I believe the tree streets were something started in Takoma Park, which was a planned Victorian bedroom community established in 1883.

>In this connection one should also note that the early street names chosen by Gilbert were those of local trees: Tulip, Cedar, Maple, Dogwood. etc.

It was an extension of the "Sylvan Suburb" ideal he wanted to create. Looks like DC took those botanical names and ran with them when the city was extended, at least according to this 1907 map. If I'm reading it right, Aspen St. used to be "Wabash."

The streets are mostly trees but not all (dahlia, fern, and geranium, for example).