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Choosing A Greenhouse 

Why do you need a greenhouse? 

Growing under glass provides a protected environment ideal for raising seedlings, overwintering frost-tender plants, growing greenhouse crops (such as tomatoes, peppers, cucumbers etc.), raising crops earlier and growing them for longer or cultivating plants that need protection all year round. Having a greenhouse lets you extend the growing season by sowing plants earlier. It also enables you to grow a much wider range of plants than could be grown outdoors. 

A greenhouse is a major investment and, once installed in your garden, is difficult to move or change. This is why it is important that you spend some time deciding which greenhouse to buy. With so many greenhouses available to choose from it can sometimes be difficult to know which to select. Hopefully this guide will provide you with sufficient information to allow you to purchase the best greenhouse for you and your garden. 

Elite Compact Greenhouse  Elite Compact Greenhouse  Elite Craftsman Greenhouse 

Frame 

Aluminium greenhouses have always been the most popular type of greenhouse, whether the frame is left natural or if it has a painted finish. You may find that a coloured greenhouse will suit your particular style of garden or will be less obtrusive (especially if your greenhouse can be seen from your home). One of the reasons they are popular is that they are virtually maintenance free as aluminium will not rust or rot even in the great British weather! Another reason for aluminium framed greenhouses being popular is that the glazing bars are relatively narrow, so maximum light can get into the greenhouse. You will also find that aluminium greenhouses heat up and cool down quicker than wooden greenhouses and they also tend to be cheaper to buy than a wooden greenhouse of the same size. 

Traditional wooden greenhouses are often regarded as being a more attractive alternative to aluminium greenhouses and they are better at retaining heat than aluminium greenhouses (it has been estimated that a wooden greenhouse will cost 20% less to heat than a greenhouse of the same size with an aluminium frame). You will often find wooden greenhouses available with glass to ground (standard with most aluminium greenhouses) but you will also find styles with half boarded sides which help them to retain even more heat, but are not suitable if you want to grow direct from the ground. However, wooden greenhouses usually require more maintenance. Frames made of softwoods such as pine are vulnerable to rot even if the timber has been treated (check to see if greenhouses of this type come with a guarantee against rot). Greenhouses made using teak or cedar are more durable as their own oils naturally protect them and they can be left to weather naturally, however these greenhouse are usually more expensive then softwood greenhouses. Wooden frames are usually bulkier than aluminium therefore they can cast more shade inside the greenhouse. On the plus side wooden greenhouses are particularly strong making them an excellent choice in exposed or elevated areas, with their weight adding to their strength and durability. 

Rion Hobby Gardener  Rion Grand Gardener  Rion EcoGrow 

Plastic resin frames are a newer development for greenhouses. They have become a popular choice as they are often more attractive than an aluminium greenhouses and are often less expensive than an aluminium or wood frames greenhouses of the same size. As plastic glazed greenhouses have become more popular (due to the fact they are ideal for use in gardens where children have access) manufacturers looked for the best frames to hold this glazing securely in place – producing a plastic / resin frame often proved to be the best way to retain glazing and create a greenhouse with outstanding safety features. 

Shape

There are basically four different shapes of greenhouse, the most common two shapes are an apex style or lean-to. 

Elite Classique Greenhouse  Palram Silverline Greenhouse  Elite Titan Greenhouse 

Apex greenhouses tend to have a door in the middle of one end, either a single or double door depending on the size of greenhouse. With greenhouses with single sliding doors you can usually decide which way the door will side during assembly of the greenhouse. You would normally create a path from the door down the centre of an apex greenhouse. These apex greenhouses are usually available in a range of different widths and lengths so you are likely to find one to suit the space you have available and your available budget. 

Elite Windsor Lean To Greenhouse  Elite Titan Lean To Greenhouse  Elite Easygrow Lean To Greenhouse 

Lean-to greenhouses are designed to be fixed to a wall or fence. These tend to be cheaper to heat than a free standing apex greenhouse as the wall they are against acts as a heat sink, absorbing heat at hot times of day and slowly releasing the heat when temperature drops. Lean-to greenhouses when built against a house wall do not normally have access directly into the house – however, you could have an entrance from the lean-to into your home but you need to bare in mind that building regulations require you to use safety glazing (toughened glass or polycarbonate) when access is directly from the house into the lean-to. If your lean-to is attached to a house wall this will make it easier for you to get mains electricity and a mains water supply into your greenhouse. 

Forest Garden Glass House  Forest Garden Glass House  Forest Garden Glass House 

Hexagonal greenhouses are nice when space is limited and your greenhouse will be in full view of the house. These greenhouses appear more attractive than standard greenhouse shapes but it can prove difficult to get staging or shelving to fit inside them. Before ordering a greenhouse of this type confirm that staging is available – some models come complete with staging which then makes them a practical space for raising plants. 

More oddly shaped greenhouses (such as domed greenhouses) tend to be both more expensive that traditional greenhouses and can be more difficult to grow in, for example adequate ventilation can often be difficult to achieve and even heat distribution can be hard with hot and cold spots forming. 

