In the Winter 1999 issue of UGJ (Vol.5; Issue 4), there was an excellent article about "The Invasion of Garlic Mustard" by Vicki Nuzzo. In it the author described the plant and the problems that the plant presents to our native plant communities. I expect that most people who do any gardening/weeding are familiar with this particular plant, but I wonder how many realize the extent of the threat posed by this seemingly innocuous weed. In this article I would like to introduce a few other notable species, which have gained a strong foothold in the landscapes of western New York and elsewhere, and, like garlic mustard, present a serious challenge to all who manage and indeed to all who enjoy our outdoor environment.
I manage a tree farm and nursery and have witnessed the remarkably rapid establishment and spread of some of these plants over the last ten years. My attempts at control have met with limited or no success, partly because of the resilience of the plants themselves, but also because my efforts have not been consistent and constant, and the propagule bank for these plants extends far beyond my small domain. A serious concerted effort will be required on the part of landscape managers, nurserymen, farmers, state and local government agencies, and the public, if we ever hope to gain some control over these invaders. But first we must know the enemy.
Number 1 on my list of planta non grata is swallowwort (Cynanchum spp.), a perennial herbaceous vine of the milkweed family. Although it has been present in the northeastern US and Canada for more than a century, in recent decades it has spread very aggressively. There are two species in New York, both similar in characteristics and ecological requirements. Black swallowwort (C. nigrum) has very dark purple flowers with small hairs on the petals. C. rossicum has pale to dark maroon flowers with no hairs. C. nigrum is also rhizomatous, while C. rossicum is not. Both species will grow in full sun to full shade. In the shade they can reach about 10 feet tall, by climbing shrubs and trees. The dark green leaves are opposite and lanceolate. Flowers emerge in the leaf axils in summer and produce small milkweed-like pods in the fall, when the leaves start to yellow. The seeds are widely dispersed by the wind and by mowers in the fall. In annually ploughed fields this plant cannot establish itself, but in old abandoned fields will quickly take over. It is aggressive in both disturbed and undisturbed soils. It seems to thrive in mowed areas.
Controlling swallowwort is very difficult. It is important to prevent seed production by cutting off flowers; however, like dandelions, they tend to flower again lower on the stems. Pulling invariably leaves root sections behind, which seems to increase the production of new plants. Chemicals, such as Roundup, should be applied before full flower, but after flower buds are developed, in late June to July. Follow-up applications will be necessary. I have seen this plant spread over the entire farm in the last 5 years. Once you recognise it, you tend to see it everywhere. A study is currently being conducted by the Central and Western New York Chapter of the Nature Conservancy to control the spread of this plant. Anyone who wants more information is urged to contact Frances Lawlor, SUNY-ESF, 350 Illick Hall, Syracuse, NY 13210; (315) 470-6760, or flawlor@mailbox.syr.edu.
Autumn olive (Eleagnus umbellata) was introduced from east Asia in the 1830's, and was extensively planted in the northeast and midwest for windbreaks, erosion control and revegetation of disturbed areas. Until very recently it was promoted as a good plant for bird food, which indeed it is, since the birds have spread the seeds all over. This plant grows to about 20 feet, usually a multi-stemmed tree or large shrub. The leaves are deciduous, alternate, oval to lanceolate with smooth margins. The top side of the leaf is dark green, while the underside is covered with silver-white scales, creating an overall gray-green effect. Small, pale yellow flowers are produced in late spring, and are quite sweetly fragrant. Prodigious amounts of pink to reddish fruit, sweetly tart, are produced in late summer. It is a very rapid grower, and thrives on poor soils. Sounds like the ideal plant, right? The problem is it quickly takes over any untended area, and not just your own local backyard corner, but everyone else's also. It can create a dense, impenetrable thicket in short order, growing in the shade of tall trees or out in the open. Mowing just encourages it. It is almost impossible to pull out, except as a small seedling. Burning it also encourages new shoot growth. About the only effective method is to cut it off at the ground and apply an herbicide to the stump. I suggest planting alternatives such as Gray Dogwood (Cornus racemosa), Nannyberry (Viburnum lentago), Winterberry (Ilex verticillata), Wax myrtle (Myrica pennsylvanica), or Serviceberry (Amelanchier spp.), all of which provide food and cover for wildlife without the aggressive tendencies.
