I use them in Spring and early Summer for hardening-off seedlings and young plants - particularly the chillis and tomatoes. To me they represent a crucial intermediate stage between the indoor seed-germination stage, and the final stage in which plants are put into the raised beds. They protect the delicate seedlings from wind and heavy rain. I am conscious though that even inside one of the greenhouses it can get pretty chilly, so I don't leave tender plants in them overnight if there is a likelihood of sub-zero temperatures.
I think that these things are excellent value for money. Prices vary a bit, but you can currently get one from B-and-Q for £19.91, or from Argos at £16.99, and no doubt from many other suppliers too. The former has a cover of plain clear plastic, whereas the latter has a cover that has a so-called "rip-stop" composition and has an overall green appearance. I suspect that it may be less good than the B-an-Q one at allowing light through.
I reckon that the life of one of these greenhouses is at least three years if you look after them properly. It is easy enough to disassemble them and store them under cover during the Winter if you want to, though I generally leave mine assembled and use them to protect some of my more tender potted plants. If you don't put them into storage, just be aware that they are extremely lightweight and will need weighting-down very thoroughly if they are not to be blown away by the Winter gales! The 2-tier ones are obviously much less vulnerable in this respect than the tall 4-tier ones. Incidentally, the covers are fitted with metal rings which allow you to secure the greenhouses to a wall, though of course this means that you would sacrifice the ability to move them around easily, which I think is a big benefit.
At present I have three of these greenhouses (I did at one stage have four, but one was comprehensively demolished in a storm last year). I'm not sure how old they are, but the covers of all three needed replacing, so on Friday I splashed out on 3 new covers. I got them from B-and-Q and they were £7.41 each.
I was going to fit the covers over the weekend, but we had visitors yesterday, and the weather today is atrocious, so they'll just have to wait till next weekend!
