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21 October 2020

How to buy the best greenhouse

Our expert greenhouse buying guide reveals the different types of greenhouses, typical prices and sizes and how to pick the best heater for it.
JH
Jade Harding

A greenhouse is a major investment for your garden and one that’s not easy to move or replace once installed, so it's important to pick the right one for your needs.

Whether you want a small greenhouse to protect trays or pots of young plants or you’d prefer a larger, freestanding greenhouse that offers enough room for you to work on your fruit and veg inside, our expert guide will explain everything you need to know. 

Keep reading to find out the optimal greenhouse materials and sizes to the best features, accessories and heaters, plus our top tips on how to build a greenhouse.

Types of greenhouses

We run through the pros and cons of some of the most popular greenhouse types:

Freestanding greenhouses

Most people opt for a 6 x 8ft or 8 x 10ft freestanding greenhouse. Typically square or rectangular with an apex roof, you can get them in a variety of sizes and they allow light in from all four sides. 

Pros 

  • Can be placed anywhere in your garden 
  • Let’s light in from all four sides
  • Lots to choose from 
  • Can be roomy enough to work inside 

Cons

  • Can lose heat much easier as all four sides are exposed
  • Unlike an attached greenhouse, you will have to find a way to get power to it

Lean-to greenhouses

Consider making use of a back or side wall with a lean-to greenhouse. The initial purchase costs are similar to a freestanding model, but it takes up less space and you only need access around three sides. It is also cheaper and easier to keep warm.

Pros

  • The building provides wind resistance 
  • Maximises unused space
  • More energy efficient   

Cons

  • Not as much space to plant a variety of crops
  • Less sides for the light to come in

Mini greenhouses

A mini greenhouse is like a glazed cupboard with shelves that sits on a patio or under a window, using a house wall or garage as protection – perfect if space is limited. It will protect trays or pots of young plants from late frosts in spring, and encourage borderline crops such as peppers to ripen. Ideally, a mini greenhouse should face east or west. South-facing ones overheat in spring or summer unless shaded and north-facing ones suit only shade-loving plants.

Pros 

  • Great for people short on space 
  • Cheaper than full-sized greenhouses
  • Most can be stowed away when not in use

Cons 

  • Can be difficult to tend to plants if you have limited mobility 
  • Limited growing space 

We've trialled lots of tomatoes using greenhouses to grown them in. Find out everything you need to know about how to grow tomatoes in our expert guide.

Wooden vs aluminium greenhouse frames

Most greenhouse frames are made of aluminium, but wooden greenhouses are also available and many gardeners find them more attractive to look at, although they need more maintenance than metal frames. 

Aluminium greenhouses

Aluminium is the most common material for greenhouses which means you’ll be spoilt for choice. Available in a variety of sizes and shapes and to suit all types of budgets, here’s some pros and cons to be aware of.  

Pros

  • Budget-friendly options 
  • Wide variety of shapes and sizes
  • Durable 
  • Can pick from a range of powder-coated colours including green, brown, black and blue
  • Plain silver is maintenance free
  • Light and easy to assemble
  • Casts very little shade so the maximum light reaches your plant

Cons

  • You’ll need to buy special fastenings to attach bubble plastic or shading material 
  • Can be difficult to extend
  • Insulation can be poor
  • Normally need to add your own strong, sturdy base

Wooden greenhouses

Wooden greenhouses are less common and typically cost more than their aluminium counterparts. Picking the right type of wood will help avoid a greenhouse that’s vulnerable to rot. Softwoods, such as deal (pine), are vulnerable to rot, even if the timber has been treated. 

Naturally durable timber – usually western red cedar – can be left to age to a silver-brown. For looks, a western red cedar greenhouse is hard to beat. Look for manufacturers that offer a guarantee against rot. Here are some other pros and cons to be aware of. 

