Dense stands of knotweed line any number of roads in Harpswell. It’s easy to identify at this time of year, when clusters of small, white-green flowers bloom along its stems. Knotweed’s height — at times up to 10 feet — and its hollow, segmented stalks cause many to mistake it for bamboo.
Well-meaning horticulturalists brought knotweed to the U.S. from Asia in the late 19th century because it grew fast and was thought to provide efficient erosion control. A 1908 seed catalog described it as “Extremely fine for ornamental effect in the hardy garden. Large, heart-shaped leaves and drooping clusters of pure white flowers.” Unfortunately, the pretty flowers don’t compensate for the problems knotweed can cause.
Appearing in early May, the first sprouts are a reddish-green color and very tender. Some people even eat them — but not enough to make it a viable crop!
Once established, a knotweed canopy can shade out other plants. The thickets release chemicals that suppress growth of nearby flora, a trait referred to as “allelopathy.”
Knotweed is versatile, too. It spreads via an extensive root network and by seeds from those lovely flowers. In addition, pieces of stems and roots will sprout wherever they fall. Do not put any part of this plant in your compost. Digging up this plant will result in a thicker stand the next year!
Knotweed grows pretty much everywhere: through asphalt, in sandy soil and swamps, on rocky banks or even in mine slag. Ironically, given its early use to prevent erosion, knotweed has been found to destabilize banks when growing alongside water.
It’s no wonder the state of Maine has designated knotweed as severely invasive.
Ridding a property of knotweed requires diligence. Mowing it regularly to deprive the plant of the sunlight that stimulates growth is one approach. Cutting it back before it blooms is another. And, finally, herbicides can be effective if applied correctly and at the right time.
The Harpswell Invasive Plant Partnership provides information about how to deal with knotweed at its website, hippmaine.org. To ask a question, email hipp.maine@gmail.com.
Fortunately, there are native plants that offer benefits in the garden and erosion control near the water. One of the best is summersweet, Clethra alnifolia.
This sturdy native shrub typically grows 3-6 feet high and spreads to 4-6 feet wide. It produces an abundance of sweet-scented flowers during the waning days of summer, when few other shrubs are in bloom, and its showy, bottlebrush-like flower clusters last for weeks. In fall, the leaves turn a brilliant golden yellow before dropping for the winter.
Summersweet is remarkably adaptable, tolerating shade, variable soil moisture and salty air, making it a valuable addition to woodland settings and coastal gardens. Its fragrance attracts a variety of butterflies and bees.
Look under “plant tips” at hippmaine.org for suggestions of native plants you can use to replace invasive plants like barberry, Norway maple, and shrubby honeysuckles.
