Most trilliums take several years to get to flowering size. Yours may be older than you think, maybe 4 or 5 years depending on how well it has been grown. Trillium grandiflorum is a woodland plant from acid and alkaline woods, so not too fussy as long as it well drained and not too dry in spring. In north America, where they have dryish summers they tend to loose their leaves by mid summer, but here they can keep their leaves much longer. I grow some in pots but they are better in the garden. They may take some time to get established, and flower for you, the first flower being smaller than the eventual adult size, which can be quite dramatic. You won't regret growing them. There is a discussion list based on trilliums, and plants for similar conditions. Trillium-L.

Brian

One more thing to be aware of, Trillium resent root movement, so they may not show signs of growth this year if its freshly potted, it might even die off the top growth it already has, but you have to keep it a full year after even if it does, because more often than not they regrow again the following April, they are touchy plants, so need a bit of patients as well as luck, its best to let them establish a bit in the pot before planting out, and plant out in autumn after its done its bit, not quite as touchy then.

Thanks very mcuh for this advice, especially for pointing me at the Trillium L list - I've joined it and had a good search of the archives. I'm now well prepared for being patient! but it will be worth it.