Do those small plastic mini greenhouses actually work?

A mini greenhouse is a very small volume of air behind a very thin cover. That combination means it heats up alarmingly fast in sun and loses that heat almost as fast after dark. Understood as a sheltered, ventilated shelf unit rather than as a small greenhouse, it is genuinely useful and very cheap.

Safety

Step by step

1. Know what it is good at.Hardening off seedlings, sheltering trays and small pots from heavy rain and wind, protecting young plants from slugs and birds, giving a few weeks of season extension at each end, and holding overwintering cuttings in a sheltered spot.
2. Know what it is not good at.Holding warmth overnight — the thermal mass is negligible and the cover is a single thin skin. It is not frost protection on a cold clear night without additional cover, and it is not a place to grow a tomato to maturity.
3. Anchor it before you use it.Tie the frame to a wall, a fence post or a downpipe at two points, and weight the base shelf with paving slabs or bags of compost. A blown-over mini greenhouse is the commonest complaint about them and it is entirely preventable.
4. Site it against a wall, out of the wind.A south or west-facing house wall gives warmth, shelter and something to tie to. Out in the open it is exposed on all sides and blows over.
5. Ventilate it every single day it is sunny.Roll the cover up or unzip it fully in the morning and close it in the late afternoon. A closed mini greenhouse in spring sun reaches temperatures that cook seedlings within an hour. This is not optional.
6. Expect to replace the cover.The PVC covers degrade in UV and split at the seams, typically after two or three seasons. Replacement covers are sold separately and cost a fraction of a new unit — check availability for your model before buying.
7. Upgrade it if you use it seriously.Replace the flimsy shelves with rigid ones, add a horticultural fleece layer inside for cold nights, put a max-min thermometer on the middle shelf, and stand a tray of water in the bottom for a little thermal mass.
8. Consider a cold frame instead.For the same money a solid cold frame holds heat far better, does not blow over, lasts for years and does most of the same jobs. It has no shelves, which is the trade-off.

Tips

• The zipped roll-up front is the weak point. Rolling it fully up and tying it, rather than half-unzipping, both ventilates properly and stops the cover flogging itself apart.
• Two layers — the cover plus a fleece thrown over the plants inside — buys several degrees on a cold night and is what makes the unit usable in early spring.
• Put the tallest plants on the bottom shelf, not the top. The top shelf is closest to the cover, gets hottest, dries fastest and scorches first.

Common mistakes

• Leaving it zipped shut on a sunny spring day — The internal temperature rises extremely quickly in a small volume behind clear plastic, and seedlings cook. Open it every morning without exception.
• Relying on it for frost protection — It has almost no heat retention. On a clear frosty night the inside is barely warmer than the outside unless you add fleece or a heat source.
• Not anchoring it — It is light, tall and presents a large surface area. In the first gale it goes over and everything on the shelves breaks.

Questions people ask