What is an invasive plant?

What is an exotic plant? Nearly all invasive plants are exotic (alien, introduced, non-native). Such plants were transported to this region, usually by humans. These ferns, trees, wildflowers, shrubs, vines and grasses did not originate in our area, the New York Bioscape. These plants naturally occur in other countries, other continents, or even other states/regions within the United States.

In the past three hundred years, people have translocated thousands of plant species. Many were intentionally moved; chosen for their ornamentation and beauty, others were used for food, while some eased the homesickness of the traveler. Other plants arrived as stowaways, hidden in packing materials, soils or ship ballast. However they arrived, most of these exotic species have not been problematic, co-existing peacefully with our native flora.

Invasive plants. A small, but significant percentage has transgressed their ecological boundaries. These exotic species have escaped from where they were planted and infiltrated parks, meadows, forests and wetlands. Once here, they propagated wildly, overwhelming and crowding out native species. Such plants are considered invasive. In our area, invasive plants can be trees, shrubs, vines, graminoids (grasses, sedges and rushes) and forbs (aka wildflowers)*.
*The good news here is that New York City does not have any exotic or invasive ferns or fern allies.

As a major commercial port, New York City is also a major portal for invasives. Many problematic plants, insects and diseases have entered the country through our fair city. This also makes our region the vanguard of invasive management and controls.

Evolution. It is important to note that the process by which these invasive plants out compete our native flora is not evolution. For one thing, in this context, evolution happens over a very long, geologic time frame, i.e. thousands of years. This contest of “may the best plant win” is a fallacy. The proliferation of natives is restricted by natural checks and balances provided by the herbivores, diseases, parasites and other native plant competitors that are part of the complexities of any ecosystem. Invasives don’t have such baggage. They flourish in their new environment because they left their constraining factors back at home. Yet to lay blame at the invaders’ doorstep is unfair. There is no such thing as a “bad plant”. The culpability for the proliferation of invasive plants in the landscape lies squarely with us humans. Habitat degradation and destruction exacerbates the spread of invasives by creating more marginal habitat for them.

Invasive species are cause for concern because within a human lifetime, they flood into an area, displacing hundreds of acres of native vegetation. Populations of our indigenous flora then decline dramatically, and overall biodiversity (the number and types of living things) plummets with it. The explosion of exotic species corrodes the vitality of native plant populations, and thus undermines the health of our natural heritage.

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How do invasive plants imperil our indigenous flora?

After destruction of habitat, the biggest threat to our local flora is invasive species. These plants inevitably find their way into natural areas either through their own vegetative devices (seeds, runners) or with the help of an animal disperser (usually birds). Once there, they quickly become established and begin their offensive. Nonindigenous plants are currently invading our nation’s ecosystems at a rate of 1,729,737.7 acres per year! For example, English ivy (Helix hedera) originally planted as “ground cover” in short time will cover not only the ground, but also nearby trees, shrubs and wildflowers; the “living fence” of bamboo (Bambusa, Phyllostachys or Pseudosasa spp.) or Japanese knotweed (Polygonum cuspidatum) will pop up in your neighbors’ yards and their neighbors’ yards in monocultural stands no natives can penetrate. The Japanese honeysuckle (Lonicera japonica) fragrancing your yard will also smother our woodland springtime wildflowers (aka spring ephemerals). What we do on our property has ecological consequences for the greater landscape.

Click here to see illustrations of how our forests are under siege from invasive plants like English ivy.

Invasive plants tend to:

• Grow quickly • Spread through prolific seed or shoot production • Be generalists, able to tolerate a wide range of environmental conditions • Support other exotics – plants, animals and pathogens • Thrive in disturbed areas, such as roadsides and vacant lots • Compete with natives for pollinators • Lack biological controls to keep them in check, like the herbivores and diseases they left behind in their homelands • Reproduce with our native congeners (“of the same genus”), thus polluting the gene pools of our indigenous flora with their exotic bad habits; such hybrid offspring are no longer consider to be native • Have longer growing seasons • Suppress or kill growth of natives through shade or allelopathic toxins • Degrade habitats for insects, birds, amphibians and mammals • Interfere with natural processes like the availability of water and nutrients, life cycles of insects and plants, rates of soils erosion and frequency of wildfires

Invasive plants also seem to displace comparable congeners. For example, wild garlic (Allium canadense) is rare in New York City, but field garlic (Allium vineale) from Europe is a noxious invasive weed. Winged loosestrife (Lythrum alatum) is infrequently found in city marshes, whereas purple loosestrife (Lythrum salicaria) pokes its magenta inflorescence into wetlands everywhere. Not only is it one of the most invasive plants in the country, it is also popular in gardens and the horticultural trade.

