There are many definitions for native plants. Several references say native plants are those that grow naturally in a particular region without direct or indirect human intervention. Other references place a historical time-line on native plants, saying they are plants which were present in a particular area prior to European settlement of that area. Others say they are plants that have inhabited a particular region for thousands of years. Even the federal government published an "official" definition in the Federal Register, defining native plants as those which are "naturally occurring, either presently or historically, in any ecosystem of the United States".