Greenhouse buyer’s guide
A greenhouse is a big investment – financially, and in terms of the time you spend tending it. So it’s important to make the right choice. Use our guide to help you weigh up the options
A comprehensive guide to choosing a greenhouse to suit your needs
Where should you put your greenhouse?
In an ideal world, your greenhouse would go on a completely level site that receives uninterrupted light from all directions yet is sheltered from strong winds, and is not too close to any trees that would drop leaves on it. Your greenhouse would be close to the house, to make it economical to run water and electricity to it. It would also have easy access, including all the way around, for ease of cleaning. Of course, few of our gardens possess such perfect conditions. But do avoid full shade and areas that are prone to waterlogging. If you want to grow plants directly in the ground, then site your greenhouse away from large established plants that will thrust their roots into it. Sloping positions will need to be levelled, so do allow for this in your budget.
What is your budget?
Alan says: “Work out your budget and buy the best you can afford. When it comes to greenhouses ‘cheap is dear’, since you will most likely have to replace an inexpensive one more quickly than one that is well made.”
You can spend anything from around a couple of hundred pounds for a little lean-to, to several thousands, depending on factors like construction materials, size and shape. The cheapest greenhouses are made of aluminium and sit directly on the soil, with beds for you to grow your plants. Factors that will increase the cost include: Construction materials and choice of glazing Size Bespoke shapes Installation Landscaping work to level the ground Installing a solid base Installing power and water Equipment inside the greenhouse
What type of frame?
Alan says: “Choose a greenhouse design that will suit your house and your garden – modern and sleek, or traditional with finials.” Aluminium Aluminium greenhouses are cheap and strong. The glazing bars are thin, so they let the most light in. They require no regular maintenance. Powder-coated aluminium is more expensive but comes in several colours and looks attractive. It will last longer than untreated aluminium.
Wooden Wooden frames look traditional. The glazing bars are wider than aluminium, so less light gets in. But wood retains heat better than aluminium – extending the growing season at the start and end of the year.
Softwood greenhouses are cheap but need to be painted/treated regularly to prevent rotting. They have a shorter life span than hardwood, but if pressure treated, they can last almost as long.
Hardwoods, such as Western red cedar, are more expensive, but they do not need regular maintenance and will last much longer.
Aluminium strips over wooden bars extend the life of the frame.
Frameless New technologies allow greenhouses to be without a frame. These let in maximum light and are easy to clean. The cost is comparable to a good-quality wooden greenhouse. Some gardeners worry that birds would fly into a frameless greenhouse.
What is the best glazing?
Glass lets in the most light, but offers little insulation during the winter. Toughened glass is more expensive, but stronger and safer – if it breaks, it shatters into small pieces rather than dangerous shards – and is ideal for doorways. Polycarbonate sheeting is good for insulation during the winter, the diffused light is great for plant growth and it doesn’t need shading. However, it doesn’t let in quite as much light as glass, and the thicker the sheeting, the less it allows in.
What is the best size?
Alan says: “Buy a bigger greenhouse than you think you will need; you’ll soon fill it!”
A high roof height will allow in more light and improve air flow, helping plants to grow straight and strong. Height is also useful for training plants up canes. You won’t have to stoop, of course, but you’ll need long-handled window-cleaning tools. It’s also easier to maintain even heat with a high roof.
A rectangular greenhouse has the tallest sides, and it is easy to make use of the space inside with staging and shelves.
A lean-to is the answer where space is really tight. It is warm, but the light is one-sided.
Domes and octagonal greenhouses allow in lots of light and are better at withstanding high winds, but the roof is generally lower than other shapes.
What sort of vents should you choose?
Good air flow is vital to cool a greenhouse and reduce humidity, to limit fungal diseases. To work out how many vents you will need, measure the floor area and make sure the roof vents are equal to 15-20 per cent of this area, and the side vents plus the door are equal to 15-20 per cent again.
Roof vents are the most effective and can be automated so that they open when the greenhouse reaches a certain temperature, although they can be slow to respond.
Louvered side vents are a useful addition, or second choice – but them clear of clutter.
The door is a good source of ventilation, so
opt for as large an opening as possible; it also helps with access when you have lots to carry.
In a large greenhouse, use fans, hung midway between the vents, to improve air flow.
Do you want a soil base or a solid base?
Greenhouses can be built with a paved or concrete base, or placed onto compacted soil (using a steel frame cemented into the ground at the corners) that you can grow directly in to.
Soil base Cheap to construct Can grow plants up full-height canes Roots can grow beyond the boundaries of the greenhouse Simple watering regime Potential build-up of pests and diseases Soil structure deteriorates Limits an area to one use Not suitable for very large greenhouses
Alan says: “A greenhouse with glass sides is best if you plan on growing crops in the soil borders – tomatoes, melons, cucumbers, or a grape vine.”
Solid base Durable and solid Easy to keep clean Staging can be moved to provide flexible height growing spaces Storage under benches is possible Free from vermin and moles Makes it possible to have a knee-high brick wall, which helps keep heat at the base and makes the construction extra-strong Expensive if hard landscaping is needed Can only grow plants in containers or raised beds
Do you want staging in your greenhouse?
These are wide, waist-high shelves that run around the edge of the greenhouse. They can be freestanding or built-in. Slatted staging is adequate and the most common. Solid staging, or trays placed on slatted staging, allows you to use capillary matting for watering. It’s useful to have an area of solid staging for potting up.
What about shading?
The simplest form of shading is a kind of whitewash that is sprayed or brushed onto the glass in late spring and washed off in late summer, but it is messy to apply and to remove. Shade netting is clipped onto the bars on the inside of the greenhouse. It must be taken down and washed regularly to remove any pests or fungal spores. The best, but most expensive, option is integral blinds, preferably on the outside so that they intercept the sun’s rays before they penetrate the glass. Internal blinds are a good second choice.
Are you going to heat it?
Electricity is a cost-effective method of heating a greenhouse, but you will need to get an electrician to install electric points for lights, heaters and propagators at the outset. Propane heaters are a good idea where there is no electricity supply. Record max-min thermometer readings to monitor heating efficiency in the greenhouse. Insulating the greenhouse with bubble wrap or double glazing will keep down the bills, but make sure the vents can open. Heat mats and propagators are a cheap way to keep a small range of plants protected during the winter months. Water storage tanks provide warm water and absorb the day’s heat and can help to keep the greenhouse frost-free overnight.
Octagonal designs are good at withstanding high winds Louvered side vents are less heavy to operate than single pane vents Roof vents are essential to reduce humidity in a greenhouse September 2017
Solid staging provides a handy area for potting up Shade blinds diffuse the sun’s scorching effect September 2017