Greenhouse buying guide

Your choice of greenhouse will depend entirely on your requirements:

• if you're looking for a greenhouse to sow seeds and/or grow cuttings and you don't have a lot of space, go for a mini greenhouse;

• if you're hoping to get your crops started early, set up a market garden or protect your plants over the winter, go for a polytunnel with a plastic cover and a steel structure and choose the right size to match the space you have available;

• sturdy and attractive, a glass greenhouse (or a polycarbonate and aluminium model) is perfect for sowing seeds, growing cuttings, starting crops early and overwintering plants;

• if you need to install close to a wall for lack of space, think about investing in a lean-to greenhouse.

Greenhouse structure materials

The material that makes up the structure depends partially on the type of greenhouse you've chosen. The following materials are common:

• wood: ideally, you should go for woods like red cedar or treated pine;

• metal: galvanised steel is a budget option; aluminium will offer better protection;

• PVC: used for low-end and small greenhouses.

Greenhouse covering materials

Just like with the structure, the material that makes up the covering of your greenhouse will usually depend on the model you go for. The following options are common:

• polyethylene: limited life expectancy (around 3 years for a mid-range covering); for a longer lifespan, go for double layer poly with UV protection and 200 gr/m² minimum;

• polycarbonate: durable (service life of approx. 10 years); opt for 4 mm minimum as any thinner and the material will lack insulating properties;

• glass: unlimited lifespan; the most insulating material of the three options.

Important criteria for choosing a greenhouse

When making your choice, be sure to weigh up the following criteria:

• windows and doors: essential for ventilation;

• dimensions: depends on your growing needs; the size of the door determines the size of the plants and equipment you can use (e.g. the type of wheelbarrow you can fit in the greenhouse).

• anchor system: greenhouses are usually fitted with a base that needs to be anchored to the ground or a concrete slab;

• heating: will always be more effective in a well-insulated greenhouse;

• planning permission: in most cases, permission won't be required but there are exceptions to the rule so be sure to read up on legislation before purchase.

Unlike traditional growing methods, having a greenhouse in the garden allows you to sow seeds and grow crops without having to deal with weather conditions such as wind, frost or hail. As such, greenhouses can be used to extend the growing season and protect more fragile plants over the winter. The transparent covering of a greenhouse provides plants and crops with the best growing conditions to guarantee healthy harvests all year long. If you are growing in the ground, it's important to prepare the soil correctly using fertiliser or manure. Alternatively, greenhouses can be used to host potted plants in planters, pots or even raised beds.

The characteristics of the greenhouse depend entirely on what you want to do with it:

• type: traditional, polytunnel, lean-to, mini;

• application: heated or cold;

• size and shape;

• materials: structure and covering;

• additional accessories: shelves, etc.;

• doors and windows: for ventilation.

To ensure your greenhouse can perform its role properly, you need to pay attention to a few criteria:

• the quality of the structure and covering determines how long the greenhouse will last;

• the size needs to correspond to your crops and the space you have available;

• the location and orientation of the greenhouse must match your growing needs. Of course, you should avoid placing your greenhouse beneath any large trees to prevent shade being cast on the structure. In terms of orientation, the entrance of the greenhouse should face north or south, and the longest side of the greenhouse should face east or west so it catches the first of the sun and is exposed to the sun's rays for as long as possible.

There are four main types of greenhouse:

1. Traditional greenhouse: attractive and versatile.

2. Polytunnels: for market gardens and early crops.

3. Lean-to greenhouses: smaller models designed to lean against a wall.

4. Small greenhouses: for cuttings and seedlings.

In order to make the right choice, think carefully about the type of growing you want to do.

A traditional greenhouse generally resembles a mini house which can be used to host all types of plants including climbing plants, potted plants and so on. These greenhouses provide a lot of light allowing your plants to grow in the best conditions possible.

