In general, spring-flowering bulbs naturalize better at the foot of trees with a taproot, like an oak, than trees with shallow, spreading roots, such as a poplar or maple. That’s because trees with shallow roots hinder the growth of bulbs, sucking all the moisture from the soil and emptying it of its nutrients, creating a dreaded condition know to gardeners as “dry shade”.
Some bulbs, though, are exceptions to this rule and will tolerate root competition. This is particularly true of Siberian squill (Scilla sibirica), snowdrops (Galanthus nivalis), glory of the snow (Chionodoxa spp. ), trilliums (Trillium spp.), trout lilies (Erythronium spp.) and winter aconites (Eranthis app.). All will grow under deciduous trees, where sun is abundant in spring, but totally absent during the summer, and aren’t the least bit bothered by tree roots. In fact, they will usually spread on their own once they’ve settled in. Try them where your previous attempts to cultivate bulbs were unsuccessful.