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Where can I find a list of plants that are invasive in New York City?

You can find such a list on Drosera’s Plant List page.

Many of the plants on this list are common components of landscapes and gardens. As responsible stewards, we must revise how we view and use them. They are not inert backdrops to our barbecues. These plants are aggressively overrunning our naturals areas, smothering native flora, destroying animal habitat and disrupting ecological processes. If any of these plants are on your property, please consider removing them. And don’t worry; there are lovely native alternatives, which you will find here too.

Warning – be prepared. You will be shocked.

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What are some examples of invasive plants in the New York City area?

The 10 plants below are just the tip of the proverbial iceberg. *Click* on the name to see photos. They have many pernicious comrades. For comprehensive lists, see above.

N.B. The following were exported from foreign lands for the U.S. horticultural trade. Be wary of nursery folks who may tell you that they have an innocuous cultivar (a horticultural variety of a species) that is not invasive. Nine times out of ten that is simply not the case. Do not install these plants in your window box, garden or yard. If they are already on your property, please consider removing them.

Norway maple (Acer platanoides) Well-adapted to urban conditions, it is often planted as a street tree. Leaf out is earlier than our native maples, thus greatly reducing sunlight for our spring ephemerals. May be confused with our native, sugar maple (Acer saccharum), but native maples don’t have the white sap that oozes from broken leaves and stalks. Still used in the horticultural trade, especially the many “crimson” varieties. Sycamore maple (Acer pseudoplatanus) is equally problematic. Japanese barberry (Berberis thunbergii) This shrub is in front of everyone’s house. Thorny, deer-resistant and a prolific seeder, it forms large stands that crowd out native wildflowers and shrubs. In our area, it is invading fields and early successional woodlands. Available at a nursery near you. Oriental bittersweet (Celastrus orbiculata) Planted as an ornamental due to its gorgeous yellow-skinned fruits that open to show an inner scarlet aril in fall. Very common and aggressive, this woody vine strangles trees. Birds disperse the berries far and wide. Crafty folks who use this vine in dried arrangements are also responsible for its dispersal. Reproduces with our native, American bittersweet (Celastrus scandens), thus destroying its ability to produce true native offspring. The resultant hybrids carry traits from both parents, which makes the offspring not native. Our native bittersweet is, of course, increasingly rare. More on hybridization. Winged burning bush (Euonymus alata) Another Asian import. Widely planted for its fiery red leaves in fall. Prolific seeder, it forms dense thickets that greatly reduce native biodiversity. Highly invasive in the New York Bioscape, it is not much of a problem in the five boroughs…yet. Wintercreeper/emerald & gold (E. fortunei), the creeping, evergreen groundcover, is also problematic. The native congener, strawberry-bush (E. americanus) is rare in New York State. English ivy (Hedera helix) This is another plant that, much to our chagrin, is on everyone’s property. It is also all over our forests, carpeting the ground with its evergreen foliage that casts complete shade. No part of this plant is used by native wildlife for food or shelter. As if all that wasn’t bad enough, it is a carrier of a bacteria that harms a wide variety of trees. One of the most invasive plants around. Its horticultural comrade, Boston ivy (Parthenocissus tricuspidata) also hails from foreign shores. Common daylily (Hemerocallis fulva) A wildly popular ornamental with thousands of cultivars. Shade tolerant, this exotic lily spreads via roots, eventually smothering our native forest lilies. Once established, its thick, tuberous roots are difficult to eradicate. This nursery offers hundreds of daylily cultivars and boldly advertises having “native daylilies”. Native to where?? For your information, dear reader, this genus, Hemerocallis, is not native to our area or our country. We have no native daylilies! This is why most nurseries, sadly, are suspect. Note too the description given of the plant, it is an excellent example of its monopolizing ways. “Naturalizing” is a euphemism for “invasive”. Purple loosestrife (Lythrum salicaria) This European import has degraded wetlands through its large, monotypic stands that muscle out native plant species. Established populations persist for decades, are difficult to eradicate and continue to spread into new territories. One of the country’s most invasive wildflowers. And yet, it is still planted in front of many homes, the resident gardeners unable to resist its stunning magenta flowers. Japanese knotweed (Polygonum cuspidatum) Introduced in the late 1800s as an ornamental, we are still wondering why. It’s not that attractive, but boy is it everywhere. Bamboo-like, it grows to 10 feet tall and forms dense colonies from its root stock (rhizomes). Ubiquitous and aggressive, particularly along riversides, but also common along roadways. It is a pernicious weed, monopolizing the banks of the Bronx River, and many, many other locations. Once established, it is extremely difficult to eradicate, making this ‘Freckles’ cultivar an insidious purchase, despite its cutesy name. Lesser celandine (Ranunculus ficaria) Sadly, this plant has been cropping up in flower beds more and more. Its large, sunny yellow, buttercup flowers and glossy green leaves are very attractive. But resist temptation! As one of the first plants to bloom in early spring, it relegates our spring ephemerals to a dark and shady future under an impenetrable carpet of leaves. Periwinkle (Vinca minor & V. major) When a garden catalog announces, “Great as groundcover!”, you can be sure the plant is bad for the local ecology, which is the case with periwinkle. This vine forms a dense, evergreen mat that smothers native plants and keeps new prospects at bay. Not known to reproduce from seed but at the rate it is planted in yards, it has no need to.

