How to Make a Plant Collection
Mounting the Specimen
For over four hundred years people have preserved plants for study by pressing and drying them. A collection of such dried specimens is called an herbarium. Plants that have been thoroughly dried and well mounted, and are protected from moisture and insects, will remain intact essentially forever. Linnaeus's collections, made in the 18th century, are still used (very carefully) by researchers today.
A specimen consists of a dried plant, or piece of a plant, or several small plants of the same kind. It comes sandwiched in a folded sheet of newspaper (the same one in which it was pressed and dried). In order to display its parts for study and to prevent damage in handling, it should be mounted.
Paper for mounting
Herbaria in the United States, and most other countries, use a standard size paper (11.5 by 16.5 inches) for mounting plants. We use University of California type, a medium-weight acid-free buffered paper.
We use a neutral-pH formulation of PVA (polyvinyl acetate: a white glue like Elmer's) for mounting specimens. We dilute it with water for general mounting and use it full strength for specimens that need to be more firmly glued, such as a woody branch that only touches the sheet in a few spots.
Arranging the specimen
First, take some time to look the specimen over. Clean off dirt, dead insects, bits of other plants, and so on. Decide which side should be up. See what parts are on the plant that must show in the finished specimen.
The specimen should be mounted in such a way that all its parts can be studied. Both sides of the leaves should show. It may be necessary to trim some parts off to make the specimen fit on the sheet or to expose important plant features.
The label is placed in the lower right corner of the sheet. An envelope for small loose parts or seeds may be folded from a quarter sheet of acid-free typing paper and glued anywhere it will fit.
Gluing the specimen
We usually use the "glass plate" method of mounting plants. A thin layer of glue is spread on an aluminum cookie sheet (traditionally a sheet of glass). If using white glue, some water can be stirred in to dilute it to the consistency you want.
The specimen is first arranged on the paper as it will be glued, and all necessary cleaning and trimming is done. Piece by piece the plant is placed into the glue, making sure all parts have touched down and picked up glue. It is then lifted and blotted on newspaper, and placed on the paper. A paper towel is gently pressed against all parts of the plant to squeeze out and blot up excess glue and to push the plant against the paper.
A thin layer of glue is spread on the back of the label with a palette knife, and the label smoothed into place and blotted.
Another method of gluing is useful for tricky specimens (like wispy grasses, which may gloop together in glue) or recalcitrant parts (such as roots or fuzzy leaves, which often seem glue-repellant). The specimen is arranged on the paper and held in place with weights. Then, working from the roots upward, the weights are removed and glue painted gently on the under side of the plant with a palette knife, and then blotted. The weight is then replaced before moving on to another part of the specimen. The weights are removed before placing the specimen for drying.
Allowing glue to dry
The specimen is covered with a sheet of waxed paper so the glue won't stick to anything else. A square of cardboard is placed over the label to hold it flat while it dries. Padding may be added to press down the flatter parts of the specimen if there are bulky parts like stems or fruits. A sheet of cardboard may be placed between specimens to distribute the weight. A board and a weight (we use a rock) top off the stack. The plants are left to dry overnight.