Basic membership is free. It allows you to view member-only content.
Community membership is for people working on the ground – gardeners, researchers, individual projects, etc who want to view all content, participate in the forum.
Corporate membership is for councils, businesses and other organisations who want to participate in the forum, list their services in the directory.
Premium membership is for corporate supporters and sponsors. As the top level of site membership, you receive all the benefits of other levels with some special extras