Growing direct from the ground  Growing on benches  Pathway down centre of greenhouse 

Size

Greenhouse dimensions are usually quoted as nominal sizes in ft and inches, with most increasing in length or width in increments of 2ft. It is important when ordering a greenhouse to ask for confirmation of the greenhouse size so that you can check you have the correct sized space to accommodate the greenhouse. 

The overriding factor when it comes to greenhouse size is the space you have available. Ideally fill the space as much as possible – going for the biggest greenhouse you can accommodate. However, other factors can play a part in selecting the correct size, as outlined below. 

How much space you have available for a greenhouse can affect the style of greenhouse you can accommodate. If you have lots of space a traditional free standing greenhouse is the right choice. If you have limited space then a lean-to greenhouse might be a possible alternative. 

What you intend to use your greenhouse for will affect the size of greenhouse you need. You need space for your plants to grow – this could be growing direct from the ground the greenhouse is sited on, it could be in growbags or large pots or it could be on top of benches. If you intend to raise plants from seeds than you’ll need to be able to stand comfortably within the greenhouse and have some working space where you can store your tools and bags of compost as you sow, propagate and pot on. And don’t forget that you need to be able to reach the back of your greenhouse so will need space for a path down the centre of the greenhouse. 

The most popular size of greenhouses is one 6ft by 8ft. This size is popular as it provides you with sufficient space to stand and work within and have staging down both sides and across the back of the greenhouse. Smaller greenhouses than this are available and when space is at a premium a compact greenhouse may be your only option. If that is the case try to make the best use of the working and growing space within – order narrower staging or purchase custom built staging to fit exactly in place without any wasted space. 

The height of a greenhouse is important as the taller it is the better the light transmission. Ideally choose a greenhouse where the eaves are at least 5ft (1.5m), ideally 6ft (1.8m) or more (RHS recommendation). 

The greenhouse ridge should be at least 2ft (60cm) above the eaves height. This ratio allows a door to be fitted for easy access and also creates a slope in the greenhouse roof which reduces heat loss and diverts rain off the roof.

Your greenhouse should also have sufficient headroom so that you will be comfortable working within it – ideally no less than 1ft (30cm) clearance between you and the greenhouse roof. 

Taller greenhouses are also invaluable when it comes to some greenhouse crops – such as tomatoes, cucumbers etc – which are happy to grow up to the greenhouse roof if they can. 

The economy of scale also comes into play when buying a greenhouse. The price per square foot will be less as the greenhouse becomes bigger. So it could be a false economy to buy a small greenhouse now with the intention of extending it later as this would be more costly than just buying the larger greenhouse at the start. 

You might also consider whether you want a wide but short greenhouse or a longer, narrower greenhouse. Wider greenhouses provide more space to grow as the path takes up less space – however, depending on what you intend to grow, this space might not be practical. For example if you want staging within your greenhouse then you need to make sure you will be able to reach across the width of the staging to reach and care for your plants. However wide spaces would be more advantageous if you intend to raise crops in growbags

Horticultural glass  Toughened glass  Polycarbonate glazing 

Glazing

There are basically four different types of materials which can be used to glaze a greenhouse: 

Horticultural Glass 

Standard horticultural glass is the favoured glazing as it’s crystal clear letting 90% of light through, does not degrade in sunlight and reflects heat generated within the greenhouse back into the structure rather than letting it escape. 

On the downside as horticultural glass is supplied in small panes (usually 2ft square) which overlap, algae can form where the overlaps are and these can be difficult to clean. You might also find that in extreme frosts that the glass might crack (you can still use the cracked glass but ideally seal with a strip of All Weather Tape). The other thing to note is that if something breaks the glass it will break into sharp shards – so this type of glazing should not be chosen if the greenhouse can be reached by small children or is in a public area. 

Toughened Glass 

Toughened glass, as its name suggests, is stronger than horticultural glass so there should be less breakages. It should be noted that once glass has gone through the toughening process it cannot be cut, so it is important when ordering panes of toughened glass to order the correct size as you won’t be able to cut it to fit. Toughened glass is heavy adding to the stability of the greenhouse making it favoured for greenhouses in exposed positions. It is still crystal clear, so lets through 90% of light, but because it has been put through a process to toughen it greenhouses glazed with this glass will be more costly. 

This type of glass is usually chosen when there is the potential that people might fall against the glass. If broken this glass will crumble into relatively safe pieces rather than horticultural glass which could break into dangerous splinters of glass. 

For most greenhouses, when toughened glass is selected, this is supplied in single sheet where possible. So, without overlapping panes of glass there is less opportunity for algae growth, making toughened glass easier to keep clean. 

Acrylic 

Acrylic or single wall polycarbonate will let 85% of light through, is lightweight and durable and is safe if anyone was to fall against it. However this type of glazing is more brittle than twin wall polycarbonate so can crack if care is not taken when assembling the greenhouse. 

Polycarbonate 

This twin-walled plastic glazing lets 83% of light through, is resistant to breakage and is good at retaining heat within the greenhouse. This is the best glazing to use if children or pets are around as it’s difficult to damage and won’t cause any harm. However, if you are raising plants which require high light levels you would be better choosing toughened glass for the glazing. 

