Quick answer: Invasive plants spread from roots and fragments, so mowing or cutting alone usually makes them denser. Effective control means correct identification, treatment timed to when the plant sends energy to its roots, follow up for more than one season, and replanting so the space does not reopen.
What makes a plant invasive?
An invasive plant is a non-native species that spreads aggressively and causes harm: crowding out native plants, damaging habitat, blocking drainage or undermining structures. New York maintains a list of prohibited and regulated invasive species that cannot be sold or intentionally spread.
Which invasive plants are common on Long Island?
- Phragmites (common reed). Tall, feathery topped reed in dense stands along ponds, marshes and roadside ditches.
- Japanese knotweed. Bamboo-like canes with heart shaped leaves. Can push through asphalt and foundations.
- Mugwort. Spreads by underground rhizomes through beds, lawn edges and vacant ground.
- Mile-a-minute vine. Fast growing vine with triangular leaves that smothers other plants.
- Oriental bittersweet and porcelain berry. Woody vines that climb and strangle trees.
- Tree-of-heaven. Fast growing tree that resprouts heavily when cut.
- Aquatic invaders. Hydrilla, Eurasian watermilfoil, fanwort and water chestnut in ponds and lakes.
Why does cutting not work?
Most of these plants store energy below ground. Cut the top and the root system responds with more shoots. Knotweed and several aquatic plants can also regrow from small fragments, so mowing or pulling can spread them to new spots.
What does a real control plan look like?
- Identify and map. Confirm the species and how far it extends.
- Check the site. Wetlands, water and property lines affect which methods and permits apply.
- Time the treatment. For perennials such as knotweed and phragmites, late summer into fall is typically most effective, when plants move sugars down to the roots.
- Follow up. Expect two to three seasons of monitoring and re-treatment for established stands.
- Dispose carefully. Bag and dispose of plant material so it does not root elsewhere.
- Replant. Native plants or turf close the gap.
Who needs invasive control?
Homeowners losing a yard edge or pond view, HOAs with common areas and basins, and commercial sites where invasives overrun fence lines and drainage. Early action costs far less than removing an established stand.
Related: Wetland vegetation management | Commercial weed control
This article is general information. Work in or near wetlands and water may require permits. Pesticides must always be applied according to the product label and New York regulations.