The herbarium of the Arnold Arboretum, established shortly after the Arboretum was founded in 1872, contains approximately 1.5 million specimens. As with the Gray Herbarium, the collection has grown steadily, but the directorships of C. S. Sargent and E. D. Merrill saw periods of particularly rapid growth.
The Arnold Arboretum herbarium is especially strong in material from the Indo-Malesian region (India to the Philippines and Papuasia), China, Japan and eastern, and southeastern Asia. The Arboretum collection is also rich in type specimens largely due to the work of staff members such as R. A. Howard, I. M. Johnston, C. Kobuski, Merrill, E. J. Palmer, A. Rehder, Sargent, C. K. Schneider, and E. H. Wilson. Several special collections reflect the interests of former staff members; these include the McKelvey Agave and Yucca spirit collection and the Shaw collection of the genus Pinus.
The herbarium also includes a reference collection of 85,000, mostly woody, temperate plants of cultivated origin, which are housed at the Arboretum in Jamaica Plain, and an associated library collection, The Library of the Arnold Arboretum.