By the time trees are tall enough to give meaningful shade, their roots begin to work on pushing up the surrounding pavement, even if they're put in their own beds.

Trees that are bred to grow faster with less-invasive root systems, like the ornamental pear, have other problems, such as the lack of a central trunk after the first 4 feet or so of height. This makes the tree vulnerable to wind damage that can drop limbs, or whole trees, on the ground nearby, and on any cars parked upon it.

A developer near here used "instant shade" trees to make a housing development more appealing. It worked until they reached the critical height at which they looked like grown trees and a typical summer storm blew through. It looked like a tornado with a tree fetish had gone through the neighborhood. The music of chainsaws filled the air for a couple of days. Maples are planted there now.

Shade is a good idea. Trees are a good idea. How about real trees around the periphery of the lot and some sort of canopy structure inside? The designer can control the strength of the structure holding the canopy better than even a skilled gardener can control the growth of the trees, which have minds of their own. The canopies themselves could be pourous (to keep them from becoming overhead ponds on rainy days) and translucent. There's a plastic fabric used for blinds in my library to let in the light but keep out the glare that might be adaptable to outdoor use, and I'm sure there are other materials such as the ones used in portable pavilions. They could be taken down in winter (as a Northerner, I think of snow a lot), just like the trees take down their canopies in the fall, except no one would have to rake them. Or slip on them when they're wet.

If you go ahead and do this, I want 5% of the royalties. ;)

Regards--
Mary Anne