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.
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.
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.