by Aina R Ferris, The Native Explorer co-founder
Let's get one thing clear before we begin. Not all non-native plants fall under the "invasive" category.
A quote from the USDA Forest Service says, "An estimated 5,000 non-native plant species have been introduced, established, and now exist in US ecosystems. Most of these species are not invasive and do not cause significant harm." * For example, did you know that tomato, wheat, and rice are not native to the US? Yet they aren't considered invasive plants.† The USDA has a list of plant-related definitions that should be helpful in identifying native versus non-native, versus noxious plant species.
Sadly, the Forest Service reports that invasive species have contributed to the decline of 42% of US endangered and threatened species. Invasives were the main culprit in the decline of 18% of US endangered or threatened species.‡
Like thugs at a line up, the following are the Top Six offenders pointed out by the Smithsonian Institute§ as invasive non-natives wreaking havoc all across the US:
Purple Loosestrife (Lythrum salicaria)
It is native to Europe and temperate Asia but was introduced in the United States in the1800s for ornamental and medicinal purposes. Found predominantly in wetlands, it is estimated that in just one year, about 2 million seeds are spread by the wind. The plant's underground stems grow approximately 1 foot long per annum.
Japanese Honeysuckle (Lonicera japonica)
Originating in Eastern Asia, this species of honeysuckle was intentionally shipped to Long Island, NY in 1806 for erosion control and ornamental use. Migratory and local birds consume its fruits and spread seeds through their droppings. Despite climate variation per state, Japanese Honeysuckle is highly adaptive to an array of conditions.
Japanese Barberry (Berberis thunbergii)
Settlers brought the European barberry to the US and used its berries for making wine and jams. Its sturdy wood was used to create tool handles. Unfortunately this species became prone to a fungus called, "black stem rust".¶ As its name suggests, Japanese Barberry originated in Japan. Its seeds however, made their way from Russia to the Arnold Arboretum in 1875, and has since replaced its European cousin.
Norway Maple (Acer platanoides)
Plant explorer, John Bartram introduced the US colonists to this tree in 1756. He probably didn't realize it would compete with the native Sugar Maple and Red Maple trees. The Norway Maple's helicopter seeds have spread far and wide. It has the potential to dominate the Northeast and Northwest parts of the country.
English Ivy (Hedera helix)
In the 1700s, the colonists desired a quick-spreading evergreen ground cover. Hence the importation of the English Ivy (which is still sold today!) This aggressive and wide-spreading vine kills trees by smothering them and limiting the amount of sunlight they receive. Its berries are consumed by birds that inadvertently spread the seeds through their droppings.
Kudzu (Pueraria montana var. lobata)
Soil erosion and deforestation were the reasons why the Soil Conservation Service allotted one million acres of US land to planting ornamental Kudzu between the 1930s and 1940s. In the Southeastern states, this plant (which originated in Japan) was also used as foraging crop. Its spread became so aggressive that vines climbed trees and smothered them. Dense clusters have also reportedly toppled trees. Shockingly, once established in an area, Kudzu grows at a rate of up to one foot a day and 60 feet annually.
In my home state, the Missouri Invasive Plant Council (MoIp) has Callery Pear (Pyrus calleryana) in its crosshairs as the No. 1 vigorously invasive species. Hover and click the MoIp link for a list of the 25 most invasive plants found state-wide.
Why are invasive non-native plants so successful? ‡
Many invasive plant species produce large quantities of seed.
Many invasives thrive on disturbed soil.
Invasive plant seeds are often distributed by birds, wind, or unknowingly humans allowing seed to moving great distances.
Some invasives have aggressive root systems that spread long distances from a single plant.
These root systems often grow so densely that they smother the root systems of surrounding vegetation.
Some plant species produce chemicals in their leaves or root systems which inhibit the growth of other plants around them.
What threat do invasives pose where I live?
Invasive species compete directly with native species for moisture, sunlight, nutrients, and space.
Overall plant diversity can be decreased
Establishment and spread of invasive species can degrade wildlife habitat
Results in poor quality agriculture lands
Degraded water quality
Increased soil erosion
Decreased recreation opportunities
These impacts change nature's balance on which all species depend.
How can I prevent the spread of invasive non-native plants where I live?
Do not pick the flowers of noxious weeds and take them home.
Drive only on established roads and trails away from weed infested areas.
When using pack animals, carry only feed that is certified weed free.
Remove weed seeds from pack animals.
Don't pick and transport wild flowers that you can't identify.
Do not camp or drive in weed infested areas.
When is a good season to start helping with invasives prevention?
This season and the seasons thereafter.
References:
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