American Community Gardening Association is a nonprofit that builds community by improving and enhancing community gardens and greening in the U.S. and Canada.
National Agriculture in the Classroom provides support to state programs by giving teachers the knowledge, appreciation, and awareness to education and inform their students. https://www.agclassroom.org/index.cfm (For school gardens)
Out Teach is a national teacher-development nonprofit working to promote experiential learning in outdoor classrooms through their professional trainings. https://www.out-teach.org/