Invasive Plant Info

Japanese Knotweed

This plant zombie found all over Swampscott is Japanese knotweed – a member of the buckwheat family, introduced into the U.S. from Eastern Asia (Japan, China, Korea) as an ornamental on estates in the late-1800s. Although once used as erosion control and sold through seed and plant catalogs, by the late-1930s knotweed was already being viewed as a problematic pest. The plant, which can grow from three to 15 feet tall, has bamboo-like stems and is sometimes called Japanese bamboo.

Identification

Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.) is an upright, shrubby, woody-appearing perennial reaching heights of 10 to 15 feet. The stems are smooth, stout, and hollow. Where the leaves attach to the stem, the stem is swollen with a membranous sheath surrounding the joints. The leaves are broadly oval to somewhat triangular or heart-shaped, pointed at the tip and alternating on the stem. The plant’s shoots come up from a network of spreading horizontal roots (rhizomes) which can reach lengths of 65 feet or more. Japanese knotweed has branched sprays of small greenish-white flowers from August to September.

Dispersal

Japanese knotweed spreads primarily by seed (transported by wind, water, animals, humans, or as a soil contaminant), stem fragments, and by shoots sprouting from its system of rhizomes.

Problem

Japanese knotweed spreads rapidly, forming dense thickets that crowd and shade out native vegetation. This reduces species diversity, alters natural ecosystems, and negatively impacts wildlife habitat. The ground under knotweed thickets tends to have very little other growth. This bare soil is very susceptible to erosion, posing a particular threat to wetland areas. Once established, populations of Japanese knotweed are extremely persistent and hard to eradicate.

Control

Single young plants can be pulled by hand depending on soil conditions and root development. If all of the root system isn’t removed, re-sprouting can occur. For thickets, grubbing with a pulaski axe or similar tool to remove all of the roots after cutting back the standing vegetation can be an effective control measure. All parts of the removed plants should be bagged and disposed of in a secure location. It is best to not mow or cut Japanese knotweed with weed trimmers as the pieces of the plant can easily get moved around and re-sprout, spreading, rather than controlling the plant.

Replacements

The following native plants can serve as a good replacement for Japanese knotweed and aid in native habitat restoration:

• New England Aster (Aster novae-angliae)

• Blue False Indigo (Baptisia australis)

• Sweet Joe-Pye-Weed (Eupatorium purpureum)

• Queen-of-the-Prairie (Filipendula rubra)

Tree of Heaven

The Tree of Heaven (Ailanthus altissima) is sometimes counter-nicknamed “tree from hell” due to its prolific invasiveness and the difficulty in eradicating it. It is found in many areas around Swampscott and should be removed and replaced with native tree species whenever possible. Leaving it in place makes it more difficult to remove, encourages its spread, and fosters the growth of thickets that crowd out and poison native plants – upsetting the balance of the native ecosystem and reducing biodiversity. Keep a sharp eye for the tree of heaven – oftentimes it grows alongside the very similar looking native staghorn sumac!

Identification

Tree of heaven has smooth stems with pale gray bark and twigs. Its large compound leaves are 1-4 feet in length, alternate, and composed of 10-41 smaller leaflets. Flowers occur in large terminal clusters and are small and pale yellow to greenish. Flat, twisted, winged fruits each containing a single central seed remain on the tree from late summer to early fall. Similar in appearance – native sumacs can be distinguished from tree-of-heaven by their completely serrated (toothed) leaf margins..

Dispersal

Tree of heaven reproduces by seeds and through sprouting. Flowering occurs late in the spring. Established trees produce numerous suckers from their roots and resprout vigorously from cut stumps and root fragments.

Problem

Tree of Heaven both grows and reproduces at a fast rate allowing it to push out native species and create large thickets. It has the ability to poison soil and root systems and damage sewer lines with its roots. To make matters worse, tree of heaven acts like a motel for other invasive species, including the spotted lanternfly, one of the most prolific and detrimental invasive insects in North America.

Control

Hand pulling is acceptable for small saplings of this tree, with mechanical removal and a “cut-and-dab” chemical treatment needed for larger trees. Make sure to bag all limbs that may contain seed pods to prevent the spread of the plant. Early research into a soil fungus that will kill the tree of heaven has been promising but no biological controls currently exist.

Replacements

The following native plants can serve as a good replacement to the Tree of Heaven and aid in native habitat restoration: hickories, green ash, butternut, smooth & staghorn sumac.