Over the past several decades, invasive plants have been spreading throughout Monhegan. Over time, several invasive plants have managed to take hold in wild areas of the island and are threatening to outcompete native plants. Although the general policy of the Monhegan Associates is not to interfere with the natural processes that occur in the Monhegan wildlands, action has been taken in recent years to control the spread of invasive plants, as they are considered a threat to the health of the island’s ecosystems.
IPANE (Invasive Plant Atlas of New England) is a great resource for people wanting to learn more about the invasive plant issue, as is The Invasive Species Network and the IMap invasives website. Monhegan Associates interns have worked with them to map Monhegan. This is a graphic representation.
Some of the “wildflowers” you see growing on Monhegan are considered to be “invasives” – and if we don’t combat them they will take over large areas of the wildlands, crowding out other varieties. Here are some of them:
Asiatic bittersweet is a deciduous vine. It has alternate leaves that are almost round and toothed. Small, green flowers bloom in the spring and early summer. The woody vine wraps around other plants, easily suffocating them. These vines may grow to be 5 inches in diameter and up to 60 feet long. Asiatic bittersweet reproduces rapidly and can easily take over an area in a short period of time. It prefers growing in disturbed areas and along the edges of forests, where it can wind its way up tree trunks while still receiving ample sunlight.
One distinctive feature of this plant is its pea-sized fruits, which mature throughout the season, turning from green to bright yellow in the fall. After the fruit turns yellow, the outer casing splits open, revealing a bright red ball inside. These berries are quite beautiful and are often used to make wreaths and other decorative items for the home. Unfortunately seeds often fall off the wreaths and into lawns, gardens, and compost piles where they take root and spread into the wild. Birds also distribute the seeds of this plant quite effectively.
Purple loosestrife grows in wet areas, often filling in wetlands completely. Magenta flowers grow in clusters on tall spikes that can reach 4 feet tall. Individual flowers are ½–¾ inches wide with five to six petals. Leaves are lanceolate, growing in pairs or sets of three. Purple loosestrife blooms in mid- to late summer. Look for it at Lobster Cove in late summer. Monarch butterflies are often seen clustered around the brightly colored flowers.
Japanese barberry was introduced to Monhegan as an ornamental plant in gardens in the early twentieth century. Since then it has spread into the forests at an alarming rate and can presently be found in most parts of the island. This deciduous shrub can get quite large, with trunks measuring 6–8 inches in diameter and branches reaching 10 feet high. The green or purple leaves are small, entire, and grow in clusters around a thorny branch. The leaves turn vibrant shades of red and orange in the fall. Yellow, pink, or white flowers bloom in the spring, and bright red berries emerge in the fall.
Although Japanese barberry prefers a certain amount of sunlight, it can make do with the light coming through a light forest canopy and therefore is able to populate the forest floor.
Black swallow-wort is a perennial vine with an ovate leaf that is dark green, almost purple. The leaves are entire and opposite or occasionally whorled on the stem. Tiny purple flowers, each with five regular petals, bloom throughout the summer. Smooth seedpods develop during the summer, producing seeds similar to those of milkweed, with a feathery attachment that carries them away from the plant on the wind. This vine quickly and aggressively suffocates other vegetation, wrapping itself around anything with which it comes into contact. Black swallow-wort typically grows in open, disturbed areas and along the edges of woodlands. It is less common in the interior of the forest, where shade limits its growth.
Black swallow-wort is of particular concern because of the threat it poses to monarch butterflies. Monarchs typically lay their eggs on milkweed plants. When the eggs hatch on the milkweed, the caterpillars eat their fill of the nutritious plant. Black swallow-wort is in the same family as milkweed but is poisonous to monarch butterfly larvae. Monarch butterflies can be fooled into thinking that the black swallow-wort is milkweed and will lay their eggs on the plant. When the larvae hatch and consume the swallow-wort, they are poisoned and die.
Japanese knotweed is a perennial, dying back completely each winter and growing again in the spring to heights of 7–8 feet. It has a straight, hollow stalk. The leaves are entire and broad, coming to a sharp point at the tip. Plants have greenish white flowers that bloom in the early fall.
Japanese knotweed is particularly fond of wet areas and will grow in dense stands along the edges of streams, ponds, and other wetland areas. It is able to reproduce by spreading rhizomes as well as by seeds. Look for stands of it throughout the village, in particular in the field area next to Elva’s Old P.O.
Multiflora rose has small white flowers, ½–1 ½ inches wide, that cover the plant in early summer. Small red rose hips, less than ½ inch in diameter, appear in the fall. This rose can grow to be 12 feet high, creating a dense and practically impenetrable thicket. Leaves are divided into seven to nine leaflets that are about half the size of those of rugosa rose. This plant was originally planted in gardens and escaped to the wild. It is found in open fields, along the road through the village, and along sunny portions of the trails. There is a large stand of multiflora rose between the Monhegan Library and Lupine Gallery.