The Pinjarra Plain features a variety of diverse flora due to being seasonally inundated with heavy soils. Key species include the threatened Synaphea sp., various banksia species and succulent flowering plants such as Xanthorrhoea. Unfortunately, the plain has been cleared for agricultural purposes and remaining vegetation remnants are small and highly fragmented. There are marri trees on the plain, with wandoo on the clay flats, paperbarks in the swamp areas and flooded gum along the watercourses. Much of the Pinjarra Plain is pastureland with drains constructed to reduce the amount of waterlogged land.

The Pinjarra Plain is the low lying, flat area found to the east of the Swan Coastal Plain and stretches to the west of the Darling Scarp , overlying the Bassendean dunes. The sediments are fluvial in origin and are the floodplain deposits of the major rivers that flow across the Swan Coastal Plain. The soils on the plain are primarily fertile clays and loams, characterised by their low relief and naturally poorly drained areas. During winter, surface water appears as seasonal wetland and swamps. The Pinjarra plain soil is relatively fertile in comparison to the nutrient-deficient soils of the Bassendean and Spearwood dunes. The soil stratigraphy of the Pinjarra Plain consists of various deposits including the Coollup, Boyanup and Wellesley units. These stratigraphic units are exposed at riverbeds on sections of the Murray River near Pinjarra.

The name Pinjarra is believed to derive from the Aboriginal word ‘beenjarrup,’ meaning ‘place of the swamp.’ Another interpretation links it to ‘Bin-jaril’ or ‘Binjarlly,’ referring to digging among the swamps and jarrah trees (Boodjar Place Names). The Pinjarra Plain was among the earliest areas cleared by settlers for pasture.