I'm not sure if you're planting trees to make the woodland or flowers to cover the ground, or both. The problem with flowers under trees is that once spring is past they get shaded out. There is research to show that bees prefer wildflowers to cultivated varieties, and if there's no choice then open single flowers, no doubles. Are you looking for wild flowers or garden varieties? Certainly bees love foxgloves but of course you will be wanting a continuous offering of flowers in every season, except perhaps winter. In the gardening for bumble bees thread already referred to by Penninehillbilly we are looking to see which flowers we grow are most attractive to bumble bees. We have only just started at the beginning of this year and I think only one person has any bees at the moment, so it will be February or March before making useful entries. Remember that bees also need somewhere to nest, unless you will be keeping hive bees, so rough grass, holes in masonry, uncut banks and so on help.
Trees liked by bees include: Willow, all the spring flowerers like cherry, blackthorn, mirabel, roses (roses of course are not trees but any woodland needs a shrubby layer as well as an understory), honeysuckle, buddleia as mentioned, alder, broom, gorse, and so many more, anything that produces pollen and nectar.
Flowers, depending on whether these are for under trees or in more open ground: Primrose, cowslip, bulbs, pulmonaria, daffodils, especially the native type, wood anemone, knapweed, meadow geranium, herb robert (which stinks), all sorts of low herbs such as thyme, marjoram, wild garlic, various mints including water mint (a small pond is good to include as bees need a drink every now and then), dandelions, clover, ox-eye daisies, yarrow, the list is pretty much endless.