American barberry is a tenacious native plant, now present on the endangered lists of several Mid-Atlantic and Appalachian states. Eradicated in some states because of its susceptibility to and transmission of wheat rust, it has generally been overwhelmed by influxes of far more invasive Japanese and European varieties. Removal strategies remain the same for all varieties of barberry, and patient digging produces the most successful results. What may differ for removed plants is their final destination (see Tips for ways to perpetuate this threatened plant without keeping it in your yard).
Don protective gear. American barberry has plentiful thorns, often grouped in threes, protruding at different angles from branches.
Cut down branches and bag them, raking area as free as possible of berries, which are one source of new plants. This reduces the development of new plants from berries eaten by birds. You may find that wrapping all or some branches in a tarp or heavy plastic bag as you cut keeps you from getting badly caught by thorns.
Water soil heavily around the plant stump to make digging easier. American barberry spreads through long rhizomes; removing all the roots is critical to preventing new plants from growing.
Trace roots and dig them out completely. Most will not be deep, but they are likely to be long and widespread. Lifting them with your shovel and following them to their ends is more effective then chopping them out. Even small remaining roots can be the source of new plants.
Pull or chop out the remaining stumps and bag them. Barberry remains belong in the garbage, not the compost pile, unless you want to remove new plant from your compost.
Revisit the site frequently and over several seasons. New shoots are most easily spotted in spring, but new plants can be removed as soon as they are detected.