Want some greenery on your block, or on another dreary block? Ask the city! Just spotted a recent posting on the Adams Morgan listserve letting people know that they can request tree planting from the city, for free.
The Urban Forestry Administration will plant trees in a grass strip or an existing tree box on either your block or another block, you just have to request it by calling 311 or going to 311.dc.gov. Sounds like a pretty great service.
The UFA's website has a lot more on types of trees available and on the benefits of trees: adding greenery, improving people's moods, screening harsh scenery, environmental benefits, and more.
UPDATE: a commenter makes the good point that you have to water the trees and that the non-profit Casey Trees provides free watering bags.