Colorado nurseries that I am aware of that specialize in rock garden plants are Rocky Mountain Rare Plants in Franktown, CO: www.rmrp.com and Laporte Avenue Nursery in Fort Collins. For others, contact the Rocky Mountain Chapter of the North American Rock Garden Society: www.nargs.org/meet/chapters.html You might be able to find where to look from at the Denver Botanical Garden which has a fantastic rock garden: www.botanicgardens.org/pageinpage/home.cfm Cheers, John P in NS

Believe it or not, tree roots can have its advantages. Soil dries quicker, and I'm guessing that will be an advantage in the UK. Even here in the USA, we have native plants that seem to always do better when grown in an area invaded by roots that are not their own. Of course, growth of the alpines will be slower, but that has the trade-off (usually) of more winter hardy plants. In the UK especially, winter wet is a serious problem, and any evergreen tree will help with that problem, helping to dry out the soil.

You don't say what your climate is, or whether you prefer natives or exotics, flowers or foliage, so it's hard to be specific. Some bulbs like shade, and others go dormant early enough for the maple's shade to be irrelevant. Some species of the genus Bellium (English daisies) form low groundcover in fairly deep shade, as do some Arenaria species. Epimedium, tricyrtis, and some species of anemone, corydalis and primula flower well in shade. And there's always ferns, many of which are very dainty.

Adding to my own list: Hebe pimeleoides- blackish stems with small greyish/bluish/silverish leaves that creeps along the ground at about 1 foot tall. Excellent for textural contrast and unique appeal, best against large rocks and contrasting foliage of other plants. Hebe "McKean"- dwarf 1 foot tall and wide dome of bright green/yellow scaly leaves, similar in appearance to a common heather plant (although in my opinion much more exciting and beautiful). Excellent near large rocks and contrasting foliage and colors. Most Hebes like very well drained soil with moderate moisture. See link for more info Cotula hispidula- feathery fluffy soft grey/silver foliage, low creeping, with interesting yellow button-like flowers on thin stems. Likes good drainage, full to part sun, and moderate moisture. Excellent between rocks, with gravel mulch under plant crown and stems to keep the feathery leaves clean and reduce chance of rot. Here is a link that might be useful: Hebe Society