Should I get a polytunnel or a greenhouse?

These are genuinely different tools rather than cheaper and dearer versions of the same one. A polytunnel is a season-extension structure for growing in the ground at scale. A greenhouse is a controlled propagation and cropping environment. Most allotments end up with a tunnel and most gardens with a greenhouse, and both are usually right.

Step by step

1. Start with what you want to grow.Crops in the ground at volume — potatoes to extend the season, beans, salads through winter, a big tomato bed — favour a tunnel. Propagation, overwintering tender plants, pot-grown crops and precise control favour a greenhouse.
2. Compare on area, not on price.For a given budget a polytunnel typically covers several times the area of a greenhouse. If growing space is the constraint, that difference dominates everything else on this list.
3. Think about ventilation honestly.A greenhouse can have roof vents, louvres and automatic openers. A tunnel ventilates through doors at each end and optional roll-up sides. Tunnels get very hot and are harder to control, which matters in a hot summer.
4. Compare heat retention.Single polythene retains heat poorly and a tunnel is difficult and expensive to keep frost-free. If overwintering tender plants is a main purpose, a greenhouse — especially a lean-to — is the better structure.
5. Factor in the cover replacement.Polytunnel polythene has a stated life of several years and then needs replacing, which is a two-person day and a recurring cost. Glass lasts indefinitely. Include that in the lifetime comparison.
6. Consider wind and the site.A tunnel is a large sail and is only as good as its anchoring and its trenched or clamped cover. On a very exposed site it needs serious groundwork. A greenhouse on a solid base is more forgiving, though it too must be bolted down.
7. Be honest about appearance and permissions.A polytunnel is highly visible and not to everyone's taste in a small garden. Some allotment sites, conservation areas and covenants restrict them. Check before buying.

Tips

• Diffuse light through polythene reduces scorch and gives more even growth through a crop canopy — one genuine growing advantage of a tunnel that is often overlooked.
• A tunnel with roll-up sides is enormously better than one without in summer. It is worth the premium and it is difficult to retrofit.
• Many people end up with both: a small greenhouse near the house for propagation and a tunnel on the plot for crops. They do different jobs.

Common mistakes

• Buying a tunnel for propagation and overwintering — It cools too fast overnight and is expensive to heat. Tunnels extend the growing season for hardy and half-hardy crops; they are poor at keeping tender plants alive in January.
• Skimping on the tunnel groundwork — The anchoring and the way the cover is secured — trenched in or clamped in base rails — is what stops the whole thing leaving in a gale. It is not the part to economise on.
• Assuming a tunnel needs no maintenance because there is no glass — The cover needs replacing on a cycle, the doors need adjusting, and the polythene needs cleaning. It is less maintenance than a timber greenhouse, not none.

Alternatives

• Polytunnel: Hoops covered with a single sheet of polythene. Very large covered area for the price, diffuse light with no scorch lines, and excellent for growing crops in the ground. The cover needs replacing every few years, ventilation is limited to the doors and any side vents, and it is much harder to keep frost-free.
• Glass greenhouse: Permanent, holds heat far better, easy to fit vents, staging, insulation and heating, and it looks like part of a garden rather than a piece of agricultural equipment. Costs far more per square metre and the glass is a hazard and a maintenance item.
• Polycarbonate greenhouse: Sits between the two on price. Better insulation than glass, safer, and it has proper vents and a rigid frame. The panels have a limited life like a tunnel cover, but it is a garden structure rather than a field one.
• Lean-to greenhouse: A greenhouse against a south-facing wall. The warmest and cheapest to heat, the shortest service run, and the smallest footprint — the right answer where space is tight.

Questions people ask