The geographic scope of this flora encompasses the dunes and wetlands of Humboldt Bay, California (see map), defined as follows.

Dunes are those undeveloped areas of the North and South Spits of Humboldt Bay located from the mouth of the Mad River in 2004 south to Centerville Beach. Undeveloped dunes include disturbed areas such as roadsides free of residences and other improvements. Residential and industrial areas are excluded, such as the towns of Manila, Fairhaven, and Samoa, the pulp mill, developed portions of the airport, etc.

Wetlands consist of those areas defined by the U.S. Fish and Wildlife Service's National Wetlands Inventory (and mapped by the agency as such), including all estuarine wetlands (eelgrass beds, salt marshes, and brackish marshes), and all palustrine wetlands (freshwater marshes, swamps, and open water) that fall within or adjacent to the historic boundary of wetlands identified in the 1850s maps of the region. Riparian wetlands that extend upstream were only included to the outer limit of contiguous non-riparian wetlands. Roadside ditches were included when they were adjacent to agricultural or other wetlands. In setting these boundaries we attempted to exclude ornamental plants that have not truly escaped and naturalized, while still capturing the presence of potential new invasives.