Answer: 20' is relatively small for a tree; you may be happier with a tall growing shrub and cutting off the lower branches to make it more tree-like. With that said, dogwood will grow in the area and will produce early springtime flowers. As for shade trees along the back of your property, you could include maples or Paulowina (Empress tree). Stewartia is a lower growing shade tree and hawthorns can be kept small with annual pruning. Both flower in the late spring/early summer.
Best wishes with your new trees!
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