Invasive species pose a serious threat to the ecosystem balance of Pennsylvania. Invasives usually out-compete native plants in the same environmental conditions. This is mostly because the natural controls that keep the invasive species in check in their native ranges are not present in our environment. Secondarily, they often have wide-ranging effects over simply crowding out native plants. Invasive species “green up“ earlier than native species and also hold leaves and fruit longer in the growing season, making them highly competitive to natives. Many invasive plants are also unpalatable to native species that need to eat plants, notably white-tailed deer. For example in areas where Japanese Stilt Grass has become dominant, deer will often selectively graze off native seedlings and other plants that would otherwise compete with the grass, allowing the grass to spread even more prolifically and harm the biodiversity of the area.
Invasive plants are categorized by the potential threat they pose to the native species or agriculture. Invasive plants that pose a significant economic impact to farmers, in cropland or forest, are placed higher than those that have yet to become “naturalized” (meaning they have not been found to spread widely on their own and are less aggressive in displacing desirable species). Some of these plants may only spread in recently disturbed areas (such as timber harvests, tilled land, or recent construction) and are placed lower on this index. With a changing climate, some invasive plants that present a problem in surrounding states may find new regions more habitable and will expand their range. While they have yet to become widespread problems in Pennsylvania, they are watched carefully, and active measures can be taken to ensure they do not become established here. Most recently this was the case with the spotted lantern fly. Though it’s not a plant, the insect is directly linked to the spread of tree-of-heaven, an invasive tree species, and active control measures on that tree were implemented to prevent the spread of the insect.
DCNR's List of invasive species
Many species of invasive plants were not always considered a problem, and some were even introduced for erosion control or landscaping . Most, however, escaped gardens or their original cultivation. Often a plant gets its start through human activities. These vectors can be as benign as European settlers wanting their homes to resemble those they left, or for purposeful reasons such as using the plant for erosion control or living fences.
While these plants served their initial purposes well, often to great acclaim by their original introducers, over time their impact on our native plant communities and ecosystems has been negative. The native plants that evolved with multitudes of native pollinators, birds, and other cute-and-cuddlies face extreme competition with the aggressive newcomers. In the case of berries from the many varieties of shrub honeysuckle, they are edible for many birds (not humans), but contain less useful nutritional value to migrating birds. This means birds depending on nutritious food to migrate now must spend more time and energy foraging than migrating. This can cause delays in their migration timeframes and exposes them to increased predation while on the move. Other invasives can alter the soil composition - either through actively introducing inhibiting chemicals or absorbing more nutrients and leaving the soil unsuitable for other plants. Purple Loosestrife, for example, can create a monoculture (as it sounds, a singular plant species that covers an entire area) that prevents important nutrients from cycling in wetlands, inhibits the flow of the water in those areas, and forms dense thickets that reduce habitat for waterfowl.
While it may seem that the ever-advancing walls of invasive plant species are unstoppable, there are a wide array of control methods to slow their march. The most effective and least impactful method is good-ol’ mechanical removal. This is when people manually pull invasive plants, chop shrubs and remove root structures, and in some cases perform controlled burns. The main downside to this process is the large effort and high cost involved in treating areas that have become infested. This makes volunteer work in mitigating invasive plants so important, as there are only so many employees that can fight the good fight.
Sometimes, precise chemical herbicides are used by highly trained, licensed applicators to target very specific plant species. These applications can take the form of targeted herbicide used on a plant-to-plant basis, pre-emergent treating of an area to prevent the germination of seeds, or inoculation of a plant with a chemical designed to kill that specific species. Invasive management never relies on widespread chemical application as this can have an inverse effect, killing off the native plants and providing a new area for invasive to take hold. Ecologist who use herbicides in conservation work always use the lowest amount of herbicide possible to accomplish invasive control.
Biologic control methods can be employed as a last resort for truly endemic infestations. Methods include bringing in introduced management species from the invasive plant’s original ecosystem, or a specialized control measure developed and bred from the originals. These measures are the riskiest because of the high risk of unforeseen consequences, so these measures undergo years of rigorous study and testing before finally being introduced. Loosestrife is a more recent example, where a beetle that consumes only this plant was studied for years in controlled settings before being released to combat the spread of the loosestrife.
Many invasives are common garden plants and some of the varieties below are seen in the midst of many landscaped areas. Many government agencies track the spread and actively control the populations of invasive plants, going so far as to declare a number of them as noxious weeds. This declaration allows for very targeted mitigation and enhanced control methods to prevent the spread of that species. In Pennsylvania, the Dept. of Conservation and Natural Resources maintains the most up-to-date listing of all invasive plants in PA. The Pennsylvania Dept of Agriculture maintains the list of plants legally designated as noxious weeds. At the federal level, the U.S. Dept. of Agriculture offers tracking of species across most of North America. All three agencies actively work in the trenches to fight the spread of many of these species into natural areas and preserve the biodiversity of our ecosystems.
DCNR Invasive plant fact sheets
USDA Pennsylvania State-listed Noxious Weeds
Below are some of the more common invasive plants you might encounter on an adventure anywhere in Pennsylvania. Some, like the flowering herbs, grow in dense clumps while others, like the trees, often can be seen solitarily.
While the number of invasive trees is drastically lower than the number of vines or herbs, trees can cause a disproportionate effect on the local ecosystem. Bradford Pear trees are extremely common in landscaping due to their resilience, wide range of tolerable soils, and fast growth. All things that make them a problem when they escape. While most varieties are labeled as sterile, an unintended cross-pollination with other cultivars makes many of these trees’ seeds viable and able to spread widely. Primarily these trees compete with other early-successional trees where their removal can be difficult and can interfere with reforestation or grassland habitat management.
