What is it?
When scientists preserve a specimen of a plant (or part of a plant) they usually flatten it, dry it, and mount it on special paper. Preserved in this way the plant specimen can be stored for many years without falling apart.
Before you start
Before collecting plants in the wild, you should understand the legal issues of the ownership of the land and its resources, and the ethical issues of possible damage to wild plant populations and to endangered species. It is legal to collect plants only with the permission of the owner of the property on which they are found.
What to collect
Picking a few leaves or flowers usually does not give a representative picture of a plant. Pieces of specimen plant material need to be large enough to show the characteristics of normal growth and development. Taking a branch, stem or even the entire plant may be required to get a good specimen. If the plant is small, take the whole thing, roots and all, or even several of them. If large, get a branch about 25 cm long, with leaves, flowers, and fruits, if possible. A "sterile" specimen (one with leaves only) may be impossible to identify. Even an old empty seed capsule can be helpful if that's all you can find.
The date the plant was collected and the location as exactly as possible. Record anything that the specimen won't show, for example, the size of the plant, flower colour, whether the plant is woody or not, etc. Note what kind of a place the plant was found, e.g., in gravel at stream edge, in shade under live oaks, in a sidewalk crack…
How to press a plant
Place the specimens between newspaper sheets and write the name of the species alongside. Alternatively, you can separate the specimens with corrugated cardboard (for air circulation) and blotter paper or paper towels to absorb moisture. Arrange the plant so that all parts show (for example, don't put the flowers between layers of leaves). Place the stack between boards and strap them tightly or place a heavy weight on top (such as a pile of heavy books). Put the stack where there is good air circulation, but not too much heat: you don't want to cook them.
Examine the plants daily and change newspapers or blotters when they become too wet. Remove plants from the stack when they are dry. You can kill insects in dried plant specimens by freezing them for three or four days, and keep them pest-free in a tightly sealed plastic bag.
Once the plants are dry enough, stick each specimen in a cardboard folder. Label them writing their Latin (scientific) and English names, along with all the other data that you previously recorded (collection date and place, description of the specimen...).