You can often find Japanese barberry (Berberis thunbergii) right in your own yard or nearby, as it has been a popular landscape plant for many decades. It is now banned from being sold in Maine, however, as it is highly invasive in our forests and can form dense stands that prevent trees and other native plants from growing. As deer and other herbivores prefer other plants, it gets a competitive boost from our large deer population. Fall is a good time of year to see the bush, as it is sporting many tiny, oblong red fruits hanging along its thorny branches. It also has very small, teardrop-shaped leaves (see photo). Barberry has also been implicated in increased incidence of tick-borne Lyme disease–see this video for more information: https://www.nbcconnecticut.com/news/local/Expert-Suggestions-for-Removing-Japanese-Barberry–448183243.html.
If you own forest or fields, taking time to survey your land for this noxious bush and removing it is worth the effort before it becomes a big job to get rid of it. The plant can be pulled by a tractor with a brush chain, or dug out by hand, which is best done after cutting it just above ground level with a brush cutter or loppers to avoid handling the thorny branches. It is also easily killed by a foliar application of herbicide (Follow directions on the label and applicable laws when using herbicide).
Related plants such as reddish cultivars of barberry and common barberry (Berberis vulgaris) are also considered invasive and are banned from being sold in Maine.
If you want to replace barberry in your landscaped area, many attractive native bushes offer color and interest similar to barberry, including American cranberry, also called highbush cranberry (Viburnum trilobum) and Virginia rose (Rosa virginiana).
For more information, visit the Maine DACF website, which includes a Japanese barberry identification guide as well as tips and videos on how to get rid of it: http://www.maine.gov/dacf/