Posted: 08/07/2013 at 15:36
Probably some simple ground cover would be best as the ground will probably get quite dry. Periwinkle, Brunnera, Epimedium and Euphorbia come to mind for that kind of situation, and are mostly evergreens. Flowers are not generally large but you can add bulbs to the mix . I assume the tree's deciduous and you could get quite a nice succession of bulbs before the canopy takes over. If you can get some organic matter into the soil to improve water retention that will help, you could even put a Hosta or Astilbe there if the soil's not too dry. Think 'edge of woodland' for suitable plants.
I'm sure others will come up with plenty of other suggestions too.