Invasive Plants of the Spring Creek Watershed
Updated: Oct 24, 2020
By Terry Melton, Ph.D., Director, ClearWater Conservancy; Secretary, Nittany Valley Environmental Coalition
Our Centre Region is blessed with stunningly beautiful ecosystems and environmental resources. In the wake of books like Doug Tallamy’s Bringing Nature Home, which encourage the general public to learn about and enjoy the native plants that they host locally, there has been enormous interest in the plant communities of the Spring Creek Watershed, one of the most studied watersheds in North America.
The challenges to our watershed are numerous, including steady strong population growth and climate change. Both of these challenges have worsened a problem that will be with us no matter how small or large our communities are: invasive plant species. Invasive plants are non-native species that grow aggressively and outcompete and destroy our native plant communities, leading to devastating changes in our ecosystems. In general, development disturbs the land, permitting these species to thrive. Climate change means that warmer temperatures over time will prevail, increasing average seasonal temperatures and encouraging non-native plants that thrive in warmer climates to move northward in their ranges. Plants that never grew in Pennsylvania before now have found a foothold in Central Pennsylvania. It is estimated that one new invasive species is introduced to Pennsylvania’s aquatic environments every eight months.
On October 30, 2017, Governor Wolf signed into law the Controlled Plant and Noxious Weed Act. The new law overhauls the old noxious weed act and broadens the power of the Department of Agriculture to regulate plants in Pennsylvania. Many of Pennsylvania’s invasive plant species have been designated Class B noxious weeds, allowing the Department of Agriculture to require control of injurious infestations. Control of invasive species is not new, but has attained a new urgency as we have begun to realize that our native species, which support, and have co-evolved with, local pollinator species, birds, and mammals, may not survive an onslaught of more aggressive plants unless we provide help. To this end, there are vast resources online to educate the public about native and invasive species, particularly differentiating them. Some of these resources are listed at the conclusion of this article.
While one online article cannot possibly provide comprehensive information about each of the plants described, the goal here is to give general information pulled from many sources, focusing on interesting facts that are not readily known. I hope that some of this interesting material will help you remember these plants as you come across them in our watershed. The plants below have been suggested for this article by local experts, whose work in ecosystem restoration qualifies them to prioritize invasive species for control or removal.
Spring Creek Watershed residents may see some invasive plant species in abundance in the following locations:
Thompson Woods Park understory: honeysuckle, privet, and oriental bittersweet.
Route 322 between Boalsburg and Tait Farm: honeysuckle, privet and autumn olive.
Route 322 highway median between Route 26 and the exit to Beaver Stadium: poison hemlock.
Millbrook Marsh: European alder.
Brush Valley Road in Oak Hall after the turn from Linden Hall Road, and along Spring Creek in the floodplain: teasel.
Shingletown Road and East Branch Road, in the Slab Cabin Run floodplain: reed canary grass.
Route 550 along Buffalo Run at the first sharp curve (on the left) heading towards Stormstown after the turn from Skytop Road: Japanese knotweed.
Many thanks to Colleen DeLong of ClearWater Conservancy for these field observations.
Autumn olive (Elaeagnus umbellata)
Autumn olive was introduced into the US in 1830 from Asia to be used as an ornamental, for wildlife habitat, as windbreaks, and to restore deforested and degraded lands. These deciduous shrubs can grow up to 20 feet tall, and have buds and leaves that are covered with silvery to rusty scales, that shed over the summer months (Photo 1). The leaves are alternate, shaped like eggs or lances, and have smooth, rippled margins with a dull green color on top. The branches can have sharp thorn-like protrusions. Aromatic, pale yellow flowers with four petals appear in June and July, and mature into abundant small red-brown to pink fruits in August through October. It is similar in appearance to Russian olive (Elaeagnus angustifolia), which is also invasive.
This drought-tolerant invasive plant thrives in a variety of soil types. It can fix nitrogen in its roots, so it is able to grow on bare mineral substrates. Like most invasive species, it outcompetes and displaces native plant species, creating dense shade and interfering with natural plant succession and nutrient cycling. Once it establishes in an area, it is nearly impossible to eradicate, but may be controlled with pulling, mowing, and herbicides.
Autumn olive does provide forage to deer, raccoons, skunks, bear, mice and birds, and nesting sites for birds. The nectar is used by bees and other pollinators. The completely ripe fruits, which are juicy and sweet, are edible and can be eaten raw, made into jam or preserves or substituted for tomatoes in recipes. The berries of both cultivars and wild varieties have been proposed as potentially commercially viable due to high levels of lycopene. Mature plants may produce up to 30 pounds of fruit per year.
Interesting fact: Because of its nitrogen fixing ability, autumn olive can enrich soil, and has been found to increase yields in fruit trees planted nearby by up to 10%.
