A greenhouse's main job is extending the growing season at both ends. It keeps frost-tender crops like tomatoes, capsicums and cucumbers safe from a late spring or early autumn frost that would kill them outdoors, and it lets seedlings and heat-loving crops get going several weeks earlier in spring than they could in open ground, since the enclosed space traps daytime warmth. In cooler regions, particularly Canterbury and Otago, a greenhouse is often the difference between reliably ripening a full crop of tomatoes and constantly racing the season.
Glass greenhouses are the traditional option, and for good reason: glass holds heat well, lets in the most light, and lasts for decades with only minor upkeep. They're also the most expensive option by a wide margin, need a permanent, level base, and are a genuine building project rather than something you set up in an afternoon. For a homeowner planning to stay put and wanting a long-term structure, a glass greenhouse is a real investment rather than a garden accessory.
Portable poly tunnels, the hoop-frame structures covered in polythene or PE sheeting, are the far more common choice in New Zealand home gardens, mostly because of cost and flexibility. A poly tunnel is a fraction of the price of glass, can usually be assembled without any real building skill, and can be moved or taken down entirely if you're renting or might move house. The trade-off is durability: the plastic cover degrades under UV over time and typically needs replacing every few years, and a poly tunnel offers noticeably less heat retention and wind resistance than glass, particularly in an exposed or genuinely cold site.
Ventilation is worth planning for regardless of which type you choose. An enclosed structure on a sunny day heats up fast, and without roof vents, louvre windows, or simply propping the door open, temperatures inside can climb high enough to stress or scorch plants rather than help them. Most greenhouse and tunnel failures aren't from cold, they're from a structure that was sealed up tight on a warm day and cooked everything inside it.
Most greenhouse and tunnel failures aren't from cold, they're from a structure that was sealed up tight on a warm day and cooked everything inside it.
For most home gardeners, a portable poly tunnel is the sensible starting point: cheap enough to try without a big commitment, and genuinely effective at giving frost-tender crops a longer, more reliable season. A glass greenhouse is worth the jump only once you're confident you'll use the space consistently and are ready to treat it as a permanent part of the garden rather than a trial.