Waltons’ ultimate guide to buying a greenhouse
Offering the optimal growing environment for the nurturing of tender plants and seedlings, a greenhouse is a must for every gardener. Here’s our step-by-step guide to help you choose the right greenhouse to suit your growing requirements and the available space in your garden or allotment.
- Why greenhouses are good
- Where to site your greenhouse
- What size greenhouse?
- Greenhouse bases
- Greenhouse materials
- Greenhouse glazing
- Greenhouse ventilation
- Greenhouse heating
- Water capture
- Greenhouse alternatives
Why greenhouses are good
Investing in a greenhouse for your garden or allotment makes sense for anyone who is even slightly serious about getting the most from their plot. It will help you to:
- Plant seeds and bring on seedlings that bit earlier in the growing season; the comparatively warm, moist atmosphere of a greenhouse is also the perfect place to plant cuttings.
- Got tender plants that won’t survive a hard frost? Depending on whether or not you choose to heat your greenhouse, this can be the perfect place to over-winter delicate plants.
- As a place to grow bumper crops of tomatoes, grapes, salad leaves and more, your greenhouse can’t be beaten – it also makes a great place to grow exotics.
- When the evenings draw in, your greenhouse maximises light and warmth, keeping plants fruiting and ripening for longer.
Where to site your greenhouse
Before thinking about the design and size of your greenhouse, give some thought to where in your garden you’re going to site it:
- If the apex of the roof follows an East – West line, you’ll maximise the available sunlight right through the day.
- Choose a place away from shady trees and sun-blocking walls or other structures. Think about the way the shadows change and lengthen through the day, and choose a spot that gets plenty of light.
- A flat site is easiest – you’ll have less work to do to make it level.
- Avoid the bottom of any slopes because these are more prone to flooding and tend to be cooler and darker.
- Make sure you leave adequate room all round your greenhouse. This not only maximises light penetration, it also makes installation and maintenance easier. We suggest you leave a gap of at least 1m between your greenhouse and any walls, shrubs or hedges.
- Do you want to install heaters, lights, or a water supply? Make sure you site your greenhouse as near as you can to any utilities you need.
What size greenhouse?
Not having a big enough greenhouse is a common complaint among gardeners. Always buy the biggest you can afford, or that your garden can accommodate, bearing in mind your garden’s overall size and planning regulations. Here are some things to consider before you decide on the final size for your greenhouse:
- What do you want to grow? Obviously the more you want to grow in your greenhouse, the bigger the structure will need to be. If you’re planning to plant seedlings, bear in mind that half-walled greenhouses don’t let the light hit the ground throughout the day – you’ll need to install a bench to grow them on.
- Most greenhouses and potting sheds don’t require planning permission before building, but do check the planning regs because, depending where you live, there may be restrictions on size and allowable location.
- Height: the higher your greenhouse, the more efficiently it will let in the light – the eaves should be at least 5’ high.
Unless your greenhouse has a level base, you run the risk of the structure distorting. Your windows and doors won’t close properly, making it harder to regulate the temperature and keep bugs and vermin at bay.
If your greenhouse has glass windows, they could crack. Your greenhouse needs a sturdy, level platform that also allows good drainage:
- Patio slab base: Easy for someone with basic DIY skills to install, a patio slab base makes an excellent semi-permanent base for a greenhouse. Rather than installing a conventional patio, install a perimeter with a path through the middle. This means you’ll be able to build in a soil border or raised beds for plants and seedlings, and still have a solid place to stand. On hot, sunny days, wet down your patio path to maintain air humidity inside your greenhouse.
- Concrete base: Laying a concrete slab means your greenhouse will have a rock-solid permanent base, but drainage could be an issue unless you create the same “perimeter with a path” base, mentioned above.
- Metal base: Some greenhouses are supplied with a metal base that you can secure to the ground, creating a level platform to support a lightweight construction.
The material from which your greenhouse is built matters for a variety of reasons including light penetration, heat retention, maintenance requirements and strength:
- Metal: A traditional aluminium-framed greenhouse is lightweight and sturdy, and its minimal structure maximises the available light. The “glass to the ground” design also boosts the amount of light that reaches soil level, making it possible to grow plants and seedlings on the ground or in raised beds. Metal greenhouses are available in a range of colours and require minimal maintenance. But metal is a poor insulator, conducting heat away from the inside and the plants that need it.
- Wood: Attractive and strong, wooden greenhouses are a good bet for people who like their garden buildings to have a natural look and feel. Though some structures are more minimal, wood is generally bulkier than metal and so reduces the amount of available light. Wood also requires regular maintenance and treatment with wood stain to prevent rot.
- Apex: A standard apex greenhouse is the most common design – and there are some very good reasons for that – but there are also options to suit smaller gardens and different growing requirements and climate conditions:
- Freestanding: A traditional freestanding apex greenhouse offers excellent all round light penetration, and can be sited anywhere in the garden.
- Lean-to: Ideal for those with limited outdoor space, a lean-to greenhouse design enables you to optimise your use of your plot by choosing a structure that butts up to an existing house or garden wall. The downside is that you get less light.
- Octagonal/ hexagonal: A less common design, but one that’s very appealing to those who like to apply some design flair to their garden.
- Glass to the ground: Whichever shape greenhouse you choose, one whose frame allows for glass panels that reach ground level will let in more light, though low-level glazing is vulnerable to breakage from bumping into it with the lawnmower.
