The PISP herbarium has an extensive collection of historical specimens dating from the 1930s. As of FY2013, the herbarium collection includes 559 specimens. The first collection was acquired by Leonard Heaton, the park’s first caretaker. Heaton also compiled an early (1935) checklist of the park’s flora. Very few park units have such strong baseline data. These historical specimens were recently remounted and are in excellent condition. Relatively few additional specimens were added to the PISP herbarium until 1997, when ZION personnel began exotic and native species inventory projects. Over the past decade, the collection has grown significantly in size as a result of new collections added during inventory and vegetation mapping projects sponsored by the NPS Inventory and Monitoring Program. As of 2013, 335 vascular plant taxa have been documented for the park since its establishment in 1923. The PISP herbarium currently has a voucher for 275 (82%) of these species. Of the 60 species missing from the PISP herbarium, 13 have been collected from the park but are housed in other regional herbaria (mostly at Arizona State University, University of Arizona, and University of Wyoming). Eleven species have been recently observed but no voucher made, and 36 species have been reported in various documents, but no voucher has been taken and the species have not been observed in recent years.