Invasive trees are non-native trees that have caused or are expected to cause harm to the environment, economy, or human health. Native species struggle to compete because they did not evolve alongside these plants. Without natural predators and very little competition for nutrients, alien species have the potential to displace native flora.
Many of these species were planted in gardens and landscapes but have subsequently spread. Invasive plants are prolific seeders, vigorous growers, and adaptable to a wide range of conditions.
Invasive species threaten biodiversity, cultural resources, property values, public safety, and infrastructure if they are not controlled.
1. Norway Maple
Norway maple is a non-native ornamental tree that has the potential to overtake surrounding woods. In woodlands, this shade-tolerant tree outcompetes the native sugar maple.
Norway maples have shallow roots and a wide shade area, so grass and other plants struggle to thrive beneath them. These trees have replaced native maples and other trees in many New York neighborhoods.
Because of their color variation, Norway maples are popular street and yard trees. Green, purple, and variegated leaf cultivars are available.
These trees can withstand poor growing conditions like compacted, low-quality soils and air pollution. They are prolific seed producers and have already established themselves in woods and forest edges.
2. Tree of Heaven
Tree of heaven was initially planted as a decorative tree before being used for medicinal reasons. They are now regarded as destructive root suckers.
The wind-dispersed samaras germinate and swallow up space as the trees grow into a large thicket. Their highly invasive root systems destroy sewers, sidewalks, and foundations. The trees release toxins into the soil that inhibit the growth of other plant species.
Trees of paradise can be spotted growing in cracks and fissures in stone or cement patios, sidewalks, near building foundations and bridges, and through stone walls. They are commonly seen as out-of-place trees in cities and towns.
This tree can reach heights of more than 80 feet. The leaves and flowers of the tree of heaven are tall and pinnate.
3. Persian Silk Tree
Persian silk trees or mimosas have many beautiful qualities, which have made them very popular trees to use in landscapes. However, as an invasive species, there are many reasons not to plant them.
Silk trees spread rapidly by seed and vegetative reproduction, displacing native trees and shrubs. They are challenging to eliminate because they produce long-lasting seeds and tend to resprout easily. Their allelopathic capabilities provide a defense mechanism that keeps other plants from growing nearby.
Mimosa branches are easily damaged because the wood is fragile and brittle. In addition to breaking, the tree develops vascular wilt and webworm, accelerating its deterioration and shortening its lifespan.
4. Russian Olive
Russian olive was grown as an ornamental tree in the late 1800s. Its fast growth and resistance to erosion led to its introduction in North America.
Although many gardeners find its fruit and fragrance pleasing, it is now considered invasive. Russian olive chokes out and outperforms native plants.
The tree reproduces primarily by seed from small cherry-like fruits consumed and dispersed by birds. Its thorns and aggressive regeneration make it hard to eliminate.
5. Glossy Buckthorn
Glossy buckthorns thrive in many environments, including woodlands, forest borders, open fields, and riparian areas. They grow quickly and produce large quantities of seeds.
Glossy buckthorn spreads mainly through seeds, which humans or birds disperse. They grow into thickets, shading young trees and displacing native understory species. Most fruits fall near the tree, creating a dense understory of seedlings.
Glossy buckthorn was introduced as a decorative plant. It is a small deciduous tree or shrub that can grow 30 feet tall. Their leaves’ rapid decay alters the soil’s pH and chemical composition. They also leaf out earlier and keep their leaves longer than native plants.
Some native buckthorns are significantly less aggressive and smaller than the introduced varieties.
6. Black Locust
Black Locust is indigenous to the southern regions of the United States. However, in other parts of the country, it is considered invasive. It is a quick-growing tree that can reach heights of 40 to 100 feet.
These trees will quickly spread after being introduced to a new region. As they grow, they will form dense canopies that will shade out any vegetation beneath them.
Black locusts are most commonly found in abandoned fields, prairies, woodlands, stream banks, and roadside ditches. Because of their extensive root system, they can produce multiple clones in a single location.
In late May and early June, black Locust trees produce large, irregularly shaped crowns covered in white-pink flowers. Because it is a legume, this tree can thrive and grow efficiently in nitrogen-deficient soils.