Photo 1. Autumn olive. Credit: James
H. Miller, USDA Forest Service, Bugwood.org.
Canada thistle (Cirsium arvense)
Bull thistle (Cirsium vulgare)
Plumeless thistle (Carduus acanthoides)
Musk thistle (Carduus nutans)
Canada thistle, bull thistle, and musk thistle are recognized by Pennsylvania as Class B noxious weeds. Plumeless thistle, a biennial, is locally invasive as well (Photo 2). Canada thistle, a perennial, was probably introduced to the US by mistake in the early 1600s and is regarded as one of the “most tenacious and economically important agricultural weeds” by the Pennsylvania DCNR. It is actually native to Eurasia, despite its name. Bull thistle, a biennial, is thought to have been introduced to North America in the 1800s, but is native to Europe, western Asia, and Northern Africa. Another biennial, musk thistle (also known as nodding thistle), was accidentally introduced to the US in the early 1900s.
Photo 2. Spiny plumeless thistle. Credit: Todd Pfeiffer, Klamath County Weed Control, Bugwood.org.
More than 60 species of thistle live in North America, and at least seven of them are native to Pennsylvania. Because thistles can be difficult to identify, a native thistle guide by the Xerces Society for Invertebrate Conservation may be useful for identifying those natives which should not be removed from the environment. Thistles are members of the sunflower family (Asteraceae) and can be annual, biennial, or perennial. True thistles have spines along the leaf margins and the flower heads are pink to purple and surrounded by spiny bracts in most species. Generally, thistles like sunny open spaces that have been disturbed by fire or animals but can also invade wet spaces with fluctuating water levels, such as stream bank meadows. Thistles may produce over 100,000 seeds per plant, which remain viable for many years, making control and eradication very difficult. Annual and biennial thistles flower only once, so they can be controlled by mowing before flowering, or by herbicide application prior to seed-set.
Thistles are a preferred plant of pollinators; over 200 species visit them, including the American bumblebee, the tiger swallowtail butterfly, and monarch butterfly. Goldfinches, chickadees and other birds love the seeds. According to plant ecologists, native thistle seeds have high protein content and moisture that is good for molting birds.
Interesting fact: Because native thistles are hard to distinguish from invasive thistles, the native species may be inadvertently eradicated in control efforts; they are also removed due to farming and development. One biological control method is use of the thistle head weevil, but this method may impact native species as well.
Note: Teasel (Dipsacus fullonum) is not a thistle but has many similar characteristics. It is a tall (up to 7 feet), spiny, non-native, invasive biennial that dies after it seeds. The dried flower heads are popular in flower arrangements; the living flowers are white to light purple. Teasel is commonly observed in roadsides and waste areas, and invades agricultural fields and pastures. In the first year, a basal rosette forms with wrinkled leaves and a pale midvein. Elimination is possible by digging up colonies while the plants are small, but mowing is ineffective because the plants grow back from the cut crown. Removal of seed heads, both mature and immature, is critical for control. A single plant forms up to 3,000 seeds that may remain viable for years. Herbicide management may be required for large established colonies, due to prolific seed production; dying second year plants with their spines effectively protect and shelter newly emerging rosettes. Teasel is quite problematic in the Spring Creek Watershed.
Curly-leafed pondweed (Potamogeton crispus)
This aquatic perennial has olive-green to reddish-brown 4-to-10-cm long crinkled leaves, alternately arranged on 1 to 3 foot branching stems (Photo 3). It grows off-shore in depths of up to 15 feet and has the unique ability to form new plants under ice in winter, so it is one of the first nuisance plants to emerge in spring, eventually forming dense monotypic stands and prohibiting the growth of native aquatic plants. This is the most abundant and widespread aquatic plant in Spring Creek. Although a native of Eurasia, Africa, and Australia, it was introduced to North America by aquarium hobbyists and is present in all states and unfortunately is widespread in Pennsylvania. Unlike other pondweeds, the leaves are always submerged, but in the spring, greenish-white flower spikes often stick up above the water surface. This species prefers shallow waters (still or flowing), high nutrient/alkaline water composition, and can survive in low light and temperatures, permitting establishment in areas where native Potamogeton species cannot live.
Photo 3. Curly pondweed. Credit: Leslie J. Mehrhoff, University of Connecticut, invasive.org.
In mid-summer the typical curly pondweed die-off may cause algal blooms. Very aggressive pond spread is often an indication of excessive nutrients coming from agriculture; consequently, pondweed control can be enhanced by reducing nutrient runoff. Grass carp, a good biological control, find curly pondweed a preferred food, but these carp must be purchased from an approved hatchery after receiving a state permit. Herbicide controls are effective as well, but research indicates that treatment must be ongoing over many years. Research from the Minnesota Aquatic Invasive Species Research Center (September 2019) indicates that simply removing pondweed may be insufficient to restore native species unless water conditions (reducing turbid water and increasing light availability) are improved. Eradication is difficult.
