These gardens are called Unity Gardens and are located all around the city. Some grow specific types of produce like the Butternut Squash Garden on Kaley Street while others are run by specific organizations such as the Homeless Center Unity Garden. Some gardens donate all of their produce to local food banks while others split the harvest between members of the community. Regardless, all gardens depend on the support of the community and can use regular volunteers. Chances are there is one near you.
For locations, events, and more information visit their website here: http://www.unitygardensinc.org/index.html