How to make herbarium vouchers
Initial reconnaissance efforts should occur in mid-summer in order to identify and document desirable, seed-producing ash populations. During reconnaissance, voucher specimens should be taken along with complete passport data. The purpose of voucher specimens is to document the populations from which seeds will be collected, as well as to verify that the species has been identified correctly. A minimum of two vouchers each of both vegetative and floral branches should be sampled per collection area and deposited in both national (U.S. National Arboretum Herbarium) and regional herbaria. If desirable, additional vouchers may be collected for the collecting team's own herbarium. The number of vouchers taken per collection should reflect the expertise of the collector, the degree of within-population phenotypic variation, and the possibility of two or more taxa occurring in the same location. A vegetative voucher specimen should comprise at least 12-14 inches long of annual growth, including at least two of the compound, pinnate leaves, both lateral and terminal buds. A "floral" voucher specimen should consist of a twig about 12-14 inches long, including at least two of the compound, pinnate leaves, both lateral and terminal buds, and one cluster of developing seeds. Having both vegetative and reproductive vouchers aids in identification, as morphological characteristics may be skewed due to the amount of annual growth that is sampled. Immediately after collection, the specimen should be pressed and labeled with the collection date, proposed taxonomic name, collection location name and/or description, and collection number. The collection number and date should be written in indelible ink on the outside of the newspaper holding the specimen. The pressed specimen should then be quickly dried at a moderate temperature to avoid molding and deterioration. Collection numbers should be assigned to each herbarium specimen following an unambiguous numbering procedure.
After specimens are dried, a computer-generated label for each specimen should be laser-printed onto archival bond paper and paper clipped to each specimen until it can be mounted by herbarium staff. An example of a voucher specimen label is shown at right. Please note, herbarium vouchers should not be mounted by collectors.
We recommend that digital images of the tree and surrounding habitat be captured. If images are captured, a consistent naming protocol should be followed so images can easily be linked to the corresponding voucher specimens. The collection number along with the collection location should be cited in the image file name or provided on the reverse side of the photograph.
During the collection of the herbarium specimen, collectors should complete data collection forms for each population vouchered, documenting the site location/description, latitude and longitude coordinates, habitat, associated vegetation, etc. This ensures that the seed lots will be properly documented in the NPGS's GRIN database. A completed data collection form is only necessary on a population basis and is not intended for each individual specimen.
Literature that further explains proper technique in sampling and handling herbarium specimens includes: