A herbarium voucher is a pressed plant that is dried and stored for future reference. As part of a complete Seed Bank collection, two herbarium specimens are collected: one housed in the Chicago Botanic Gardens herbarium and used to verify the species of each accompany seed collection and the other is sent to the Smithsonian in Washington DC for redundant storage. Specimens are placed inside a folded sheet of newspaper. Each specimen should be pressed separately. Sharing newspaper for multiple collections can be problematic as dry parts can fall away from the plants and loose parts are often difficult to match up with their parent plant when the time comes to mount those plants separately on herbarium mounting paper.
A good herbarium specimen should be displayed, upon pressing, to show the features of a plant that best help to identify it. Usually this means that the plants should be in flower or fruit – often Seed Bank specimens represent the plant in fruit because they are collected at the same time as the seeds are collected. Sometimes a plant is better identified when in fruit, such as the sedges in the genus Carex, but usually plants are easier to identify when in flower so, if seed collectors are out scouting for seeds to collect and come across potential collections in flower, it might be prudent to collect herbariums specimens then instead of later.
For herbaceous plants, collecting the whole plant is best. Sometimes the owner of the collecting site may not want the roots extracted. In that case, take as much of the above ground plant as possible. For woody plants, a flowering branch small enough to fit on a standard sheet of herbarium mounting paper (11 ½ X 16 ½) will suffice. A plant can be folded in a configuration to fit on a standard herbarium sheet. It is best to try and fold the plant to appropriate dimensions while it is still fresh and pliable because after they dry out, they become brittle and break into a big mess if the specimen needs to be manipulated to fit on the herbarium sheet by the person mounting the specimen. If a plant is too big to fit on a herbarium sheet, even when folded, the specimen can be divided into sections and mounted on multiple sheets. It is often helpful for those using the herbarium sheet for identification to be able to see both sides of a leaf so it is best to have leaves oriented so both sides are visible when pressing the plant.
Although plant presses are handy, you can still effectively press plants without one by positioning them correctly in newspaper on a flat surface and placing something heavy over them such as large books. For transporting specimens to the Seed Bank, sandwiching them between sheets of corrugated cardboard will keep them in good shape during transit.