Included in this category are glass and polycarbonate greenhouses. Polycarbonate is a type of clear, hard-wearing plastic made to look like glass. The advantage of this material is that it is weather-resistant and provides good thermal insulation. Traditional greenhouses are stationary structures that cannot be moved around. It is therefore essential to choose the perfect spot before installation.

Polytunnels are tube-shaped structures made up of a galvanised steel structure and a plastic covering (usually made of polyethylene).

Easy to set up and move around, these models are usually used to get an early start on crops or for market gardens. Polytunnels can, however, also be used to store plants over the winter. Unlike traditional greenhouses, polytunnels are portable structures.

Lean-to greenhouses are different from other models in that they are designed to be set up against a wall. They can also be placed in a less-than-optimal spot for plants where you may not have full sun as they store heat better than traditional models or polytunnels (which have to deal with more wind exposure).

They can be used to overwinter plants or grow tropical plants, and can even take the form of a mini greenhouse set up on a balcony.

Mini greenhouses can be used to keep cuttings and seedlings, or to protect small plants from frost. Cold frames and pop-up greenhouses will both work well for these applications. Mini greenhouses can be set up on a balcony and are best for seedlings while cold frames are great for allowing plants to build up their energy stores before transplanting.

Usually rectangular in shape, these models are designed to take up little space on a balcony or patio.

Even if you don't have a garden, you can still enjoy the pleasures of gardening all year round. No matter the same or layout of your balcony, you're sure to find a mini greenhouse to slot in. 

Mini greenhouses can in a wide range of shapes and sizes. The smallest, made up of a rectangular base and a transparent covering, are best for seedlings. Mini greenhouses on stands or legs are great if you don't want to crouch down. Cold frames feature a slanted, transparent roof to let in as much sun as possible. Vertical greenhouses, made up of a metal frame with a removable cover, can be fitted with a series of shelves. It is even possible to find mini lean-to greenhouses for the look of a traditional glass or polycarbonate greenhouse in miniature!

Some mini greenhouses come on castors for easy transportation. Thanks to the reduced size of these structures, they have limited maintenance needs and can be a good budget alternative to a full-sized greenhouse. 

These greenhouses come in a range of materials. In terms of structure, you can choose between plastic, galvanised steel or aluminium. Alternatively, you can go for wood which is attractive but does require maintenance. The cover of these greenhouses is made of plastic; choose between PVC and polyethylene. If you pick a model with a hard roof, you can opt for polycarbonate for a hard-wearing and less costly alternative to glass, but bear in mind it won't last as long. When it comes to picking glass, the main options will be horticultural glass and tempered glass, the latter of which is safer. Depending on the model you choose and its size, mini greenhouses can work really for starting seedlings, growing herbs and/or vegetables or creating the ideal conditions for some tropical plants, such as orchids. They are also handy for keeping plants safe from downpours or frost in winter.

The aim of all greenhouses is to keep temperatures above zero, even in winter. However, they can be split into three categories depending on their temperature: cold greenhouses, temperate greenhouses and heated greenhouses. The main factor at play involves the greenhouse's capacity to retain heat which all depends on the level of insulation and therefore the material that makes up the structure.

Cold greenhouses

Cold greenhouses provide an indoor temperature of 10° to 15°C. However, this temperature can drop to below zero if the greenhouse isn't heated. Cold-hardy vegetables and other plants will grow happily in this type of environment and you should be able to start planting about four weeks earlier than you would outdoors. These greenhouses are primarily designed to protect plants from the harshest weather conditions.

Temperate greenhouses

A temperate greenhouse is kept at a temperature of around 10° to 18°C but, thanks to a heating system, will never drop below 5°C. These greenhouses can be used to host all types of plants over the winter, especially exotic or cold-sensitive varieties.

Tropical greenhouses

Tropical greenhouses are kept at temperature of above 8°C at all times and are equipped with a heating system and high-quality insulation. Tropical plants will do well in this type of greenhouse.

