot more maintenance. Wood needs treating regularly against damp, and the thicker framework can cast large shadows into the greenhouse.
That said, a wooden greenhouse will have a definite “wow” factor, and will likely compliment a traditional garden well. You’ll also find that they’re much more insulating than other options too and can reduce heating costs when growing exotic species year round.
It used to be the case that greenhouses have a very formulaic look. Four walls, an apex roof, and one or two doors. However, with new materials coming to market in recent years, manufacturers have been able to get a lot more creative.
Of course, the material you use for the framework will have a big impact on the aesthetic appeal of the greenhouse, as will your decision to go for a brick plinth base or metal base. However, manufacturers offer different styles that can not only boost the structure’s visual charms but also its functionality.
Traditional Greenhouse: A traditional greenhouse will be a square or rectangular, detached structure. It will most likely have an aluminium frame that sits on top of a metal base or brick plinth. Traditionally-styled plastic greenhouses and wooden greenhouses are also popular. The most common glazing choice is glass but polycarbonate and polyethylene options are available for some manufacturers’ traditional designs.
Octagonal or Heptagonal: Another popular style of greenhouse is octagonal or heptagonal designs. These products work really well in the corner of a garden. They often feature a glazed hip-roof for additional visual appeal and have an aluminium or wooden frame.
Lean-to: Lean-to greenhouses are perfect for smaller gardens. Lean-to designs have three walls and a roof. The structure’s fourth wall will be the wall of your property. If you have a south-facing wall in your garden, it would likely be an excellent spot for a lean-to greenhouse. Both wooden and aluminium frames are popular choices for lean-to greenhouses.
Lean-to greenhouses might not let in as much light as the other styles but the additional brick wall will store heat during the day. This will raise the temperature of the space during the night.
Other shapes: Some manufacturers sell dome or other odd-shaped products. Whilst they might have stronger visual appeal, non-conventional shapes are usually more difficult to assemble and maintain. They do not seem to provide any discernible advantage for the gardener.
Greenhouses get hot in summer. You want to make sure yours has adequate ventilation. For a 6′ by 8′ space, you should choose a model that has both roof and side vents. As a general guide, you should aim to provide ventilation that covers around 20 percent of the total floor area. Both louvre vents and hinged roof vents are available, as are automatic opening systems. It’s worth remembering that any ventilation that cannot be closed will act as a heat leak during colder months. You therefore might favour hinged vents over louvre ventilation options.
Different greenhouse coverings will offer different insulating properties. We’ve provided the following details of typical products from different manufacturers. The R-Value refers to how well the product will insulate a space. The higher the number, the more insulating it is.
Meanwhile, the U-Value refers to how much heat will escape through the material. Here, we are looking for a lower number.
|Polyethylene film , single layer||0.83||1.20|
|Single Pane Glass, 3mm||0.95||1.05|
|4mm Double Wall Polycarbonate||1.43||0.70|
|6mm Double Wall Polycarbonate||1.54||0.65|
|8mm Double Wall Polycarbonate||1.60||0.63|
|10mm Double Wall Polycarbonate||1.89||0.53|
|Double Pane Storm Windows||2.00||0.50|
|8mm Triple Wall Polycarbonate||2.00||0.50|
|3.5mm Twin-Wall Polyethylene||2.10||0.48|
|5mm Twin-Wall Polyethylene||2.30||0.43|
It’s common to see both hinged and sliding doors in modern greenhouse designs. If you’re working with a wooden greenhouse frame, you will probably go for hinged doors. Those using aluminium frames can choose either. When thinking about the door to the greenhouse, make sure to remember that you will often be carrying plant pots or other large items through it. Therefore, choose a model with an appropriately wide door.
Staging is the name given to the shelving on which your plants will live. Typically, a greenhouse will feature staging on either side of a central walkway leading from the door. Staging allows you to organise your growing space more efficiently. It will typically be made from either aluminium, wood, or both. Ideally, you want to use slatted staging so as to allow for light and air to circulate and excess water to drain.
At the height of summer, the sun’s rays can be too much for plants in a greenhouse. To cool the space and protect your tender plants, consider adding polyethene shading to the greenhouse. This temporary solution can be added and removed depending on weather conditions.
There are loads of different accessories that growers can include in their greenhouses. Some will make the space even for perfect for tender plants. Meanwhile, others will increase the efficiency with which you can work in there.
We’ve included a list of the most common greenhouse accessories below:
- Water butts.
- Automatic venting systems.
- Non-horticultural lighting.
- Grow lights.
- Grow light timers.
- Lights rotation systems.
- Greenhouse shading.
- Potting benches.
- Misting wands and other watering accessories.
- Automatic irrigation systems.
Greenhouse Delivery and Assembly
Different retailers and manufacturers will have different delivery policies. Some might include home delivery and others may charge extra for it.
WhatShed Tip: If it’s within budget, or a free inclusion, we always recommend taking a company up on any offers they might make with regards delivery. Components often get damaged in transit and if a company causes this damage, they’ll replace the piece for you. They might not be so keen to do so if you broke the item, however!
Most manufacturers will expect the customer to take care of the installation themselves. Be sure to check out the installation procedure described by the company before purchase. Generally, they try to make it as easy as possible to erect a greenhouse but it’s worth being sure you can manage it prior to buying.
You can expect an average traditional greenhouse build to take around four to five hours. This doesn’t include any prior site preparations you might need to make (for example, flagging or concreting an appropriate level area).
Some companies might offer an installation service at an additional cost. For those suffering from mobility issues or leading excessively busy lives, having the company assemble a product can be a real godsend.
The style of greenhouse and materials it’s made from will have a direct impact on the amount of maintenance you will need to commit to. Whilst all greenhouses need cleaning periodically, different design choices will make the process either easier or harder.
For example, cleaning is much easier with whole pane toughened glass glazing. As mentioned earlier, algae can build up between overlaps required by standard horticultural glass, making cleaning a much more involved job. Polycarbonate materials with multiple layers might also suffer from algae buildup between the individual panes.
Wooden greenhouse owners will need to periodically treat the timber itself. This will require a thorough coat of a suitable wood-preserving product about once every year.
Of course, if a sheet of glazing material does break for whatever reason, the building’s insulating ability will be seriously compromised. You should make it a priority to replace any missing panels as soon as possible to continue gardening year round in the space.