Propagation of curly pondweed has several mechanisms. After the summer die-off, seeds drop to the bottom of the pond or stream. It also spreads vegetatively from winter buds called turions that float downstream; waterfowl ingestion and excretion of the turions may disperse the plants. Recreational water activities also spread it. In August 2019, Somerset County experienced a massive fish kill due to the combined factors of excessive heat, oxygen depletion, and extensive overgrowth of curly-leafed pondweed, suggesting that warmer temperatures caused by climate change will have significant impacts on pond ecosystems.
Interesting fact: Pondweed has recently been discovered to have been a component of the diet of Paleolithic hominids.
European black alder (Alnus glutinosa)
The European black alder is a deciduous tree growing up to 60 feet tall. It has simple rounded alternate leaves with shallow, toothed margins. The distinguishing feature are the male flowers, or catkins, that grow up to 4 inches long; the erect green female flowers mature into egg-shaped nutlets, similar in appearance to small pine cones in October (Photo 4). It has a similar appearance to native alders such as smooth alder (Alnus serrulata) and green alder (Alnus viridis). Native to Europe, Asia, and North Africa, it enjoys acidic soils and colonizes stream sides and floodplains. Its ability to fix nitrogen via the bacteria Frankia alni allows it to colonize poor soils.
Photo 4. European black alder. Credit: T. Davis Sydnor, The Ohio State University, Bugwood.org
European black alder reproduces mostly by seeds that are full of small air pockets, well known for their floating ability, which are dispersed via wind or water, so downstream colonization is problematic. While the seeds are a winter food for birds and the tree itself feeds mammals, it often forms large stands, and therefore can easily displace native species by blocking sunlight. The trees’ dense network of roots can cause increased sediment in waterways. Cutting the tree results in regrowth from the stump and fallen branches can re-root. Control is via mechanical, chemical, and biological methods. Numerous insects and diseases are known to infect it.
Historically, the wood was useful for underwater foundations and for manufacture into paper and fiberboard, tanning and dyeing, for smoking foods, and for wood carving (including Fender guitar bodies). In fact, many of the piles under Venetian buildings, including the Rialto Bridge, are made of alder wood. While the pollen is a major source of allergy, the leaves and bark or European black alder have been used in folk medicine as treatment for sore throat, muscle aches, skin ailments, and infestations from lice and scabies.
Interesting fact: In research studies, seed extracts have been found to act against some human pathogenic bacteria, including methicillin-resistant Staphylococcus aureus (MRSA).
Exotic bush honeysuckles (Lonicera spp.)
While there are five exotic bush honeysuckles found in the northeast US, the two most common Pennsylvania species are Morrow’s (L. morrowii) and Tatarian (L. tatarica) (Photo 5). These dense, upright, deciduous shrubs can reach 20 feet in height and were introduced as ornamentals and for erosion control in the 1700s. While both species have opposite oval leaves, Tatarian honeysuckle has smooth, hairless, bluish-green leaves, while Morrow’s honeysuckle has downy leaves. Fragrant flowers are arranged in pairs that bloom May through June; Tatarian flowers are pink to crimson and others are white, aging to yellow. Round berries are deep red, yellow, or orange, maturing in September and October. To differentiate invasive bush honeysuckles from native species, note that the leaves of exotics appear earlier in spring, and hold their leaves later in fall. Native species Lonicera diervilla and Lonicera canadensis have yellow flowers; L. diervilla prefers dryer, rockier soils, but L. canadensis prefers the understory of moist, rich forests.
Photo 5. Tatarian honeysuckle. Credit: Richard Gardner, Bugwood.org
Because exotic bush honeysuckles are shade-intolerant, they are mostly observed in forest edges, pastures, roadsides, and other disturbed habitats, but can still invade forest understories. Morrow’s can also invade bogs and lake shores. The berries are eaten by deer and over 20 species of birds, which widely disseminate them, but invasive spread can also occur via vegetative sprouting from branches and roots. This invasiveness results in shading out native plants, altering soil chemistry, and reducing soil moisture. Because pollinators prefer the nectar of these invasive honeysuckles, there is reduced pollination and seed-set among native honeysuckle species, and their berries are relatively nutrient-poor for migrating birds compared to those of native honeysuckle species. Fortunately, young plants are shallowly rooted, permitting hand removal in light infestations, and burning or herbicide treatment for heavier infestations has been shown to be helpful, however, all root fragments must be removed to prevent regeneration.
Interesting fact: Goat herds have been used to reduce the overgrowth of honeysuckle in some areas, permitting humans to access and remove the remainder of the plants by hand.
