Greenhouse vs polytunnel

Two ways to create protected growing space

A greenhouse is usually a rigid framed structure with glass or solid plastic panels. A polytunnel generally uses a curved framework and flexible film covering. Both can extend protected growing and support crops such as tomatoes, cucumbers and seedlings. The better choice depends on land, access, maintenance and how you expect to grow—not a claim that one is universally “warmer”.

Use actual model drawings in the comparison. A small walk-in greenhouse in a suburban garden and a long tunnel on an allotment occupy different scales. If the site can only accept a 6 × 4 footprint, a theoretical low cost per square metre from a large tunnel is irrelevant.

Usable floor and headroom

A greenhouse's straight walls can make shelving and upright staging easier, although the roof slope still limits tall plants near the eaves. A curved tunnel may have generous central height but less standing room at the edges. Compare the usable growing width at plant height, not just the ground footprint.

Draw paths, crop rows and access to the back. For tomatoes, allow for mature foliage and harvesting on both sides. A path that looks generous in spring can disappear by August. Our practical greenhouse layouts are useful even if the final structure is a tunnel.

Light and summer climate

Greenhouse glazing varies: clear glass, clear polycarbonate and diffusing twin-wall panels produce different light patterns. Polytunnel film also varies in transmission and diffusion. Cleanliness, orientation, neighbouring shade and condensation can matter as much as material labels.

Both formats can overheat. Greenhouses may use roof vents, side vents, doors and shading; tunnels often rely on door openings, side ventilation or roll-up sides depending on design. Check what the specific kit provides. A door at each end is useful but not a substitute for monitoring and a workable ventilation routine. See the ventilation guide for the principles.

Base, anchoring and exposure

A greenhouse typically needs a precise level foundation to keep the frame square and glazing secure. A polytunnel needs its own correctly specified anchoring system, ground preparation and access. Neither should simply be set on a windy site without following manufacturer instructions.

On an allotment, tenancy rules may govern structures, foundations and appearance. In a domestic garden, check boundaries, nearby trees, planning constraints and whether the structure can be maintained from outside. A permanent hard base may be undesirable if the plot arrangement could change.

Maintenance and lifespan planning

Glazing can crack, shift or need replacement clips; frames and gutters need cleaning and checks. Tunnel film can tear, abrade or eventually need replacement, while the frame and doors also need attention. Ask how replacement panels or covers are specified and supplied for the exact model. Include the maintenance route and future replacement in your whole-project budget.

Do not assume a low initial kit cost guarantees the lower long-term cost. Conversely, do not buy a rigid glass greenhouse simply for permanence if you need flexible crop rows and can properly maintain a tunnel.

Which suits your garden?

Choose a compact greenhouse when space is limited, you want defined staging and a rigid structure fits the garden and base. Consider a polytunnel when you have enough length for productive rows, permission to erect one and a credible anchoring and cover-maintenance plan. If unsure, mark both full footprints, including door and exterior working space, on the ground. Then compare model-specific delivered-and-installed quotes rather than generic format claims.

For greenhouse sizes, see what size greenhouse do I need? and our 8 × 10 guide. If the choice is still between a very small enclosure and open beds, the cold frame comparison may be more relevant than either large structure.

The RHS greenhouse-alternatives overview is useful background for this format decision. The comparison should still finish with the exact greenhouse or tunnel instructions, especially where wind exposure and anchoring are the limiting factors.