Contact: Kerrie Kyde, Maryland DNR | firstname.lastname@example.org
ANNAPOLIS, MD (March 1, 2005) – A bird in the hand might be worth much more if the bush is one of three common exotic invasive bush honeysuckles. Studies show that robins’ nests in Amur honeysuckle (Lonicera maackii) are more likely to experience predation than nests in native shrubs like Viburnum. When robins and wood thrushes competed for nesting sites in Amur honeysuckle, predation on thrush nests increased, too (Schmidt and Whelan 1999). Exotic bush honeysuckles have no thorns and a branching structure that may allow predators easier access to songbird nests. For this reason, and the shrub’s ability to shade out regenerating tree seedlings and understory herbs, the Maryland Invasive Species Council has chosen bush honeysuckle as the March Invader of the Month.
As many as seven species of exotic bush honeysuckle have been found in the mid-Atlantic region, but the three most common, Amur, Morrow’s (L. morrowii) and Tatarian (L. tatarica), were all introduced into the United States as ornamental plants. They have increasingly spread into natural areas in the Northeast and Midwest states, where they can form dense thickets that compete for space, light and nutrients with both native trees and wildflowers, reducing the numbers and diversity of those plants. Although they can invade undisturbed forest, they are more often found in hedgerows, forest edges, in forested floodplains and other disturbed areas with higher light levels.
Bush honeysuckles are upright, deciduous multi-stemmed shrubs that can grow to 15 ft. They have simple, opposite, entire, sometimes hairy leaves that emerge earlier in the spring than leaves on native shrubs, giving the invaders a growth advantage early in the season. Their flowers are tubular, whitish-yellow or pink, less than 1” long, growing in pairs in the leaf axils. Flowers usually appear in April, although the bushes can bloom as early as March and as late as June, depending on the species. Their fruit, appearing in the summer, is usually red, rarely yellow. Exotic bush honeysuckles can usually be distinguished from our native shrub honeysuckles by their hairy styles, the upright extension of the seed-forming structure deep in the center of the flower, and by their hollow stems. There is one more exotic honeysuckle, Japanese honeysuckle (L. japonica), that is a very common mid-Atlantic invader, but it is a vine not a shrub.
Invasive honeysuckles reproduce almost entirely by seed, which is widely disseminated by birds and small mammals. Exotic honeysuckle fruit may not be a preferred food source for birds, but Tartarian and Amur honeysuckle fruits often remain on the bushes late into the winter, when other food for overwintering birds is gone. For migrating birds, the carbohydrate-rich but low-fat fruit of exotic honeysuckles does not provide the nutrients needed to support long distance flight as well as native fruits.
Because of their abundant seed, high germination rates and adaptability, exotic bush honeysuckles are difficult to control. Smaller plants can be grubbed out mechanically; larger bushes can be removed with a weed wrench. Repeated cutting back to the ground during the main growing season may deplete the plants’ resources enough to kill them, but because honeysuckles resprout readily, this treatment can take several years. Treatment with herbicide is usually necessary to control mature infestations. Glyphosate, a non-selective herbicide, or triclopyr, a selective chemical for broad-leaved plants, can be used in foliar sprays (2% strength) late in the growing season, or painted or sprayed directly onto the cut stumps of the bushes (20-25% strength) from late summer through to dormancy.
For more information about other Invasive Species of Concern, visit the Maryland Invasive Species Council.