Another problem woody plant is honeysuckle, specifically the shrubby varieties that fill our woodlands. These are mostly Eurasian imports, which can be distinguished from native honeysuckles by their hollow stems; Tartarian honeysuckle (Lonicera tatarica), with smooth leaves and pink or white flowers, followed by red or orange berries, and Morrow honeysuckle (L. morrowii) with downy leaves and white flowers which fade to yellow, followed by red or orange berries. An additional difference between these two is the berries on L. tatarica are on long stalks, while those of L. morrowii are on short stems. There is also a hybrid of the two. Two other imported honeysuckles are European fly honeysuckle (L. xylosteum) with yellow flowers, followed by red berries, and leaves which are hairy underneath, and Amur honeysuckle (L. maackii),which has dark green leaves with hairs along the veins, white flowers fading to yellow and bright red berries held close to the stem. Many of these shrubs are attacked by aphids in the summer resulting in deformed, puckered clusters of leaves on the terminals, which often persist as unsightly brown clusters of dried leaves through winter. All of these shrubs are aggressive colonizers of abandoned fields and woodlands, being quite tolerant of shade, although fruit production is higher in full sun. The fruit is relished by birds which make excellent propagators. They can form dense cover shading out native herbaceous perennials and woody shrubs. They are usually the first to leaf out in the spring, thus getting a head start on everything else, and about the last to drop in the fall, which gives them a competitive edge. Like many plants they were promoted as wildlife species, erosion control, and for their ornamental value. Control is only effective if the roots are removed, since vigorous resprouting will occur after pruning. Herbicide applications to cut stumps is effective, and fall foliar sprays provide some control.
Probably the most beautiful of the alien invaders is Purple Loosestrife (Lythrum salicaria), an herbaceous perennial, identified by its showy magenta flower spikes in late summer/fall. The plant was introduced in the 1800s from Europe, where it is typically a minor component of the wetland plant community, presumably due to some indigenous control mechanism not present in US wetlands. It has been and continues to be a popular landscape plant and is still being sold in New York, although at least 13 states have banned it as a noxious weed. Several cultivars are available, which are promoted as sterile, but which are now known to be fertile when crossed among themselves and with wild populations. The plant has flowers with 5 to 7 petals in dense clusters on terminal spikes, each of which can produce up to 300,000 seeds, and densities of 80,000 spikes per acre have been reported in Virginia. The plant can also reproduce readily from root and stem segments. The leaves are opposite or in whorls of three, lanceolate and with smooth margins. It will form dense monospecific stands in wetlands, especially those disturbed such as by dredging operations, and crowd out all other plants, cattails, grasses, sedges, and rushes, which are favoured by wildlife as food sources and shelter; loosestrife itself provides little in the way food or shelter. It has been known to clog drainage ditches and provides poor spawning habitat for fish and amphibians. Control is difficult, like most of these invasives, in part due to the prolific seeding and the longevity of seed in the soil. Hand-pulling, cutting, burning, water-level manipulation, and herbicides have all been used with some success. Long-term monitoring and control efforts are required until the seed bank in the soil is depleted. The first step, of course, is to stop selling and planting Purple Loosestrife. Alternatives might include Physostegia or Liatris.
Anyone who has tried to bushwack through just about any woods in western New York has probably been snagged and torn by multiflora rose (Rosa multiflora). This plant can grow to 10 feet tall and produce huge impenetrable thickets. The simple white flowers are attractive, with their stamens and pistils forming a protruding column from the center. Small red hips are produced in the fall and persist through December, when birds take most and spread the seeds. Multiflora rose can be distinguished from other roses by the feathery stipules (leafy growths at the base of the leaf stalks). The thorns are sharp and recurved, almost like barbs. A native of east Asia, it thrives in old fields and woodlands, in sun or shade. Several states in the northeast and midwest have classified this as a noxious weed. Control consists of physically removing the roots or using herbicide. As an alternative, try the native Rosa virginiana, a much tamer plant with single pink flowers, maturing at about 6 feet, and not bothered by black spot.
These are some of my favorite bad guys, which I personally have to contend with. There are many others, such as Oriental bittersweet (Celastrus orbiculatus), (beware the buyer of American bitterwseet; it may actually be Oriental), Black Locust (Robinia pseudoacacia), Japanese knotweed (Polygonum cuspidatum), European buckthorn (Rhamnus cathartica), Japanese barberry (Berberis thunbergii), and even some of the all-time big sellers in the horticultural world, such as Norway Maple (Acer platanoides), which is planted on just about every street and is now invading our woodlands and outcompeting Sugar Maples, and burning bush (Euonymus alata), which is becoming a problem in open woods and pastures across the northeast. (To witness the proliferation of burning bush, I suggest the reader take a walk in Durand-Eastman Park from Log Cabin Road to the dam of Trott Lake, where the slopes flanking the trail are essentially a monotypic understory of Euonymus). I would also include black walnut (Juglans nigra) in this list of planta non grata, since I have a plethora of the weeds, thanks to squirrels, and it is just not a friendly plant, due to its juglone toxin, which kills just about everything within its root zone, except for a few tough individuals; but that's another story!
As of last summer there is now the Invasive Plant Council of New York State, Inc. They can be reached at 415 River St., 4th Floor, Troy, NY 12180 cwinters@tnc.org
A good reference for the identification of the woody plants is The Shrub Identification Book by George W. D. Symonds, 1963, William Morrow & Co., New York. Fact Sheets on most of the above-mentioned plants are available from the Virginia Native Plant Society, P.O. Box 844, Annandale, VA 22030. More information can be obtained from the Cornell Cooperative Extension at (716) 652-5400 or the USDA Natural Resources Conservation Service at (716)652-8480.