The worst pest is the one you don’t notice until it has moved in, made itself at home, and won’t give up the remote. Common buckthorn (Rhamnus cathartica) is that pest.
This small tree was introduced to North America as an ornamental shrub (and is still sold in some garden centers). Unfortunately, it escaped from cultivation and is choking many forests and woodland areas.
In spring, buckthorn has clusters of yellow-green flowers near the base of leaf stalks. The smooth oval leaves are pointed at the tip and have three to five pairs of veins. Twigs end in thorns. In the fall, small black fruits form on female plants and leaves stay green longer than on other deciduous trees. Glossy buckthorn (Rhamnus frangula), a relative that’s also invasive, has leaves with more veins, thornless twigs, and fruit that ripens from red-brown to dark purple.
There are several native look-alikes, such as Carolina buckthorn (Rhamnus caroliniana), that do no harm.
Damage
Buckthorn is such a fierce competitor for light and moisture that it wipes out native plants and degrades wildlife habitat. Since the seeds travel by way of hungry birds, the buckthorn hedge in your yard could be responsible for the buckthorn forest invading the river bottom 2 miles from your house.
Control
To get rid of buckthorn, pull out small trees and cut down larger trees. Dig out stumps or spray them with an herbicide containing glyphosate or triclopyr.