Answer: Japanese maple (Acer palmatum) grows to a height of about 10 feet in 15 years, 25+ feet in 50 years. It has lovely delicate foliage that turns bright red in the fall. The plant is relatively trouble free and needs little in the way of maintenance.
Flowering dogwood (Cornus florida) grows to a height of about 10 feet in 18 years, 20 feet in 35 years. The beautiful flowers (actually bracts) are a wonderful welcome in early spring. They can sometimes be infested with borer insects (look for holes in the trunk) and a disease called anthracnose (look for dark spots on the leaves.)
I think these are both good choices for your site. Just be sure to keep the newly-planted trees well watered during dry spells, especially during their first few seasons of growth. Other than that, your trees should be fine! A layer of compost around the trunk, extending out to the dripline, will supply the tree with slow-release nutrients. You may want to mulch around them--to be sure you don't damage them with a mower or string trimmer.
Almost any plant would be an appropriate neighbor. Just don't plant anything too close to the trunk; preferably, don't plant under the canopy of the trees, so the roots don't have to compete. There are all sorts of shrubs and perennials that would look wonderful--azaleas, low-growing junipers, daylilies, columbine, there are so many choices. I would go to the library and get some books with color photos, and see what appeals to you. Good luck!
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