In DC, GROW contacted the National Alliance of Land Trusts (do I have the name straight?) and decided to adopt the combined liability (for the organization) and open land insurance that the Land Trust folks created. I am not sure what the base rate is (about $750, including the organization liability coverage and 50 acres of open space).
The gardens here on National Park Service property are required to hold their own insurance and most of them seem to be with Erie. I think they pay about $200-$250 per garden. The gardens here on DC Parks & Rec property are covered by the agency. Other gardens on land of the housing agency are required to carry their own insurance. So its a mixed story here. Please post your other responses to the list -- or a summary after you hear back. I'd like to see how others do it. Judy Tiger