A new study by the U.S. Forest Service found that planting trees along the perimeter of a rental property increase the rates the landlord could charge by $21 a month.
Planting right on the property increase the value by at least $5 a month.
The survey was done by combining the price of rental listings on Craigslist for Portland, Oregon with tree data gleaned from Google Earth.
Of course, it was sponsored by the Forest Service, known dendrophiliacs, so there is a risk of bias.
The effect of urban trees on the rental price of single-family homes in Portland,
Oregon (PDF) [Urban Forestry & Urban Greening]
Plant a Tree, Boost Your Property Value [GOOD] (Thanks to c-side!)