Washington State Nursery and Landscape Association
Includes a directory of members by location.

Association of Professional Landscape Designers
Includes a directory of members by location.

American Nursery and Landscape Association
United States national organization of landscape professionals.

Directory of gardening related products and services
A great shortcut to finding local and not-so-local sources of information and services.

Washington State Extension Hortsense pages
Provides descriptions of common plant problems and remedies.

Cornell University Plant Pathology Dept.
Includes information on common diseases and remedies (select Extension from menu).

International Society of Arborists
Includes a directory of member arborists ("tree trimmers") around the world.