Japanese barberry (Berberis thunbergii)
Japanese barberry was introduced into the US as an ornamental in 1875, promoted as a substitute for European barberry, which had been used as hedgerow material, and for dyes and jam. This plant has dense, prickly foliage consisting of small red-to -purple-tinged oval, alternate leaves, and this foliage has led Japanese barberry to become a popular, hardy, and inexpensive landscaping plant (Photo 6). In mid-spring to early summer, drooping clusters of yellow flowers develop into bright red oblong berries. It tolerates a wide variety of habitats, including woods and wetlands, but its shade tolerance is of particular concern for forested areas. Seeds are dispersed by birds and small mammals, and have a high germination rate. It can also spread vegetatively via branches touching the ground.
Photo 6. Japanese barberry. Credit: Richard Gardner, Bugwood.org.
Japanese barberry’s thick, prickly stands harbor black-legged (deer) tick populations because it offers protection to deer mice, the primary winter host for black-legged ticks. A 2011 University of Connecticut study found that the number of ticks carrying the Lyme spirochete were more than 10 times greater in barberry thickets than areas without the bushes. Fortunately, Japanese barberry could be eliminated if people would stop purchasing it as a landscaping ornamental. At this time, it is proposed to be banned by the Pennsylvania Controlled Plant and Noxious Weeds Committee (decision pending), and is banned at garden centers in Ohio, New York, Maryland, and many other states. However, New York State’s Department of Environmental Conservation recently approved the sale of four sterile cultivars of the Japanese barberry. Correct labeling will be important in assuring that homeowners plant the sterile version in the future. In the 2016 USDA survey of invasive species, Japanese barberry was the seventh most commonly observed invasive species.
Interesting fact: Reduced native tree diversity is observed on plots with barberry stands. In addition, an August 2019 study published in Environmental Entomology, from Washington State University (Pullman, WA) and Great Hollow Nature Preserve and Ecological Research Center (New Fairfield, CT), indicated that forests that have been heavily invaded by Japanese barberry have a significantly lower abundance and species richness of leaf-litter and foliage-dwelling invertebrates than areas of the same forest that are relatively free of Japanese barberry. The cascading negative impacts of ecosystem change are then propagated through many other forest species, including birds and small mammals.
Japanese knotweed (Fallopia/Reynoutria japonica)
The bane of many a watershed enthusiast, Japanese knotweed is a shrub-like herbaceous perennial that can grow to over 10 feet (Photo 7). The wide triangular leaves have a straight-across base and resemble elephant ears on stems that are segmented like those of bamboo. Branched clusters of greenish to white summer flowers have a strong floral scent. The base of the stem or each joint is covered by a membranous sheath called an ocrea. Japanese knotweed prefers a moist, open, and partially shaded habitat and is found in a variety of soil types and acidities. It tolerates high temperatures, high salinity, drought, and floods. Underground rhizomes spread readily to form dense colonies which outcompete nearly every other native species living nearby. A native of Asia, it was introduced in the 1800s as an ornamental and has been used for erosion control. Widespread in Pennsylvania, it is one of the most difficult invasive species to eradicate.
Photo 7. Japanese knotweed. Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org.
Slate Magazine provides fascinating reading on the threat that this species poses to wetlands and riparian communities. In the UK, for example, banks will not issue a mortgage on property where Japanese knotweed is present unless a management plan is in place to control it. Homeowners describe knotweed encroaching through floors and walls, heroic efforts to excavate soil from building sites where it is endemic, and the frustrations caused by the potential of a plant that can regenerate from a single half-inch piece of root. While mechanical and chemical control methods are useful, they can take years; all pieces of the plant must be collected when removal is the chosen option. New York City has spent over $1 million to treat just 30 acres of knotweed since 2010.
In the realm of biological controls, the sap-sucking psyllid Aphalara itadori has been licensed by the UK government. This insect successfully defoliates knotweed, depleting its energy supply and significantly reducing growth. Aphalara itadori is the first successfully licensed European bio-control agent. In consequence, the United States has advised the public “that the Animal and Plant Inspection Service has prepared an environmental assessment relative to permitting the release of Aphalara itadori for the biological control of Japanese knotweed…we have reached a preliminary determination that the release of the biological control organism will not have a significant impact on the quality of the human environment.” This assessment was provided for public comment in 2019 and over 300 comments from scientists and the public have been received to date.
Interesting fact: Some opposition to reduction of knotweed populations has been expressed by the bee-keeping community; the species is a source of nectar when in flower.
Japanese stiltgrass (Microstegium vimineum)
This highly aggressive annual, herbaceous, sprawling grass resembles miniature bamboo (Photo 8). It germinates in springtime and grows slowly through the summer months to a height of 12-24 inches. The pale green narrow leaf blades are lightly hairy, and are distinguishable by a pale silvery strip of reflective hairs on the mid-rib. This latter feature permits differentiation from a native perennial, Leersia virginica (whitegrass). Japanese stiltgrass is thought to have been introduced in 1919 into Tennessee, when it was used as packing material for porcelain shipped from Asia.
Photo 8. Japanese stiltgrass. Credit: Bruce Ackley, The Ohio State University, invasive.org.
Stiltgrass reduces growth and spread of native species and alters insect communities. It occupies a wide range of habitats like open woods, floodplain forests, wetlands, fields, roadsides, ditches, and gardens, and readily invades disturbed land. Invasions typically begin along roads, where propagules (portions of the plant capable of regeneration) are dispersed into favorable light and nutrient habitats, and then creep into woods and fields. Japanese stiltgrass spreads by roots originating from nodes on the trailing stems. Seeds disperse widely via flowing water, floods, human movement, deer browsing and contaminated materials. Unfortunately, the seed bank is long-lived (at least 5 years), meaning that repeated and determined eradication efforts are required, and plant parts containing seeds should be removed, bagged, and incinerated. One plant can yield 1000 seeds which set in late August and September. It can be closely mowed late in the season to prevent seed setting; herbicides are useful, especially grass-specific applications, which have been found to have a kill rate of 87% or greater with repeated applications. There is currently no known biological control agent for stiltgrass, though research is going on in this area. Per the US Forest Service’s 2015 inventory of invasive species, Japanese stiltgrass was the sixth most commonly observed invasive species.
According to Penn State Extension, some native species that may outcompete stiltgrass include ferns that will naturalize, including ostrich fern (Matteuccia struthiopteris) and sensitive fern (Onoclea sensibilis). In sunny areas, mountain mint (Pycnanthemum muticum) or beebalm (Monarda spp.) have been observed to compete successfully. According to Wikipedia, stiltgrass can serve as the host plant for at least two species of butterflies, the Carolina Satyr and the Mitchell’s Satyr (an endangered species not found in Pennsylvania).
Interesting fact: Research has shown that stiltgrass invasion is statistically correlated with higher ecosystem tree diversity. High tree diversity creates diverse micro-habitats that favor invasion of shade-tolerant species like stiltgrass. This research also showed that natural forest regeneration is conducive to stiltgrass invasion, so artificial regeneration assistance, such as planting and supporting native trees and shrubs through their establishment may help prevent overwhelming stiltgrass invasion after severe land disturbances.
Multiflora rose (Rosa multiflora)
This invasive perennial shrub grows to over 10 feet tall, and has a mounded form with curving canes that arch from a single base. Multiflora rose colonizes roadsides, fields, open woodlands, and forest edges, and prefers sunny areas. The white to pinkish flowers bloom abundantly in June; flowers mature in late summer to fleshy pink to red hips that are highly nutritious and are enjoyed by birds and mammals (Photo 9). Multiflora rose was among the first plants to be listed on Pennsylvania’s Noxious Weed list. It can form dense thickets, which due to thorns, are largely impenetrable, and where the arching branches touch the ground, new roots can form and establish new plants. Near trees, the plant can grow like a vine, climbing the trunk. It was introduced from Asia in the 1700s as rootstock for the grafting of ornamental roses and later was used as a living fence for pastured animals, erosion control, and as cover for wildlife. To distinguish multiflora rose from the over a dozen native rose species of Pennsylvania, look for the flowers which are five petaled and cluster in branches. In addition, the base of the leaf where it attaches to the thorny stem is fringed. The hips contain, on average, 7 seeds that can remain viable for up to 20 years in the soil. These seeds are dispersed by animals that forage on the hips.
Photo 9. Multiflora rose. Credit: Ansel Oommen, Bugwood.org.
Control of multiflora rose via different methods is necessary. For example, mowing the crowns to remove the majority of thorns can make treatment of the stumps via herbicide more feasible. According to PSU researchers, multiflora rose may be under attack from rose rosette disease, a viral condition which is working its way across the US from the west. Unfortunately, rose rosette disease affects native rose species as well. It moves via an eriophyid mite; an infected plant can be recognized by “witches’ brooms” terminal branch clusters and small reddish leaves and shoots. There is no known treatment or cure for infected plants. Where multiflora rose in the ecosystem is killed by this disease, it is rapidly replaced by exotic honeysuckles or invasive olive species. Like many invasive species, its attractive features such the fragrant lovely blooms and showy hips reduce the average homeowner’s inclination to remove the invader. Some success in control has been achieved with grazing goats. Research is being conducted on possible additional biological control methods such as the European rose chalcid wasp (Megastigmus aculeatus). This wasp lays its eggs into the developing ovule of the rose and the larvae consume or damage the seeds.
Interesting fact: Multiflora rose is a significant pollinator plant, and the hips are used for tea, jelly, jam, and syrup. They are rich in Vitamin C and should be collected after the first frost, when they are softened and at their sweetest.
Narrowleaf cattail (Typha angustifolia)
Narrowleaf cattail is an aquatic perennial that was introduced from Europe in the ballast of early ships. This non-native species produces distinctive furry brown spiked flowers that are divided into separate male and female parts (Photo 10). This invasive plant, which has narrow leaves up to 6 feet long, can hybridize with a native broadleaf cattail (Typha latifolia) to produce a hybrid species (Typha x glauca). The fruits are distinctive, appearing as cigar-shaped tubes up to 6 inches long, and contain downy seeds about 1 mm in size. Dense stands that form along shores can crowd out or shade out species that may shelter or feed waterfowl, and it is believed that the species produces chemicals that prevent the growth of other species, a characteristic known as allelopathy. A single plant can produce over 200,000 seeds annually that may remain viable for 100 years. According to the USDA, while narrowleaf cattail is a very undesirable plant due to the aforementioned reasons, all parts of the plant are edible when gathered at the right stage. The base of the stem can be boiled like a potato, and the young flower stalks can be steamed. The pollen is used as a type of flour for pancakes or as a thickener, and the rhizomes are sweet and can be eaten raw, baked, roasted, or broiled, or dried and pounded into flour. Because of its high nutritional value, this ubiquitous plant could be a significant source of food for world populations. Native peoples of North America used it for weaving, ropes, boat building, bedding, caulking, tinder, and insulation.
Interesting fact: Apache Indians used narrowleaf cattail pollen in female puberty rites.
Photo 10. Narrowleaf cattail. Credit: Rob Routledge, Sault College, Bugwood.org.
Oriental bittersweet (Celastrus orbiculatus)
Oriental bittersweet is a deciduous woody vine with a twining habit. Introduced into the US in the 1860s as an ornamental plant, it has attractive yellow and orange berries (Photo 11). The glossy rounded alternate leaves display finely toothed margins and finely pointed tips. Female plants produce small clusters of greenish flowers in May and June, becoming fruits that ripen from green to yellow capsules, and open in the fall to reveal three red-orange seed coatings. The vines can grow to large size with thick trunks up to 4 inches in diameter. To distinguish oriental bittersweet from native American bittersweet (Celastrus scandens), note that the flowers and fruits grow only at the tips of the small clusters of branches in the native species, rather than along long stretches of the stems in the invasive species. The native form also has dark orange fruit capsules.
Photo 11. Oriental bittersweet. Credit: Chris Evans, University of Illinois, Bugwood.org.
Oriental bittersweet infests forest edges, riparian corridors, woodlands, fields, hedge rows, and other areas with habitat disturbance. It covers and climbs anything nearby, eventually killing the underlying tree by girdling the trunk or causing uprooting or branch breakage when soils are saturated or during heavy wind. It out-competes the native bittersweet, and inter-species hybridization between native and invasive bittersweets has been observed in the wild, suggesting that the genetic identity of native bittersweet could eventually be lost. Dispersal is primarily by prolific seeds via birds, or by root suckering from existing exposed or buried roots. Control is via multiple methods, however cutting the vine without herbicide treatment of the stump will encourage root suckering and regrowth.
Interesting fact: The berries are beautiful and stand out in the woods after fall leaf drop. Because the vines are used extensively in wreaths, humans take some blame for moving the plants to new areas.
Phragmites or Common Reed (Phragmites australis)
Phragmites is a perennial long-lived grass that can grow up to 15 feet tall in stands that outcompete all other vegetation. While the common reed is native to North America, a non-native strain introduced from Europe in the late 1700s has aggressively replaced the native strain. During the growing season, the wide, pointed leaves are gray-green, and in late July and August panicles, the bushy clustered flowers, appear purple or golden (Photo 12). Hairs on the seeds at maturity give the panicles a fluffy appearance.
Photo 12. Phragmites or common reed. Credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org.
Phragmites is found along the borders of streams and ponds and in tidal and/or marshy areas. The spread of this species is primarily via roots and seeds, and portions of the plants that break free are widely transported via water and can establish new downstream colonies. Abundant research is being done on the control of Phragmites, due to its invasiveness in wetlands where restoration efforts are ongoing. Control via multiple methods is required, due to deep roots and rhizome viability. Phragmites may be confused with reed canarygrass (another invasive plant).
Interesting fact: Many uses have been described for the common reed in human culture, including thatching, and more recently biogas production. Culpeper’s Complete Herbal (1653), says of the common reed that “the roots …are applied to the place draw out thorns, and ease sprains; the ashes of them mixed with vinegar, take scurf, or dandrif off from the head, and prevent the falling of the hair.”
Poison hemlock (Conium maculatum)
Poison hemlock, a biennial native to Europe and North Africa, has an appearance similar to wild carrot, with divided compound lacy-appearing leaves that taper to a point and also appear similar to ferns (Photo 13). Umbrella-like flat-topped white flowers bloom in mid-summer. The most distinguishing feature is hollow, ribbed stems with purple spots; the plant grows up to 10 feet tall. The crushed leaves have a rank odor. Now found in nearly every state of the US, it is a pioneer species that is opportunistic in disturbed sites where it can quickly colonize an area.
