Hello, I'm a UMD alum working to start a small community garden in DC....
It is possible to have a successful community garden despite rats. Do everything you can to reduce the rat population. Look at the following links below. You may also want to visit other community gardens to see what works for them.
Contact information for Rats in DC http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0CEoQFjAH&url=http...
Managing Rats in City Gardens http://www.bbg.org/gardening/article/managing_rats_in_city_gardens