What to collect:
Be conservative and collect only what you need. It is a good idea to use the following rule of thumb: never collect a plant when you can see fewer than 6 individuals in the area. Select vigorous, typical specimens. Avoid insect-damaged plants. Make sure the plant has flowers and/or fruits. It may be a good idea to collect extra flowers and fruit for identification purposes. Sterile plants are very difficult to identify.
Roots, bulbs, and other underground parts of herbaceous (non-woody) plants should be carefully dug up, and the soil removed with care. When collecting shrubs and trees, clip one or two small branches. For each specimen you intend to collect, gather sufficient material to fill a herbarium sheet (11" x 17") and still leave enough room for the label. Plants too large for a single sheet may be divided and pressed as a series of sheets.
It is good practice to collect in duplicate. This means that if possible, you should collect sufficient material to make more than a single herbarium specimens. This serves multiple purposes: 1) as an insurance policy in case one specimen were to be lost or damaged, 2) allows you to then send one of the duplicates off to a botanical expert for identification, in which case this is commonly done as a gift to their herbarium, and 3) if you are intending to export your collections from a country, it is often required that you first deposit a duplicate in that country's national herbarium.
Note: If you are not able to press specimens during collection, place them in a labeled plastic bag (see field log section below for information on collection number) and add a sprinkle of water.
Where to collect:
A choice of collecting locality depends on the purpose behind your collection. If you are collecting specimens to donate to a local herbarium, your backyard may not be an ideal choice. Most local herbaria specialize in native and naturalized plants and have fewer collections of the hybrids and varieties that are bred specifically for the horticulture industry and generally are not seeking specimens of these plants. Some good locality choices might be undeveloped lands or areas that are not maintained and are likely to contain plants that have adapted to survive on their own in the “wild”. However, if you are interested in creating a herbarium for your own purposes then it is entirely up to you to choose where and what to collect. It is a good idea however to always obtain permission from the owner of any property on which you intend to collect plants, and be aware that collection in National Parks and preserves is likely only possible with special permission.
If you intend on collecting plants outside of the United States, be aware that most countries require that you obtain a special plant collecting permit. It is always best to contact the national or local herbarium closest to your chosen collection location and request their advice with these permitting procedures. Note that most countries will require an export permit if you wish to ship the specimens back to the United States, and that the United States requires an import permit. Herbaria are equipped to deal with transactions such as these and can be of immeasurable assistance with these logistics.