When the Harpswell Invasive Plant Partnership was launched a few years ago, we consulted with the invasive plant specialists at the Maine Natural Areas Program. They recommended including the two invasive buckthorns on our list of 20 target plants for Harpswell. Yet during the seven years that we have been working, the only plants we have not come across are the buckthorns. Have we been missing something?
We are hoping that homeowners and other residents and visitors can help us find out just how much buckthorn may be in Harpswell. Why? Because both common buckthorn and glossy buckthorn are severely invasive in Maine. They crowd out native plants and reduce the number of birds, butterflies and other wildlife.
Autumn is a good time to identify buckthorn, because it often keeps its leaves after other plants have dropped theirs. Both common and glossy buckthorn can be large shrubs, more than 20 feet tall. Common buckthorn has dark green, oval leaves with distinct, deep veins. The leaves appear almost opposite each other along the stem and the edges have small teeth.
If you scrape away the dark-gray outer bark, the inner bark is orange. Twigs usually end in a sharp thorn and thorns sometimes appear along the branches. The fruit is small and changes from green to purplish-black. In spring, the yellow-green flowers have four petals and grow in small clusters. Common buckthorn occurs on uplands, though it can also be found in wooded wetlands.
Glossy buckthorn also has dark-green oval leaves with distinct sunken veins, but they alternate along the branch, have smooth edges and are very glossy on top. The bark is grayish-brown with small white spots or lenticels, and no thorns. The fruit changes from red to black. In spring, flowers are in white clusters with five petals. Glossy buckthorn occurs in all wetlands but can also move into uplands.
Maine also has one native species, alder-leaved buckthorn. Alder buckthorn is smaller, only growing about 8 feet tall. The oval leaves are alternately placed along the stem and are very glossy, with as many as eight pairs of deep veins. Leaf edges have a double serration. The bark is covered with grayish-brown hair when young. There are no thorns and the greenish-yellow flowers have no petals. The fruit is bluish-black. Alder-leaved buckthorn occurs in wetlands.
The other native shrub that often keeps its leaves late into fall is winterberry, but its fruit is bright red, so it can be distinguished from buckthorns. Hawthorns also have thorns like some buckthorns, but not at the tips of branches. More information is available at hhltmaine.org/hipp, maine.gov/dacf/mnap and gobotany.nativeplanttrust.org.
If you find any buckthorn in Harpswell, HIPP would appreciate learning about it. You can write us at hippmaine@gmail.com.