Poison hemlock is found along roadsides, fields, ditches, marshes, meadows, and other low-lying areas, but it prefers shaded areas with moist soil. Research has shown that mowed roadsides contain high levels of nitrogen from fallen cuttings, making these areas an attractive habitat for the nitrogen-loving poison hemlock. Because it is a biennial, it produces flowers in the second year, but in the first year an attractive shaggy mound of leaves appears. Seed dispersal is the primary form of reproduction.
Wild carrot (Daucus carota), wild parsnip (Pastinaca sativa) and water hemlock (Cicuta maculata) are similar with lacy leaves, but wild carrot has a hairy stem. Although water hemlock does have the purple mottling on the stem, it also has a cluster of fleshy taproots at the base.
Photo 13. Poison hemlock. Credit: Ansel Oommen, Bugwood.org.
Poison hemlock is highly toxic to humans and livestock due to piperidine alkaloids. Its toxicity is well-known in history: the poison was used to execute prisoners, with Socrates being the most notable example. The plant’s toxins lead to respiratory failure; symptoms of exposure are trembling, salivation, lack of coordination, dilated pupils, and weak pulse. Some people have an allergic reaction to handling it, resulting in a rash, however, this rash should not be confused with the extremely unpleasant blistering photosensitivity rash created by cow parsnip (Heracleum lanatum). The plant should never be composted.
Control is largely chemical but to mechanically dispose of the plant, protective gear must be worn. First year plants can be removed at any time, but second year plants should be treated and removed before flowering occurs. The plants should not be burned due to possible release of toxins.
Interesting fact: Numerous reports of exploding populations of poison hemlock in Pennsylvania in the last two years are attributed to large amounts of rainfall combined with a persistent seedbank.
Privet spp. (Ligustrum vulgare, Ligustrum sinense, etc.)
Privet, a member of the olive family, was introduced from Europe and Asia in the early to mid-1800s as an ornamental. This shrub is semi-evergreen and forms thickets that can grow up to 15 feet tall. Generally, the leaves are opposite, elliptical or oval, and are found in two rows along the stem. The upper side of the leaf is lustrous green, and the underside is pale green with a hairy midvein and may be rounded at the tip with small indentations (Photo 14). Small white flowers grow in clusters at the end of stem, June through July, and mature to small shiny blue-black berries in mid-fall.
There are various invasive privet species, difficult to distinguish from each other, found along roadsides, lowland forests, hedgerows and other disturbed areas. Privet adapts to various habitats, though it prefers full sun. Dense thickets can easily out-compete native shrubs. It reproduces via seed dispersal from birds, as well as vegetatively. All Ligustrum species are banned from New Zealand due to asthma and eczema in some people exposed to it in abundance. There, the privet lace bug (Leptoypha hospita) is being used as a biological control.
Photo 14. European privet. Credit: T. Davis Sydnor, The Ohio State University, Bugwood.org.
Researchers have determined that the abundance and diversity of butterflies will increase following privet removal to almost the same abundance as that of a similar forest community with no history of privet invasion. Dense privet invasions have also been observed to decrease the diversity of native honeybee colonies.
Climate change will increase the spread of privet northwards over time; indeed, privet was once only a species of concern in the Southeast US. While mechanical methods of removal are possible, herbicides are the most effective tool, with foliar applications in dense thickets applied in early spring and late fall to avoid native species. Mowing may be ineffective, as stumps and roots re-sprout. For single plant treatments, cutting the stump and applying herbicide to the surface or into a hole drilled into the stump is effective. New research has indicated that privet can be well controlled with lower concentrations of herbicide, but these must be perfectly timed to the growth cycle.
Interesting fact: There is a native Florida privet, Forestiera segregata, but no Pennsylvania Ligustrum native.
Purple loosestrife (Lythrum salicaria)
Currently classified as a Pennsylvania Class B noxious weed, purple loosestrife is an upright perennial growing up to 10 feet high with an attractive purple flower spike (Photo 15). The long narrow leaves are opposite or whorled in groups of three on stalkless stems that have smooth edges and are occasionally covered with fine downy hairs. The flowers with five to seven petals are pink to purple-red, blooming June to September. The mature plants have 1 to 50 square woody stems coming from a central taproot and are sometimes confused with blue vervain (Verbena hastata) and blazing star (Liatris spp.), both desirable natives; differentiation is possible from the blooms. Introduced in the 1800s from Europe and Asia, it was widely cultivated for its ornamental and pharmacological values, but has little value as forage, nectar, or shelter when balanced against its invasiveness.
Photo 15. Purple loosestrife. Credit: Richard Gardner, Bugwood.org.
