How to Make Voucher Specimens of Plants

The following are instructions for making a pressed plant voucher. Vouchers aid in identification -- so that control work can begin on the correct species. For complete information, see the overview of the Invasive Plants of the Future project.

A voucher specimen is a dried plant sample consisting of pressed leaves, stems, flowers, roots and/or fruits that is used by experts to verify species identification. Vouchers are valuable because they provide the physical evidence to confirm the presence of plant species in specific locations. They have a variety of uses, such as documenting the occurrence of rare plants or revealing the geographic spread of invasives over time. Once received by a herbarium (a plant specimen "library"), vouchers may be mounted, labeled and kept for future reference and research.

Equipment needed

1. Plant Press (or any device in which to flatten and dry specimens. Instructions for building a press are below.)
2. Invasive Plant Report Form [PDF 123KB] (or equivalent listing of specimen-related data).

Basic steps for preparing vouchers

1. Collect plant in the field.
2. Record specimen data on Plant Report Form.
3. Press immediately, or transport temporarily in a plastic bag and press as soon as possible.
4. Dry quickly and completely.
5. Send specimen and Plant Report Form to the WI State Herbarium at UW-Madison.

What to Collect

Select one or more healthy plants that are typical of the population. Take samples of the whole plant, if possible, or enough leaves and stems to show leaf shape and size, opposite or alternate branching, and buds. If possible, include flowers and/or fruits, which may be needed to confirm a plant's precise identity. For grasses and grass-like plants, try to include roots. For large specimens, fold stems into a V or N shape. Thick stems may be cut in half lengthwise. For small plants, collect several and press together. Show upper and lower surfaces of leaves and flowers. Press flowers with the blossom open, and if possible slice one in half lengthwise to show internal structures. Be sure to press the plant before it wilts.

Pressing Plants

Use a standard-sized (12 x 18 inch) plant press if you have one. Or, make your own portable plant press. Specimens must not exceed 11.5 x 16.5 inches (though large plants often are divided up and glued to multiple sheets) as that is the size of herbarium mounting sheets.

Plant press dimensions can be as small as 9 x 12 inches. This makes it easy to carry the press in a backpack, as well as to send specimens by mail (in large, business-size envelopes).

Specimen Information

For each specimen or field observation, basic information about the occurrence is needed. For all specimens collected, make sure that all documentation is linked to the specimen. Some collectors write data on the specimen packaging or on a sheet enclosed with the sample. Others use a notebook with code numbers that correspond to a specific sample.

Use the Invasive Plant Report Form [PDF 123KB] to submit information, or create your own that covers these categories: state, county, date, plant name. Estimate the size and density of the infestation as well. Include landmarks such as nearest city, road, intersection, power lines, lake edges, or other natural/cultural features. Provide a habitat description, such as forest interior, forest edge, old field, prairie, wetland, lakeshore, crop field, pasture, disturbed ground, urban setting type. Tell if found on public or private land. Provide the collector's name, address, phone number, and email so they can be reached for more details. Enclose a completed form with each specimen. If the landowner or land manager is known, provide his/her name and contact information.

Accurate information about location is essential. Provide exact geographic coordinates using a GPS unit, topographic map, or the Wisconsin Gazetteer. If you have access to the internet, you can use TopoZone.com (exit DNR) to find the precise location on a digital topographic map. When you click the cursor on the exact collection site, its coordinates (choose UTM or Latitude/Longitude) are automatically displayed in the text above the map. Include a printed or photocopied map with a colored dot showing the spot.

Mail specimen with its report form to:

Wisconsin Invasive Plants Reporting & Prevention Project
WI State Herbarium - U.W.-Madison
430 Lincoln Dr.
Madison, WI 53706

For more project information contact:

Brendon Panke
Invasive Plant Education, Early Detection, and Mapping Specialist
WI Dept. of Natural Resources
P.O. Box 7921
Madison, WI 53707-7921
Phone: (608) 267-7438