High Quality Content by WIKIPEDIA articles! Walnut Grove is a census-designated place (CDP) in Clark County, Washington, United States. The population was 7,164 at the 2000 census.Based on per capita income, one of the more reliable measures of affluence, Walnut Grove ranks 78th of 522 areas in the state of Washington to be ranked. According to the United States Census Bureau, the CDP has a total area of 3.8 square miles (9.8 km²), all of it land.