Another highly invasive tree is the tree-of-heaven. As mentioned, these trees are one of the preferred food and reproduction sites of the spotted lantern fly. While visually similar to natives black walnut and staghorn sumac, tree-of-heaven produce the distinct aroma of burnt peanut butter. In fact, this scent is one of the key methods of identifying among other visually similar trees and shrubs. The leaves are also different. Walnut and sumac have fine teeth along the edges of their leaves. Tree-of-heaven has smooth leaf edges with 2 distinct lobes near the bottom of the leaf.
Slightly smaller, but still possessing a woody stem are the invasive shrubs or bushes. The many varieties of bush honeysuckle were originally used as ornamental landscaping plants, but due to their lack of natural controls they are free to grow and reproduce rapidly. They can be easily identified in the early spring as they produce leaves before most natives, and in late summer and fall you can spot them by the shiny red-to-orange berries. While native bush honeysuckles exist, they are dwarfed in size and outcompeted by the invasive species.
Another invasive is multiflora rose, which produces a fruit at the base of each rose as well as thorns, just like its namesake. The curved thorn easily dislodges from the plant and is designed to inflict as much discomfort as possible, discouraging natural consumption by Pennsylvania’s native wildlife. This plant was introduced as part of a program to have farmers plant “living fences” instead of barbed wire. Unintentionally, the plant escaped these narrow hedgerows and now can be found across most roadsides, farm fields, and forest edges.
Japanese Barberry, still widely available as a landscaping plant, produces small red berries, but instead of curved thorns the plant is covered in half-to-three-quarter inch long spikes. The plant arches from a central root mass and forms a characteristic wave of cascading branches from one plant to another. If spines and toxic berries were not enough, barberry also chemically alters the soil and is one of the main nurseries for the lyme spreading deer tick.
Many of the common “wildflowers” that can be seen along roadways are in fact invasive species. Most common among the early spring invasives are garlic mustard plants. Garlic mustard is appropriately named as the early plants can be harvested and ground into a paste that smells like garlic (after the heads of the plants mature the seeds can be ground into a punchier version of the same flavor). Taste aside the plant is an aggressive competitor to native ephemeral wildflowers, often crowding out established plants and emitting chemical inhibitors preventing new ones from germinating. Fortunately the plant has very shallow roots and is easily pulled from areas where it doesn’t belong, but this task is a large one requiring a great deal of time from ecosystem stewards. Garlic mustard also poisons the eggs of butterflies that lay their eggs on it because the native plants they need to lay their eggs have been displaced.
Later in the season invasives like crown vetch begin to bloom with their signature white-to-pink-to-purple color. Originally introduced as erosion control (individual plants can cover 100 square feet in under four years of growth) crown vetch likes to invade recently disturbed areas, regularly mowed areas, and forest edges. The dense matts it forms inhibit the rooting of most other plants. Though labeled as an invasive in Pennsylvania, it is used sporadically in agriculture as a grazing plant or for erosion control, its original intent.
Continuing down the line, the more simple invasive vines and grasses are harder to recognize due to their resemblance to many landscaping cultivars (and many originated as such) but represent just as serious a threat. A recent and very dangerous invasive is Japanese Stilt Grass. Stilt grass forms a bamboo-like leaf structure with the base of the plant elevated above the ground on root-like stilts. It produces a lush green carpet of small plants early in the summer, but by late summer and fall the plant creates brown, hay-like, smothering thatch. The plant quickly outcompetes native plants and spreads across forest floors which can cut off nutrients and moisture to trees. Japanese Stilt Grass was used widely as packing material by porcelain manufacturers and was likely introduced by being discarded after shipping wares.
English Ivy, still widely available as a landscaping plant, was one of the many species introduced during European colonization. Many old homesteads still feature this plant even if all other evidence of human habitation has disappeared. This plant causes issues when it begins to climb trees, killing branches by smothering leaves and adding weight which increases the damage potential to the tree during storms. On the ground its evergreen leaves perpetually shade out other competitors.
A special designation of invasive plants, the noxious weeds, pose a serious threat to humans either through chemical interactions (such as giant hogweed) or through sheer potential damage to agriculture (such as mile-a-minute). PA noxious weeds are illegal to sell or plant within the state and are aggressively controlled if found. In the case of the Giant Hogweed, the plant can tower several feet into the air and produce equally enormous umbels. Even though it is related to delicious carrot, the plant creates a photo reactive compound that causes symptoms similar to a chemical burn when it makes contact with skin and can actually cause blindness. Fortunately this plant is very rare as extensive control measures are used as soon as it is identified.
Mile-a-minute on the other hand is fairly common by comparison. Growing at up to 6 inches a day this ground cover forms a similar mat to English ivy, but at a much faster rate.
For the adventurer finding an area littered with invasives is not terribly uplifting, but the following areas demonstrate locations you can visit to see how modern invasive management policies are used to keep these nuisances from damaging some of our most important ecologic areas. These areas utilize the hard work of volunteers to maintain the native populations and mitigate the spread of invasive species.
Alan Seeger, as part of the Rothrock State Forest has the aid of several volunteer organizations that maintain the trails and help remove invasive from them that could be tracked to ecologically sensitive areas.
Millbrook provides hands-on education on invasive species management from school-aged children to adults.
The Mount Nittany Conservancy actively works to defend the mountain from encroachment of popular escaped landscaping plants and preserve many of the original regrowth on the mountain.
All along the Spring Creek Canyon multiple groups work to keep invasives at bay on the land as well as in the aquatic realms.
ClearWater and our conservation partners control invasive species on the Barrens to Bald Eagle Wildlife Corridor and all our riparian buffer project sites annually.
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