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Invasive Plants: Invasive Plants

Iris pseudacorus (yellow flag iris); photo courtesy of Flickr cc/ Katja Schulz
Iris pseudacorus (yellow flag iris); photo courtesy of Flickr cc/ Katja Schulz

What is an invasive plant?

An invasive plant is one that takes over a local habitat and alters the existing ecosystem. These plants damage or crowd out native species, forcing wildlife to adapt (often unsuccessfully) to the altered surroundings. In the worst case, the habitats may turn into a barren monoculture. This sounds like a drastic scenario, but when you begin to notice severe infestations, you can see how large areas of land can be profoundly altered by an invasive plant.

One easy-to-see example is purple loosestrife (Lythrum salicaria), which has taken over many wetland areas of the Northeast. You can spot it all over: in roadside ditches, along streams and ponds, and covering many wet areas. What was once a pretty ornamental plant with tall spikes of vivid, magenta flowers has become an aggressive thug. It still looks pretty, but purple loosestrife has choked out native, wetland plants that once provided food, shelter and nesting sites for many native and migrating birds and other wildlife.

Most invasive plants are exotic introductions (non-native plants) that entered this country either deliberately or accidentally and do not pose environmental problems. But there are a few hundred invasive plants that threaten native plants and habitats that could cost the government millions of dollars to control or eradicate.

Common Traits of Invasive Plants

  • copious fruits and seeds which are efficiently dispersed
  • effective, speedy establishment and growth
  • crowding out of native plants

Some of the Top Invasive Plants in the NYC Region

Botanical Name

Common Name

Acer platanoides
Norway Maple
Aegopodium podagraria
goutweed
Ailanthus altissima
tree of heaven
Alliaria petiolata
garlic mustard
Ampelopsis brevipendunculata
porcelain berry
Berberis thunbergii
Japanese barberry
Celastrus orbiculatus
Oriental bittersweet
Eleagnus umbellata
autumn olive
Euonymus alatus
burning bush, winged euonymus
Iris pseudocorus
yellow flag iris
Lonicera japonica, L. morrowii, L. tatarica
honeysuckles
Lythrum salicaria
purple loosestrife
Phragmites australis
common reed grass
Polygonum perfoilatum
mile-a-minute
Reynoutria japonica
Japanese knotweed
Rhamnus cathartica
common buckthorn
Rosa multiflora
multiflora rose

 

Some of these are still sold in nurseries because they have attractive characteristics. Do Not Buy Them!  And please discourage others from buying and planting them.

Ailanthus altissima (Tree of Heaven) spreads aggressively via wind blown seed; photo courtesy of Flickr cc/ Wayne National Forest
Ailanthus altissima (Tree of Heaven) spreads aggressively via wind blown seed; photo courtesy of Flickr cc/ Wayne National Forest

Tackling Invasive Plants in Your Home Garden

  • The best way to tackle perennial or annual thugs is to pull them out by hand. Often it will take several years of weeding to eradicate the problem; viable seeds will remain in the ground and some invasive species (such as garlic mustard) are biennials that operate on a 2-year life cycle.
  • A few important tips for hand weeding: pull out plants just prior to flowering when they are most vulnerable (all their energy has gone into flowering). Try to remove the entire root system. Try not to disturb the soil around the plant, which will only encourage re-seeding.
  • For larger plants (saplings and shrubs) use large tools such as a weed wrench that will pry the plant out of the soil. Otherwise, try girdling the stem by cutting a 2-inch strip around the base of the stem.
  • Pour boiling water on the root system of plants that are in hard-to-reach places or try organic products such as Burnout™.
  • Frequent mowing of areas that are covered in weeds will help control problems. For woodland areas, try using a weed whip; just be careful not to damage the bark on neighboring trees. 

Remember, when you are eradicating invasive plants from your property try not to be too overzealous and pull out natives along with them. After you remove an invasive, remember to either mulch or re-plant the area immediately so that it doesn't get re-colonized.

If you are planning on reseeding or planting the area, then do so as soon as possible, from several days to a week after removal. Plant thickly to fill in the empty space and to make it difficult for the invasive to reestablish itself.

Difficult areas can be filled with native plants that are effective colonizers. One example for a shady wooded area is hay scented fern (Dennstaedtia punctilobula), which is unstoppable and will rapidly cover an exposed area. Be sure when making these decisions that you are not replacing one problem with another.

If you are not planting immediately, fill the space with a nurse crop, a short-lived crop that can smother out potential problems. Such cover crops are planted to prevent erosion, reduce weed infestation and, in the case of leguminous crops, to fix nitrogen in the soil. A mix of hairy vetch and winter rye is a good late season combination. If you are clearing an area in the summer, buckwheat is an excellent warm weather cover crop that can be turned into the soil at the end of the season.

If you are reseeding a lawn make sure to use a seed mix that includes some species of grasses that establish quickly such as perennial ryegrass in the fall.

Below is a list of some native alternatives to invasive plants. You can also substitute  non-invasive, garden-friendly, exotic plants in your garden. For example, if you are enamored of the stately yellow flag iris (Iris pseudacorus), try planting the moisture-loving Japanese iris (Iris ensata) or the moisture-tolerant Siberian iris (Iris sibirica). These last two are not native, but they don't spread out into the landscape. Or try our beautiful, native, blue flag iris (Iris versicolor). One of the merits of planting natives is that they attract local wildlife (birds, butterflies and bees) to your garden.

Native Alternatives to Invasive Plants

 

For Acer platanoides (Norway maple)
Acer rubrum (red maple), Acer saccharum (sugar maple)
For Berberis thunbergii (Japanese barberry)
Clethra alnifolia (summersweet), Cornus sericea (red twig dogwood), Itea virginica (Virginia sweetspire), Myrica pensylvanica (bayberry)
For Eleagnus umbellata (autumn olive)
Amelanchier arborea (serviceberry), Aronia arbutifolia (chokeberry), Fothergilla major (large fothergilla), Hamamelis vernalis (witch hazel), Ilex verticillata (winterberry), Lindera benzoin (spicebush)
For Euonyus alatus (winged euonymus or burning bush)
Fothergilla gardenii (dwarf fothergilla), Vaccinium corymbosum (highbush blueberry), Viburnum dentatum (arrowwood)
For Lythrum salicaria (purple loosestrife)
Agastache foeniculum (blue giant hyssop), Asclepias incarnata (swamp milkweed), Echinacea purpurea (purple coneflower), Eutrochium purpureum (Joe Pye weed), Monarda didyma (beebalm)

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