Purple loosestrife is highly invasive due to the millions of seeds produced in a single plant and underground stems that advance about one foot per year. In addition, the seed germination rate approaches 100% and the seed bank is persistent. Its habitat is wetlands and riparian corridors, brackish water areas such as ditches, and other moist soil areas. Purple loosestrife is threatening federally endangered orchids such as the eastern prairie fringed orchid (Platanthera leucophaea), reducing habitat for waterfowl, clogging waterways, and disrupting nutrient cycling. It was added to the Pennsylvania Noxious Weed Control list in 1997.
Originally, garden varieties of purple loosestrife were considered to be sterile, but studies have shown that they are capable of pollen and seed production. In fact, most infestations are the result of garden escapes. Removal methods include mechanical methods and herbicide application, but one more successful approach has been the use of beetle combatants which do not endanger nearby native species: Galerucella camariensis (leaf eaters), Galerucella pusilla (leaf eaters), Hylobius traversovittatus (root miners), and Nanophyes marmoratus (seed weevil). These predators can spare wetlands from herbicide usage and have been used in Centre County.
Interesting facts: Purple loosestrife tea was allegedly used as treatment for intestinal problems and bacterial infections. Culpeper’s Complete Herbal (1653) states that a tea prepared from purple loosestrife “doth cure the quinsy, or king’s evil in the throat.”
Reed canarygrass (Phalaris arundinacea)
Reed canarygrass is a cool-season perennial grass reaching 9 feet tall that forms dense stands, crowding out all native plants. There may be both a native ecotype and a more aggressive Eurasian ecotype, which was originally planted for erosion control (Photo 16). The long tapering leaves are rough on both sides and are light green to straw color. A transparent ligule, or membranous strap, is present at the junction of the leaf and stalk. The flowers form purple to green panicles high above the leaves in early summer. To differentiate between reed canarygrass and the non-native orchardgrass (Dactylis glomerata), look for reed canarygrass’s transparent ligule.
Photo 16. Reed canarygrass. Credit: Leslie J. Mehrhoff, University of Connecticut, invasive.org.
This wetland plant prefers saturated soils where standing water does not last for extended periods, such as roadside ditches, shallow marshes, and wetland meadows. Its growth is enhanced by agricultural nutrient runoff. Seeds and creeping rhizomes cause aggressive propagation which is facilitated by waterways, animal movement, human activities, and machines. A long-lived seed bank often defeats eradication attempts. Unfortunately, in some areas, reed canarygrass is still being planted for erosion control. Control methods include burning, two mowings to reduce seeding, and herbicide application, but site treatment should be planned for at least five years. Eradication efforts can be intensive and complex, especially when considering which native species can replace and outcompete reed canarygrass. Penn State reported in 2017 that state gas well pads are being heavily colonized by invasive grasses such as reed canarygrass and Japanese stiltgrass, which are brought to the sites on heavy truck tires.
Interesting facts: Minnesota artist Anna Haglin makes paper from reed canarygrass and embeds native plant seeds in the paper. The idea is for the paper to be eventually buried to reconstitute the land with native seeds. In Finland, reed canarygrass is being formulated into a biomass fuel and shows potential for bioremediation (the removal, degradation, and containment of contaminants in soils, surface waters, and groundwater).
Watercress (Nasturtium officinale)
Watercress is an aquatic or semi-aquatic perennial in the mustard family that was introduced into North America in 1831. It is found in the cold, alkaline waters of springs and spring runs, making it a natural fit for Spring Creek Watershed. Found spring to fall, it is a floating or creeping plant with small white flowers that are enjoyed by hoverflies (Photo 17). It has hollow stems and leaves that are divided into 3 to 9 leaflets; the terminal leaflet is the largest and nearly round. The flowers resemble a cross, hence its designation as a member of the “cruciferous” family. It is considered an introduced species in North America, and is listed as a noxious invasive weed in 46 states.
When watercress is damaged it releases mustard oils that can dissuade various aquatic herbivores such as snails, caddis flies, and amphipods from eating it, however it is readily consumed by ducks, muskrats, and deer. The first cultivation for food was recorded in the UK in 1808 and it was grown extensively in the clean, free-flowing streams of southern England during the 1800s. Propagation is via seeds and stems in waterways. Removal efforts have been minimal and primarily mechanical due to its overall establishment and reluctance to use herbicides in flowing waterways.
Interesting fact: The bitter, peppery leaves are a nutrient-dense food containing high amounts of vitamin K. One cup of watercress has 106% of vitamin K’s recommended daily allowance; however, wild-collected leaves should be carefully cleaned to avoid giardia.
Photo 17. Watercress. Credit: Shaun Winterton, Aquarium and Pond Plants of the World, Edition 3, USDA APHIS PPQ, Bugwood.org.
Dr. Melton is the founder and former CEO of Mitotyping Technologies, a State College laboratory specializing in the analysis of mitochondrial DNA forensic samples. She is a Master Gardener and has been the volunteer caretaker of the Native Plant Garden at ClearWater Conservancy since 2009. She is on the board of ClearWater and is the secretary of Nittany Valley Environmental Coalition.