I just came across Garden for the Environment, a San Francisco group that maintains a demonstration garden in SF and offers classes on gardening, composting and other topics:
Garden for the Environment maintains a nationally acclaimed one-acre urban demonstration garden and offers environmental education programs about organic gardening, urban compost systems and sustainable food systems. Since its founding in 1990, the garden has operated as a demonstration site for small-scale urban ecological food production, organic gardening and low water-use landscaping.
I’ve always wanted to learn some gardening basics (it’s been a long time since I grew vegetables behind my Dad’s flat when I was a kid), so this is definitely on my list of things to check out.