International Society of Arboriculture (ISA)
Trees Are Good (valuable information on planting and maintaining trees)
SelecTree (information on selecting the “right tree for the right place”)
Center for Urban Forest Research has the latest research pertaining to urban forestry.
Oregon State University Extension Center contains more gardening and tree care information
University of Washington, College of Forest Resources. Nature in cities, policy, planning and urban forestry and human benefits
University of Illinois at Urbana-Champaign; Landscape and human health laboratory