- Half-wall: Also known as ‘dwarf wall’, opting for a foundation and low wood or brick wall as the base for your greenhouse, offers the advantage of being more sturdy than the “glass to the ground” option. A brick or wooden wall acts as a heat sink, trapping heat from the day’s sunshine to help regulate the internal temperature during the cooler hours of the day, and overnight. However, a dwarf wall blocks light at soil-level, reducing the vertical growing room. You’ll need to build a bench at window height if you want to grow seedlings.
Long gone are the days when the only glazing option for your greenhouse was glass – easily cracked or shattered, it was a liability for anyone with children – now there are far better options to choose from.
- Safety glass: Laminated so that if it does get broken, it won’t shatter. Safety glass means you benefit from glass’ chief advantage – namely that it doesn’t degrade in UV light – while making it safe for gardens in which children play. Glass also works best in tall windows, because unlike plastics, it won’t bow over time.
- Polycarbonate: 200 times stronger than glass, like all plastics, this glazing solution weighs about half as much as glass, is ultrasafe, and offers better insulation capabilities than glass. Polycarbonate can be UV treated on both sides, so it doesn’t degrade in sunlight. Like with all plastics, you must take care when cleaning it because it’s easy to scratch. Polycarbonate is the most expensive of the plastics.
- Acrylic: 10 times stronger than glass, and can be UV protected on at least one side. Its price point is midway between the more expensive polycarbonate and the cheaper styrene.
- Styrene: Twice as strong as glass, styrene is an affordable all-round option. Shatterproof styrene is also lightweight, offers great light penetration, has good insulation properties and is robust. Styrene isn’t typically UV protected, so it will eventually discolour and will have to be replaced. This is the cheapest of the plastics.
Heat and light make up two aspects of the greenhouse growing environment. The third is ventilation. All greenhouses should incorporate adequate movement of air into their design, plus the ability to let excess heat escape so plants don’t wilt and die. Here are some of the solutions available:
- Hinged roof windows: A vital component of any greenhouse, hinged windows fitted into the apex of the glazed roof, allowing air to circulate to stop the temperature soaring on sunny days.
- Side vents or louvre windows: These are vents fitted to the gable walls of your greenhouse. Not every structure has them, but they’re a great addition because they help renew the air in your greenhouse. Vents help to stop the air getting stuffy, and they stop a moist atmosphere from degenerating into damp which causes mould and rot.
- Automatic windows: If you’re not around to monitor the temperature in your greenhouse, you may wish to consider retrofitting an automatic window opening system which features a gas-filled tube which, as it heats up, pushes out a piston to open the window. As the temperature falls, the piston withdraws, reclosing the window.
Heating your greenhouse during the cooler months is a must if you use your greenhouse to grow exotics. But even if you just want to overwinter tender plants, you’ll need some sort of heat source to keep the temperature above freezing, or above the dew point to guard against damp and rot:
- Electric heating: Expensive to run but with the distinct advantages of controllability, the absence of flammable gas or liquid, and without noxious fumes to worry about, electric heating is perhaps the gardener’s preferred option.
- Gas/paraffin: Cheap to run, but not easy to control and when left unattended these heaters do have the potential to cause fire. Both these options produce fumes and, if they’re not burning properly, carbon monoxide, which can prove deadly. If you fit gas or paraffin heating, you must install a carbon monoxide alarm.
- Ground source heat pump: An environmentally-friendly form of heating in which warmth from the ground is absorbed into buried, fluid-filled pipes. The heat is concentrated by a compressor before being piped into your greenhouse. It’s an expensive option and difficult to retrofit, but worth consideration if your focus is on sustainability.
It’s a really good idea to think about collecting the rainwater runoff from your new greenhouse roof. Fitting gutters and installing a water butt to store the water is an easy job, and you’ll reap the reward in lower water bills, and a watering option if there’s a hosepipe ban. You could even connect your butt to an irrigation system to keep plants happy on even the driest of days.
- Potting sheds: A potting shed is ideal for anyone whose primary use for a garden building is germinating seeds and bringing on seedlings. The wooden walls offer far superior insulation compared to a greenhouse, helping to keep the temperature warm and stable – a definite plus if you’re growing young plants. There’s also plenty of storage for your garden tools and equipment, making a potting shed a good choice if you’re short on space. If you’re considering this option, do make sure you can position your potting shed so that the glazed side gets plenty of sunlight – south is the best orientation.
- Frame/PE cover: If you’re on a budget, then a steel frame and polyethylene cover could be a good solution for you. Plants are protected from the elements, and the PE film allows sunlight to penetrate.
- Grow house: A small, wardrobe-sized unit could be an ideal tool store and propagation area. Taking up less room than a greenhouse, but still capturing all that vital sunlight for your seedlings, a Victorian-style growhouse is a great alternative for those with limited space.
- Cold frame: For those with little space to spare, a cold frame is a great solution, giving just enough room to bring on a few trays of seedlings.
- And finally, for those with more space and altogether larger growing ambitions, there’s the option of installing a polytunnel.
Selecting a greenhouse is an investment in the future of your garden and has a major impact on what you’ll be able to grow both now and in the future. We hope that having read through our guide to choosing a greenhouse, you’ll feel confident enough to choose the right building to suit your needs.
You should now have an idea of the size and type of greenhouse you need, and the options and upgrades available when you want to buy. For more help, call our friendly sales experts on 0800 029 1000.
- Find out how to build a solid base for your greenhouse here: How to build a shed base.
- Find out if you need planning permission here: Guide to Planning Permission
- Waltons help pages
- Waltons blog
Browse our full range of greenhouses, sheds, log cabins, insulated garden rooms, playhouses and summer houses by visiting waltons.co.uk.