Pros

  • Tends to be more in-keeping with a garden
  • Can be stained to whatever colour you’d like
  • It’s easy to attach bubble plastic or shading material to the frame with drawing pins 
  • Better insulation 

Cons 

  • Can be expensive 
  • Normally need to add your own strong, sturdy base – especially to keep the wooden frame from being in contact with the damp ground.
  • Can be vulnerable to rust 
  • It needs regular maintenance 

If a shed is more suited to your needs then here's our expert shed buying guide

Can I buy a plastic greenhouse?

Glass is the main glazing material used in greenhouses, hence the alternative name ‘glasshouse’. Horticultural glass is inexpensive and easily replaced, but if you are concerned about breakages or safety, you can opt for a plastic greenhouse. 

Plastic greenhouses have twin-walled polycarbonate sheets. Polycarbonate greenhouses let in sufficient light for plant growth initially, but, over the years, they can become cloudy and do not age well. Toughened glass is a better option, particularly if the greenhouse is on show.

Toughened or safety glass is stronger than horticultural glass and, if it does break, the resulting pieces are not so sharp. Choosing this option adds about £200 on to the cost of a small greenhouse.

Another option is self-adhesive safety film. This is a transparent film that is applied to horticultural glass. If the glass is subsequently broken, the film holds the fragments together. However, as the film does not reduce breakages, it could prove relatively expensive over the option of choosing toughened glass from the start.

Our controlled-release fertilisers reviews reveal which the products that will keep your plants looking good all summer long. 

Cheap greenhouses

The cheapest greenhouses will almost always be smaller, aluminium models and will set you back around £200 or more. 

Anything under £200 is most likely to be a mini greenhouse or a cold frame.  

Larger, wooden structures and greenhouses with ready-made shelving and additional accessories can cost anywhere between £1000 and £4000. 

Greenhouse sizes

The size of your greenhouse will be limited to what you can fit in your garden. So for most of us it will be fairly modest, especially if you live in the city. Larger structures will also typically cost more so even if size isn’t a factor, budget might be. 

Your best bet is to opt for the biggest greenhouse you can. Width is an important dimension because this will determine the benches and aisles. 

Most people opt for a 6 x 8ft or 8 x 10ft freestanding greenhouse. This is wide enough for a central path, with shelving on one side to hold a propagator and potted plants, and a greenhouse border on the other side, so tall cropping plants, such as cordon tomatoes and cucumbers, can be planted direct into the soil.

12 feet wide is a semi-professional width, and anything above this is a typically for more commercial use. 

One thing to remember is to make sure you can get in and out of the door easily and that you feel comfortable once inside (low greenhouses can feel claustrophobic).

If space is super-limited then you might want to opt for a mini greenhouse that will sit under a window or against the wall.

