Five Invasive Plants You Should Never Grow
Invasive plants are plants that disrupt and take over natural habitats.
They include both introduced species or native ones that grow rampantly. They can be threatening anywhere, but are most dangerous in wetlands, fire-prone areas, dunes, and areas where rare plants are found.
Invasive plants tend to become established and to thrive where a natural ecosystem is disrupted, such as a construction site, cultivated areas, or after a forest fire.
In some national parks, visitors are restricted because even foot traffic can destroy native plants and open an area to these insidious plants.
Where Do Invasive Plants Come From?
Plants from other parts of the world are generally welcomed into our gardens.
It is when these plants cause serious ecological disturbances as they 'escape' that a problem is caused. Certain fast-growing invasive plants can choke out native plant life, reducing biodiversity and altering habitats for all life, including native plant species and insects, birds, and animals that depended on the native plants.
Many of these plants arrive by accident in agricultural shipments.
Other plants have been introduced because of their beauty or some other horticultural attribute such as drought-tolerance, fast growth or hardiness.
Because of their natural adaptations, these invading plants are hard to banish from the garden even after they are known to spread rampantly. They can take over huge areas of roadsides and forests, overwhelming natural species.
What Are Invasive Species?
Invasive plants are ones that:
- Produce many new plants each season.
- Tolerate many types of soil and weather.
- Spread easily, usually by wind, water or animals.
- Grow rapidly, displacing slower growing native plants.
- Spread rampantly when free of the natural checks in their native area.
Most invasive plants are ones that have been introduced to an ecosystem where they are not native. Five plants that are classed as invasive in North America (and in other parts of the world) are purple loosestrife, oxeye daisy, kudzu, Scotch broom, and English ivy.
Purple loosestrife, native to Europe, was carried to North America by settlers as a pretty addition to their flower gardens. It also was inadvertently transported as seeds in soil used as ballast in ships.
Originally intended as a flower in North American gardens, but because of its aggressive growth, it escaped into waterways and roadsides. Purple loosestrife takes root in marshes, spreading quickly and choking out native species.
It can block water flow in ditches and irrigation canals. It's now classified as a noxious weed in some provinces and states, which means it's illegal to sell or own.
Since it was introduced, purple loosestrife has spread westward and can be found across much of Canada and the United States.
This article shows how to identify, control and remove purple loosestrife.
The Oxeye Daisy
The oxeye daisy is native to Europe and Asia.
It is a rhizomatous perennial, and aggressively invades fields, where it quickly forms dense populations, crowding out native grasses. It can grow to 1 meter in height, its coarse lower leaves forming a mat.
The tall stems hold daisy-like flowers which form hundreds of seeds at maturity.
As it invades pastures and croplands, crop yield drops dramatically. Grazing animals tend to avoid it because of its bitter taste and unpleasant odor.
It is often confused with the ornamental Shasta daisy, which is a larger more robust plant, with strap-like leaves. The oxeye daisy is considered an aggressive weed in forty countries.
The US Department of Agriculture has published an informational pdf describing in detail the problems caused by this invader, and how it can be removed.
Kudzu: the Vine That Ate the South
Kudzu is referred to as the mile-a-minute vine, and the vine that ate the South.
As you travel through the south-eastern states, huge mounds of green are kudzu covered trees, fences, and abandoned buildings.
Introduced to the United States in 1876 by the Japanese at the Centennial Exposition. it's large leaves and sweet-smelling blooms attracted American gardeners. They adopted the plant as an ornamental. It was also promoted as forage for animals, and plants were sold through the mail.
During the depression, it was valued for erosion control and planted widely. However, because of the suitable climate in the southern states, the vines were soon out of control, growing as much as a foot a day, climbing trees, power poles, and anything else they could contact.
Kudzu can grow up to sixty feet a year, and eventually kills trees by blocking the sunlight. Many herbicides seem to have little effect and those that do need application over several years to be effective.
Kudzu is a perennial, so grows for many years. It spreads by producing seeds as well as rooting where the nodes contact the soil. The roots can grow to a depth of three feet, and be several inches in diameter.
If you're a victim of this aggressive vine, here's an article on how to kill kudzu.
Scotch broom is native to the Mediterranean areas of Europe. It was intentionally introduced into the farm of Walter Grant in 1850, on Vancouver Island.
This fast-spreading shrub with its sulphur-yellow blossoms quickly spread up the east coast of Vancouver Island, then invaded the Gulf Islands and onto the mainland along the West Coast.
It was encouraged as a bank stabilizer by highway departments because of its rapid growth and deep roots. It is a strong competitor to various plants such as Garry oak and reforested conifers; and competes aggressively for light, moisture, and nutrients on any disturbed sites.
Each of the multitudes of blossoms produces from one to several seeds, which are ejected forcefully as they ripen and split open. The seeds can remain live for years, making total eradication very difficult.
The Island Conservancy has published information on how to remove broom plants, how to dispose of them safely and how to restore the sites.
Ivy can be a useful addition to a garden or landscape - as long as it is kept under control. It is a vigorous vine, growing both along the ground and climbing vertically. However, when it escapes to forested or uncultivated areas, it will climb trees, searching for more sun.
Ivy kills the support tree's branches by blocking sunlight from the tree's leaves. The host tree will often die completely from this continual aggression and weakening. In addition, the added weight of the vines makes the infested and often dead tree more likely to blow down during storms or with heavy snowfalls.
On the ground, ivy forms a dense mat, killing any native plants it covers. It is also a carrier of the plant pathogen Xylella fastidiosa (bacterial leaf scorch), which is harmful to elms, oaks, and maples.
The leaves and berries of English ivy are dangerous when ingested, as they contain glycoside hederin. Symptoms include diarrhea, stomach upset, breathing difficulty, coma, muscular weakness, and hyperactivity.
Take care when removing ivy from trees by using long sleeves and work gloves. Do not try to pull down long vines from trees, as dead branches may come with it. Instead, use loppers to cut the vines at shoulder height, and remove all the growth below the cuts and the roots from the ground around the tree. The remaining growth up the tree will die, but unfortunately, the damage has been done and the tree may have to be removed.
More information on how to control English Ivy can be found in this publication from the NC State Extension department.