Invasive plants are plants that disrupt and take over natural habitats.
They may be introduced species or native ones that grow rampantly. They can be threatening anywhere, but most dangerous in wetlands, fire-prone areas, dunes and areas where rare plants are found.
Invasive plants tend to thrive where a natural ecosystem is disrupted, such as a construction site 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.
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 adaptations, these plants are hard to banish from the garden, even after they are known to be invasive. They can take over huge areas of roadsides and forests, overwhelming natural species.
What is an 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. 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 originated in Europe and was carried to North America by settlers for their flower gardens, and as seeds in soil used as ballast in ships.
It was planted as a flower in North American gardens, but because of its aggressive growth, it escaped into waterways and roadsides. It takes root in marshes, reproducing 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.
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, with 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 is decreased. 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 agressive weed in 40 countries.
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.
It was introduced to the U.S. in 1876 by the Japanese at the Centennial Exposition. It's large leaves and sweet-smelling blooms attracted American gardeners, who adopted the plant as an ornamental. It was 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.
It 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, 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.
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 multitude 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.
Ivy can be a good 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 vertically. However, when it escapes to forested or uncultivated areas, it can climb trees, searching for more sun.
Ivy kills the support tree's branches by blocking sun from the tree's leaves. The host tree will often die completely from this continual agression and weakening. The added weight of the vines makes the infested 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 the growth below the cuts and from the ground around the tree.
- Ecology and Control of Oxeye Daisy
- Purple Loosestrife: What you should know, what you can do | Aquatic Invasive Species | Minnesota Sea
Scotch broom costs the state of Oregon about $47 million each year by its impact on natural resources, particularly on timber production.
- Ivyout: How to remove ivy
Tips and techniques to help you remove ivy.
- How to Kill Kudzu | eHow.com
How to Kill Kudzu. The kudzu is an extremely aggressive vine that grows at a rate of almost a foot a day and quickly encompasses an ever-expanding area, killing everything as it goes. Once promoted to stop soil...