Answer: Kwanzan cherries do need a well drained location so would be a poor choice for a wet site. The usual choices for wet soil areas would be red maple (Acer rubrum) or river birch (Betula nigra) however these are large trees rather than small scale flowering trees. Unfortunately I am unable to think of a small narrow flowering tree that would do well in the space as described. Your local county extension and/or professionally trained and certified arborist might have some suggestions based on a more detailed understanding of the planting site and your design goals. Alternatively, a degreed landscape architect or professional with experience in correcting grading problems might be able to suggest ways to improve the drainage so that you could plant cherries as originally intended. (I might mention though that Kwanzan cherry at maturity is neither narrow nor short.) Sometimes a drainage problem is easily solved, while in other cases they are quite complex, but it might be worth looking into. Good luck with your tree planting.
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