On the downside polycarbonate can be more difficult to clean than glass and it can become cloudy and discolour over time. Being very lightweight you also need to ensure that the greenhouse frame is designed to keep it securely in place (as with the design of the Rion greenhouses where are the edges are covered by the frame) or that extra fixings are used (such as the option for Bar Capping with Elite greenhouses). 

Greenhouses can have glazing to the ground, can have timber or aluminium panels part way up the greenhouse walls (usually panels are about 2ft (60cm) in height or can be fixed to a wall (known as ‘dwarf wall’ greenhouses). Those whose walls are not glazed to the ground will naturally retain more heat than those which do. However, if you intend to grow from the ground this type of greenhouse would not be the best choice. 

Orientation 

The orientation of your greenhouse becomes more important as the size increases. For a standard 6ft x 8ft greenhouse, being almost square means that orientation is not such a worry. However, if you have a long greenhouse, such as 8ft x 16ft, then the orientation does start to be a factor. 

So, if you’re planning a large greenhouse ideally it should be facing east-west (the ridge line of the greenhouse lies east to west) with the door at the western end as this means the greenhouse will be fully exposed to the sun from the south, especially important in the winter months as it will capture more light. This makes this orientation best suited if you intend to use your greenhouse for mainly over-wintering tender plants. 

North-south facing greenhouses with doors at the south end will benefit in the summer months from sunshine on both sides but not the full force of hot midday sun along the length. This makes this orientation best when you intend to grow greenhouse crops such as tomatoes, cucumbers, peppers etc.

Lean-to greenhouses that are south facing could capture too much sunshine and overheat, whilst a north facing lean-to may not get sufficient sunshine.

Ideally avoid having the greenhouse door on a north facing side as this would let in too much cold air. 

Position

Ideally your greenhouse should be sited where it will capture maximum sunlight but is sheltered from strong winds. If your garden is in an exposed position consider where hedges or fences are which could offer some shelter to your greenhouse - if you haven’t got a hedge or fence to shelter the greenhouse from strong winds you would be advised to create one. 

Try to avoid siting your greenhouse near trees as this not only cause shadows within the greenhouse but will also shed leaves onto the greenhouse so more cleaning will be required. And in the worse case if a tree were to fall it could devastate the greenhouse. 

Consider whether you should position your greenhouse near your home or not. Position close to your home and it should be easier to access power for lighting, propagating and heating. If you want to install electricity to your greenhouse it’s best to plan this before the base is laid so it can be done at the same time. This also applies if you want to have mains water to your greenhouse. 

Select a space in your garden where there will be space to walk around your greenhouse when it’s in position – this not only makes it easier when you’re erecting the greenhouse, it also makes maintenance and cleaning easier. 

It’s also important to check that the ground your greenhouse will be standing on is free draining – this is even more important when you intend to grow direct in the ground in your greenhouse. If your greenhouse is to go onto hard standing you still need to ensure free drainage of water. Flag stones are naturally free draining as water can escape through the gaps between the slabs. If you are making a solid concrete plinth with a membrane for your greenhouse then it won’t be free draining and you may find that puddles form on the floor when water gets into the greenhouse – remember no greenhouse is 100% waterproof! 

Greenhouse on soft ground  Flags laid for a greenhouse  Greenhouse on a brick base 

Foundations 

To successfully build a greenhouse starts at the foundations, get those right and you’re on the right track! 

Ideally the surface it’s going to stand on should be level – an uneven surface will make greenhouse assembly more difficult, the greenhouse will not be as sturdy once completed and over time the greenhouse glazing could crack or fall out and the doors and windows could become difficult to open and close due to them being aligned incorrectly. 

Therefore if you’re putting the greenhouse directly onto the ground you’ll need to make sure this is level first. If your greenhouse comes with an integral base or an optional factory made base, these are easily attached to soil or grass as the greenhouses will be supplied with brackets for each corner of the greenhouse. Use these brackets to secure the greenhouse securely in place by digging a hole under each corner of the greenhouse and anchoring the corner bracket with concrete. 

If you want your greenhouse to be fixed to hard standing then the most popular foundation for a greenhouse is flag stones. Make sure they are level before you start erecting the greenhouse. Ideally have the flag stones covering an area 4” larger than the base dimensions all the way around – this enables you to drill into the flagstones to anchor the greenhouse without drilling too close to the edge when the flagstone might break. For easy access for cleaning and maintenance make the flag stones fill an area 12” bigger on all sides than the greenhouse. And if you want to use a water butt to collect rainwater make sure the base area is big enough to accept the stand for the water butt as well. 

Your other options would be to make a concrete foundation for your greenhouse. This concrete could be laid similar to the flagstones above, making the concrete larger on all four sides of the greenhouse. Alternatively you could create a concrete plinth which it the exact size of the greenhouse enabling the cill to fit over the concrete edge - if this is the case remember to ask your greenhouse supplier for exact base measurements so you create the correctly sized base.

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