Choosing a Greenhouse: A Complete Buyer's Guide

A greenhouse extends your growing season at both ends and gives tender plants a fighting chance through a British spring. Bought new they are not cheap once you add the base and the glazing, which is why picking one up ex-display or at end of season makes real sense. Get the frame, the glazing and the siting right first, then chase the deal. Start with the greenhouses listings to see what shapes and sizes come up.

Aluminium versus timber frames

The frame is the decision everything else hangs off. Both materials work well, they just suit different gardeners.

• Aluminium. The common choice and the easier one to live with. It never rots, needs no painting or treatment, and the slim bars let in plenty of light. It is lighter to handle, which matters if you are collecting and rebuilding an ex-display frame. Powder-coated versions come in green or black if you do not want bare silver. The main downside is that aluminium holds no heat and can feel a bit utilitarian.
• Timber. Usually cedar or a treated softwood. It looks smart, suits a traditional or walled garden, and the thicker glazing bars hold a touch more warmth. The trade-off is upkeep, since it needs treating to stop it weathering, and it is heavier to move. Cedar is naturally rot-resistant and the pick if you want timber with less maintenance.

For most people buying on a budget, an aluminium frame is the sensible starting point. It is cheaper, lasts decades with no attention, and turns up ex-display more often.

Glass versus polycarbonate

The glazing changes how the greenhouse performs and how safe it is around children and pets.

• Horticultural glass. The traditional glazing. Best light transmission, holds heat well, and stays clear for years. It is heavy, it can break, and toughened safety glass is worth the extra if you have kids or a football in the garden. Ex-display glass greenhouses need careful transport, since a cracked pane is a faff to replace.
• Polycarbonate. Twin-wall polycarbonate is light, close to unbreakable, and the air gap gives better insulation than single glass. It diffuses light, which spreads it more evenly and reduces scorch, though it lets through slightly less overall. It can discolour or come apart at the seams over many years. It is the safer and easier option for a windy or exposed spot, and it survives dismantling and rebuilding far better than glass.

If your priority is maximum light and a long life, go glass. If it is safety, insulation and surviving the move home from an ex-display sale, go polycarbonate.

Sizing

Almost everyone wishes they had gone bigger. Plants, pots, a staging bench and a bag of compost fill the space faster than you expect.

• 6x4. The starter size. Fine for a few tomato plants and some seed-raising, tight once you add staging.
• 6x8. The sweet spot for most gardens. Room for staging down one side and growing bags down the other.
• 8x10 and up. A proper growing space with room for borders, staging and taller plants. Worth it if you grow seriously.

Check the eaves and ridge height too, not just the floor area, since headroom decides whether you can work comfortably and grow tall crops like tomatoes and cucumbers.

Siting and base

Where you put a greenhouse matters as much as which one you buy. Pick an open, sunny spot away from overhanging trees, which drop leaves, cast shade and drip sap. A south or south-east aspect gets the most light. Avoid a frost pocket at the bottom of a slope, and keep it out of the worst of the wind, or fix it down well if you cannot.

The base has to be firm and dead level, or the frame will not square up and the door will jam. Options are much the same as for a shed:

• A perimeter of paving slabs, leaving soil borders inside for planting.
• A full slab or concrete base for an all-staging setup.
• The manufacturer's steel base kit, bedded onto a level, compacted surface.

Most greenhouses need anchoring, so plan how you are fixing it down before you build. An unanchored greenhouse can shift or lift in a strong wind.

Ventilation

Ventilation is the part beginners underestimate. A greenhouse can cook on a sunny day and reach temperatures that stress or kill plants. You want more airflow than the single roof vent most cheap models come with.

• Roof vents. Let hot air escape at the top. An automatic vent opener, which works off a wax cylinder and needs no power, is well worth adding.
• Louvre side vents. Pull cooler air in low down, creating a flow through the greenhouse. Check whether an ex-display model has one, and whether it can be added.
• The door. A propped door helps on hot days, but vents do the steady work.

When you look at an ex-display greenhouse, count the vents and check the openers move freely, since seized vents are common on units that have stood a while.

Buying ex-display and at end of season

Greenhouses follow the same clearance cycle as the rest of the garden range. Late summer into autumn is when display models get cleared and prices fall, and it is also a sensible time to build, giving you a working greenhouse ready for spring sowing. A display greenhouse has usually stood outside, so check the glazing for cracks, the frame for straightness and the vents and door for smooth movement. Confirm all the glazing clips and fixings are present, since these small parts go missing and are fiddly to source.

Because ex-display.co.uk links you straight to the seller, you can ask about missing clips, transport and whether it comes dismantled before you commit. Get the frame and glazing right for your garden, prepare a level base, and buy at end of season, and a greenhouse is one of the best-value things you can add to a garden. Browse the current greenhouses listings, or head back to the main garden section to compare against sheds and other buildings first.