Greenhouse features and accessories

  • Greenhouse vents - you will need at least two vents in the roof and one at the side to give a good flow of air. The number of vents depends on the size of the greenhouse, but consider paying extra for additional ones. 
  • Automatic vent-opener - contains a cylinder of wax that expands and opens in heat, then closes when cooler so you're not reliant on being there to open and shut the vents as the temperature changes. 
  • Shelving or staging - greenhouse shelving, also known as staging, will display plants and provide a surface for you to sow seeds, take cuttings etc. Both wooden and aluminium staging is available so you can get the same type as the greenhouse frame. Staging that is easily removed can be useful if you plan to use the floor space later in the season for growing plants in pots or growing bags.
  • Slatted staging - good for letting light through the lower shelves and can encourage good air circulation which can reduce the risk of fungal diseases. 
  • Solid staging - is needed if you want pots to sit on capillary matting to make watering easier.  
  • Shading - shading is important to help keep things cooler. Shading blinds or netting can be fitted outside or inside the greenhouse or you can paint on a white shading wash, which looks a bit unsightly in the summer but soon washes off. 
  • Heated propagator or heated pad - many seeds and cuttings benefit from some extra heat. Look for a model with a thermostat as this will allow you to preset the temperature you would like. Whichever you choose, you'll need an outdoor socket with an RCD to provide the power needed to run them.
  • Min/max thermometer - keeping an eye on the temperature of the greenhouse will help you ensure your plants aren't getting chilled or cooked. You can choose from traditional models that sit in the greenhouse or battery-powered ones with a remote sensor whose readings you can view from the comfort of your house.
  • Modular trays - have individual compartments for a number of plants within the same tray and help to avoid root disturbance when you take the plants out. 
  • Recycled pots - if you're concerned about using plastic, either look for recycled pots (many garden centres offer these) or use a soil-block maker which creates blocks of compost to grow your plants in. 
  • Clean labels - are invaluable for identifying what you're growing. These can be cleaned with wirewool or sandpaper at the end of the season so they can be reused. A pencil is the simplest way of marking them. 
  • Potting tray - is a useful accessory as you can stand trays and pots in it while filling them with compost and then simply tip the excess compost back into the bag so it's not wasted.
  • Irrigation system - to keep the plants watered while you're out. You can run these from the mains or from a water butt using a water-butt pump to move the water. 
  • A watering timer/computer - will allow you to set the water to switch on and off without you needing to be there. 
  • Self-watering pots - with a water reservoir. 
  • Capillary matting - which will absorb water when you wet it and gradually release it to plants so can help keep plants on staging moist. You'll need a solid base under it though.

We'd always recommend that you use a Best Buy compost

Where should I put a greenhouse? 

In practice, there are probably only one or two suitable places in your garden, so the size of greenhouse you opt for will be limited by that – which is why most people end up with a small one. Here some top tips to picking the perfect place for your greenhouse:

  • The greenhouse should be sited where there is plenty of light, but not exposed to high winds.
  • A sheltered spot is good, but avoid placing it under trees.
  • There needs to be access space around the greenhouse for cleaning, as well as room for acclimatising greenhouse-raised plants to the outside (eg space for a cold frame).
  • A heated greenhouse gives more scope for raising a wider range of seeds and cuttings earlier in the season. The nearer the greenhouse is to the house, the more economical it will be to get electricity to it. You must, by law, hire a professional electrician to do this. 

If you’re looking for a recommended tradesperson you can trust, visit Which? Trusted Traders to find someone near you who has been through our rigorous background checks.

Greenhouse heaters

You'll need an electric fan heater rated at least 2kW to guarantee keeping a standard 6ft x 8ft greenhouse frost free in most parts of the UK. If you opt for a gas or paraffin heater it'll need to be about 20 per cent more powerful to compensate for the heat lost via the greenhouse vents left open to allow poisonous gases and water vapour out and oxygen in. 

Lining the glass with bubble insulation will also help to retain the heat and keep your greenhouse snug in the depths of winter. 

Types of greenhouse heaters

Electric greenhouse heater 

If you greenhouse has an outdoor socket with an RCD nearby, a thermostatically controlled fan heater is ideal. They're cheap to run and the fan not only helps distribute the heat but also encourages air movement which in turn reduces the risk of fungal diseases that favour still, damp air.

Bottle gas greenhouse heater

If you don't have an outdoor socket, a bottle gas heater with a thermostat is a good option. Bear in mind that they're heavier and bulkier than electric heaters. They're also trickier to switch on and have no fan so there may be cold spots in the greenhouse, particularly at floor level. 

Gas heaters produce water vapour so you'll need to leave vents open for it to escape or you could run into problems with fungal diseases. Good ventilation also ensures that the heater doesn't run short of oxygen and start giving off toxic gases such as carbon monoxide.

Paraffin greenhouse heater 

These heaters need to be filled up very frequently, and safety-wise there's not much to recommend as they tend to be easy to knock over and they're a fire risk.

Compare gas and electricity prices now using Which? Switch, our independent energy comparison website. 

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