HI Sharon, I think your guess is correct - your flowering plants need more nutrients to compete with those tree roots. The quick solution is to sprinkle some slow release plant food like Osmocote into the soil now. It will release fertilizer all summer long so you won't have to remember to feed the plants when you \"slack off\" for the summer. The long term solution is to remove the plants (perhaps this fall) and add six to eight inches of compost rich topsoil so the plants can grow above those invading roots. You may need to add a boulder or log edging to keep this now raised bed in place. Than you can replace your old plants after dividing them up. There is no way to kill the tree roots without damaging the trees or plants, but you can garden above them. Now about the shade. I would use shade tolerant perennials like astilbe, brunnera, hosta, crane's bill geranium, and lamium groundcover. For annuals use impatiens, lobelia, begonias and coleus. Try growing the annuals in large pots under the trees if you don't want to raise the level of the soil. Be sure to try some bright white flowers to really brighten up this shaded area. There are a few perennials that will bloom in part shade and compete with tree roots. Look for Japanese anemone that flower late in the summer and fall with tall, pink, or white blooms. For spring color the woodland bulbs like wind flower, lily of the valley bulbs, and dwarf daffodils. It will be lovely. Keep growing, Marianne Binetti