Abstract
Chorizanthe robusta var. hartwegii (Scotts Valley spineflower, Polygonaceae) is a narrow endemic plant restricted to a specialized microhabitat (exposed bedrock in California prairie) in Santa Cruz County, California. This taxon and the nominate variety were listed as endangered under the U.S. Endangered Species Act in 1994. Three occurrences of C. robusta hartwegii exist on four properties in a recently urbanized area at the northern edge of the city of Scotts Valley. Ten of 80 known colonies are now likely extirpated. In 2014 the primary threats are habitat alteration due to adjacent land uses and developments, and invasive plant species (non-natives) and accumulation of thatch. Using international standards, C. robusta hartwegii is critically endangered: area of occupancy, 0.39 ha; extent of occurrence, 1.3 km<sup>2</sup>; landscape, severely fragmented; and quality of the habitat, continuing to decline. With these attributes, C. robusta hartwegii faces an extremely high risk of extinction. As a matter of urgency, we recommend reducing and controlling invasive plant species and thatch using a combination of methods: mowing with biomass removal, cutting by hand tools with biomass removal, pulling by hand with biomass removal, controlled grazing, prescribed mini-burns (testing 1 m<sup>2</sup>), and spot-treating invasive grasses with a grass-specific herbicide.