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Where can I find out more about invasive plants and native alternatives?

At Drosera's resources page and our plant lists page.

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What can I do to stop the spread of invasives?

It’s in our hands. There must be a groundswell of interest from the plant-loving public to ban the sale of invasive plants and to use natives in their stead, because the incentive won’t come from the trade. Nurseries and landscapers are concerned about regulations that would restrict their plant palette. They do not support programs curbing the sale of invasive plants that are already wrecking havoc on our environment. The industry is willing to educate its customers about invasive plants and their alternatives, but from our years of experience, they don’t know enough about invasive ecology to do this effectively. For example, for natural area restorations we may request blackhaw shrubs (Viburnum prunifolium). When out of stock, some nurseries have offered instead other exotic viburnums (congeners), such as Siebold’s viburnum (Viburnum sielboldii). This shrub has been noted as increasingly invasive. For more information click here. Also, see common daylily (Hemerocallis fulva) example above.

•For information on how to rid the landscape of these nuisance species once they are established, see Weeds Gone Wild, IPANE, & TNC Weeds. The best way to fight them is to not allow them an initial foothold - see below.

Go native. The best way to stop the spread of invasives is to just say no. Remove all invasives from your property and flower pots. Buy indigenous plants from native plant nurseries and/or catalogs that you know to be responsible, having raised their plants from seed. (Beware of the “nursery grown” label, which means the plant was taken from the wild and then grown in a greenhouse). Share your newly gained knowledge with friends and family. Then proudly announce the arrival of natives with signs to educate others.

Be vigilant. Local, state and federal governments may keep a watchful eye on invasives, even going so far as to declare some unsuitable for the horticultural trade. But by the time that happens, the damage has already been done. We must be vigilant in our own gardens and replace inappropriate species with appropriate alternatives. Keep a watchful eye on exotic species that run rampant on your property. If they “jump the fence”, it’s time to pull the plug.

Know your Latin. When purchasing plants from growers and retailers, ask that plants be labeled with their scientific names to correctly identify what species you are getting. It helps to be prepared with an idea of what you would like to purchase, Latin name in hand.

Be informed. Many invasive plants are favored by the nursery and landscaping trades and so become more widespread. The majority of these tradespeople don’t understand the ecological ramifications of invasive plant use. Therefore it is incumbent upon us, the consumers, to educate ourselves before purchasing such plants. (They may also tell you that they have non-invasive versions [cultivars] of an invasive species…but don’t believe it! While very few cultivars may possibly be non-invasive, these have not been scientifically tested. It is not worth the risk!)

Ask for alternatives. Nurseries and plant purveyors are responsive to requests for native plants species offerings. Once the public begins to demand indigenous options for their gardens, we will soon see natives popping up all over.

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