If you're hoping to make an investment that lasts, don't skimp when it comes to the greenhouse structure. A few different materials are commonly used to make up the structure including wood, steel, aluminium and PVC.

Wooden greenhouses

A stylish and elegant option, wooden greenhouses are eye-catching but the material will degrade with exposure to moisture and heat. However, if you've settled on wood, go for species such as FSC-certified Scots pine or Canadian red cedar. Wood is a highly insulating material giving it an edge over other materials.

Steel greenhouses

Steel is generally used to make up the frame of polytunnels and cold frames topped with plastic covers. In order to prevent rust, the material is galvanised meaning it is coated with a layer of zinc. These galvanised steel tubes can come in a range of thicknesses. However, don't invest in anything too thin or the structure may bend in strong winds. A minimum thickness of 3 mm is recommended. 

Aluminium greenhouses

Aluminium boasts all the qualities you want from a greenhouse structure. It is strong meaning your greenhouse is guaranteed to last and wind-resistant yet light meaning your structure will be easy to install, no matter the size. What's more, the properties of aluminium won't alter as it oxidises.

All crops need light and warmth to grow. 

• Polyethylene coverings will protect crops from the wind and provide them with the warmth they require. However, this plastic film will quickly become less efficient as it is exposed to UV rays and will need to be changed around every three years. The longest lasting polyethylene can last up to 10 years, but this all depends on the thickness and the quality of the material. For a cover able to stand up to poor weather conditions, go for a reinforced or double-lined plastic with UV protection. The thickness of this material ranges from 80 to over 300 microns for the thickest models. The thicker the polyethylene, the more hard-wearing the cover.

• Polycarbonate coverings are lightweight and airtight. These coverings are strong and can even survive under the weight of 30 cm of snow. Twin-wall polycarbonate or thick polycarbonate (4 mm) will provide superior insulation. These materials can retain up to 40% more heat than single-wall or thin polycarbonate coverings. Another advantage of this material is that it blocks almost all incoming UV radiation which can be harmful to plant growth. To ensure your covering keeps performing as expected, it should be replaced every 10 years. Polycarbonate is less expensive than glass coverings.

• Glass greenhouses can be made of horticultural or tempered glass. Horticultural glass is transparent and retains more or less the full heat of the sun while also blocking UV rays. On the other hand, tempered glass is more resistant to changing weather conditions and scratches. If you want to heat your greenhouse, go for thick glass. You'll need laminated glass for the roof (though this is usually standard) and double-glazing for the walls. Horticultural glass is fragile meaning tempered glass is the material of choice for most given its strength and limited risk of breakage. Tempered glass is able to withstand hail and is about five times more resistant than ordinary glass and if it does break, it won't form sharp pieces. This material is generally chosen for its long service life which should prevent you from having to replace your greenhouse sooner than you'd like.

A stylish option, glass greenhouses can feature rectangular or polygonal panels and may be freestanding or lean-to. 

These greenhouses allow the most amount of sunlight to enter – up to 90% – while also distributing heat evenly. 

They usually feature an aluminium or stainless steel structure and UV-protected glass walls. The glass panel covering is also designed to withstand scratches, corrosion and chemicals. They can be made up horticultural glass (a less expensive option) or tempered glass which costs a little more but is safer as it won't break into a thousand pieces if smashed. It is therefore ideal if you have children.

Another advantage of glass greenhouses is that if one panel break, it can easily be replaced without you having to buy a new greenhouse. 

Some models feature glass walls and a polycarbonate roof for better insulation and light infiltration. 

But be careful – the structure and base of a glass greenhouse must be strong enough to withstand the heavy weight of these structures. It's also important to bear in mind that glass is shock-sensitive and not hugely weather-resistant. Glass greenhouses can reach high temperatures making them particularly well suited to exotic plants. They can also be heated and used over the wintertime. In this case, it's best to go for a laminated glass roof for better insulation.

