Callery Pear (Pyrus calleryana)
Introduced as an ornamental, the Callery pear is a medium-sized tree that used to enjoy widespread use as a rugged and attractive street tree. It flowers heavily, producing copious quantities of small fruit that birds enjoy eating. Birds can then carry the seeds far and wide, depositing them in the natural areas that the Callery pear is now known to invade. This plant has become a serious invader due to its suckering habit and ability to spread widely, allowing it to form dense thickets in disturbed places and natural areas that completely exclude other plant life. This plant can be controlled with mechanical removal or even herbicide. Still, the best method of control is to stop the planting of this tree and remove any existing individual trees to prevent further dispersal.
Norway Maple (Acer platanoides)
The Norway maple is an introduced species that grows into a large deciduous tree, up to 60 feet in height. The Norway maple has been planted as a street and garden tree since the mid-1700s and used as a replacement for elms lost to Dutch Elm Disease. Because of its widespread horticultural use, the Norway maple has begun to naturalize in forests. It can outcompete native plants and change the way nutrients cycle through the forest ecosystem. It is even capable of invading the dark interiors of mature forests, and once it grows tall enough will shade out native trees. Norway maples should never be planted, and any wayward seedlings pulled. Mature trees can be killed by cutting or girdling. Herbicides can be effective for control but are not as effective as other control measures.
Tree of Heaven (Ailanthus altissima)
The tree of heaven, Ailanthus altissima, is one of the worst invasive trees in the Mid-Atlantic. It is allelopathic, meaning that it can release chemicals into the soil that prevent other plants from growing properly. They can spread rapidly due to the massive numbers of wind-born seeds each tree produces and thrive in disturbed areas. The tree is very tough, suckers profusely, and grows quickly, making effective control difficult. Unfortunately, the tree of heaven is best controlled with herbicides, which can be painted directly into cuts on the tree to prevent collateral damage to other plants.