7. Barberry

The barberry shrubs (Berberis spp.) have invaded North America from two sides. One type, Berberis vulgaris, is from Europe; the other, Berberis thunbergii, is from the Far East.

According to a recent report, these invasive plants are established in 31 US states and four Canadian provinces. They are fiercely armed and covered with the thorns that have made them so desirable in many a hedge.

Berberis thunbergii, also known as Japanese barberry, can escape cultivation so fast that much of the Midwest has named it one of the most dangerously invasive plants. Moreover, most experts strongly suggest that it should never be planted at all.