Polycarbonate greenhouses can be an interesting alternative to glass models. Polycarbonate is a clear plastic material. It is strong, lighter than glass but hugely hard-wearing – it will practically never become misshapen and is inflammable. Polycarbonate greenhouses can take the form of a traditional, freestanding greenhouse, a lean-to greenhouse or even a cold frame. 

Single-wall polycarbonate greenhouses are much more shock-resistant than glass models, but offer the same transparency. Alternatively, twin-wall polycarbonate, which is designed to trap warm air between two sheets of polycarbonate, offers better insulation allowing you to maintain a constant temperature inside the greenhouse. Bear in mind, however, that twin-wall polycarbonate is less transparent than glass. 

It is also possible to find greenhouses made of a combination of materials – for example, glass walls and a polycarbonate roof – to combine style, transparency and insulation.

In terms of structure, polycarbonate greenhouses can feature a wooden frame, for a tradition look, or PVC tubes for a budget-friendly option. Steel frames come in a multitude of formats while aluminium will offer a longer service life. 

Easy to assemble at home and less costly than glass greenhouses, polycarbonate greenhouses come in a range of designs. However, you will have to pay more attention to condensation in the wintertime than you would with a glass greenhouse. Furthermore, these structures don't tend to last as long and polycarbonate may lose its transparency over time. Polycarbonate greenhouses work just as well for seedlings, cuttings and getting an early start on the growing season as they do for overwintering plants.

Greenhouse sizes

When it comes to picking a greenhouse size, you need to think of your crop requirements and whether you will be using the structure for all or part of the year.

Market gardeners estimate that greenhouse growing should account for about 10% of the total growing area. Home gardeners with vegetable plots of around 200 m² can therefore easily make do with a 20 m² greenhouse but the most important factor to consider is the needs of your crops.

Bear in mind that if you have plants or crops with different requirements, it can be a good idea to invest in two greenhouses.

Greenhouse designs

Greenhouses should be both attractive and practical in design:

• a greenhouse with vertical walls offer the most amount of growing space and height;

• a greenhouse with sloping walls are best for sunlight infiltration and work well in very windy regions;

• polygonal greenhouses are best for potted plants as they do have limited room for growing.

Main types of greenhouse base

The base of the greenhouse is designed to hold the structure steady and anchor it to the ground. It helps to strengthen the structure, hold it firmly in place and raise the structure by a few centimetres. Installing your greenhouse on a pre-constructed base will prevent you from having to dig a foundation – you just need to level the ground to install the structure. Any greenhouse measuring less than 6 m² can simply be set up on the ground in a wind-free spot. However, ground anchors are still recommended.

Greenhouses measuring over 6 m² will need a stronger foundation such as concrete footings or reinforced concrete depending on the size of the structure and the number of anchor points required. If you aren't growing crops directly in the ground, you can also pour a concrete slab. If so, make sure that the slab measures a few centimetres more on each side than the greenhouse itself.

When choosing a greenhouse base, opt for a moisture-resistant material such as aluminium which won't weaken as it corrodes.

Greenhouse accessories should be chosen to match the requirements of your plants:

• sun shades can be installed to protect your plants from burns upon contact with direct sun;

• shelves will increase the amount of space you have for plants in the greenhouse; arrange the shelves over several levels to match the sun requirements of your crops;

• heating can be used to keep temperatures above zero during the coldest part of the year.

Just as with garden sheds and other small garden buildings, you won't usually need planning permission to construct a greenhouse. However, there are a few exceptions to the rule. You will need to request permission in the following cases:

• your greenhouse will be set up in the front garden and isn't replacing an existing garden building.

• the total height of the greenhouse is over 2.5 metres and is closer than 2m to the edge of the boundary of your property.

• you live in a listed building, national park or an area of outstanding natural beauty.

It's a good idea to contact your local planning authority before you invest in your greenhouse to double check that you do not need permission.

• How to secure your greenhouse

• How to overwinter your plants

• How to heat a greenhouse

• How to set up a polytunnel