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HomeDanger: Invasive Plants Are Not "Just Weeds"

Danger: Invasive Plants Are Not "Just Weeds"

Contributed by Joanne Lavengood, Master Gardener

 
 

"Weeds are flowers too, once you get to know them". This quote by A.A. Milne came to mind when I was asked to write an article on invasive plants. I have always subscribed to the notion that weeds are simply the wrong plant in the wrong place. Most of us know that weeds are opportunists and will invade our lawns and gardens for their great growing conditions if we don't stay ahead of them. If this is true, then is there really a difference between invasive plants and weeds?

 

Quite simply, yes, and the difference is startling.

 

The Massachusetts Invasive Plant Advisory Group (MIPAG) defines invasive plants as "non-native species that have spread into native or minimally managed plant systems in Massachusetts, causing economic or environmental harm by developing self-sustaining populations and becoming dominant and/or disruptive to those systems." Weeds, on the other hand, are usually thought of as native and non-native plants that cause problems with crops in commercial and home gardens, and have a negative aesthetic impact on our lawns and landscaping. They are not usually invasive.

The term "noxious" is a legal designation used specifically for any plant species that have been determined to be major threat to natural areas. They are listed on a noxious weed list and regulated by the USDA. Massachusetts has a list of plants that are not allowed to be sold, traded, purchased, and distributed or any other related activities that include their cultivars, varieties and hybrids. This list is available on the web: http://www.mass.gov/eea/agencies/agr/farm-products/plants/massachusetts-prohibited-plant-list.html. These plants, outside their native environment are really, just that bad.

 

 Four comparisons can be made to differentiate between agricultural or garden weeds, and invasive plants which include noxious weeds.

  

1. Introduction: weeds are usually introduced accidentally by people, animals, equipment or seed contamination. Invasive plants can be introduced accidentally, but in the past were introduced intentionally for ornamental or aquarium use, forage, food, fiber, medicinal or soil stabilization purposes. Ironically, some invasive plants were introduced because they were not only ornamental, but were easy to grow.

  
2. Disturbance: weeds require human disturbance to become established and thrive. Invasive plants benefit from human disturbance, but it is not essential, and they can take over stable native landscapes.

  

3. Persistence: Weeds will not persist without the required human disturbance, which is usually accomplished through soil tillage or irrigation. Invasive plants, once they are introduced, will survive and spread on their own without any form of assistance whatsoever.  

  

4. Life form: Weeds are primarily annuals or herbaceous perennial species. They are rarely aquatic. Invasive plants include all growth forms, including aquatic, vines, parasitic, herbaceous or woody plants. They have annual, biennial, or perennial life cycles.

  

The most important biological difference between invasive plants and garden weeds is the ability of invasive plants to disperse, establish, and spread without human assistance or disturbance.   

 

We all have heard about the environmental dangers of these plants, but their true impact on our natural landscapes and ecosystems is rarely grasped. In fact, they are one of several reasons that are driving some of our rarest plants and animals closer to extinction. Our ecosystem is carefully balanced, and all forms of life within the ecosystem are affected when an invasive plant comes for a visit from its botanical home. The invasive plant brings along things such as adaptability, aggressive root growth, quick germination, seed longevity, aggressive canopy coverage and chemical warfare called allelopathy (the release of a biochemical, known as allelochemicals which reduces seed germination and seedling growth on susceptible plants).   

 

One reason these plants happily move in for a permanent visit is because they don't have any natural enemies. Back in their own native ecosystems, they were part of a community of living and nonliving things that all worked together. Each species had their own niche. The checks and balances were in place and everything and everybody was happy. When a plant is moved for one or more of the reasons listed above, it is placed in another ecosystem that is foreign to it. If the plant doesn't have invasive, thug-like tendencies, it will either survive and behave itself, or die. This plant, although not native will not be found on an invasive species list. 
 
The damage that invasives do when they discover they have landed in an ecosystem without natural enemies is subtle....at first. Since they are adaptable, and do not have little holding them back, the plant with invasive characteristics will do what any thug would do, and that is take over the place, gradually displacing slower growing plants. Native vegetation is suppressed or killed (allelopathy), which in turn affects our wildlife that is dependent on the native species. Waterways become clogged with several invasive species of milfoil and others affecting aquatic life, vines grow into a dense, tangled mats of vegetation that are thick enough to kill the herbaceous vegetation underneath, which local insects, birds and animals rely on for food and shelter. Insect and bird disruption interferes with the natural seed dispersal of native plants and native plant-pollinator relationships. The mile-a-minute vine (Polygonum perfoliatum), in addition to growing into the dense tangled mat of vegetation, also has allelopathic qualities, not common in all plant species. Another vine, Oriental Bittersweet (Celastrus orbiculatus), originally grown as an ornamental, will girdle shrubs and trees. Some plants can be parasitic, living off a host plant and eventually killing it. Dodder (cuscuta spp.), is one example and is listed on the Massachusetts Prohibited Plant List. Finally, Massachusetts has many more herbaceous and woody invasives which in addition to being on the Prohibited Plant List, can be found on MIPAG's website: www.massnrc.org/mipag. Some of the more familiar ones are Japanese Knotweed (Polygonum cuspidatum), Purple Loosestrife (Lythrum salicaria L), Japanese Barberry (Berberis thunbergii), and trees such as the Norway Maple (Acer platanoides). These plants form dense thickets that exclude native plants.

 
Cow Parsnip
Cow parsnip
 Purple Loosestrife
Purple loosestrife
Japanese knotweed in bloom
Japanese knotweed in bloom

 

Prevention, Education, and Management are the three main components of invasive plant control.

  
1. Prevention involves keeping potential invasives from entering an area and becoming established.

  
2. Education and public awareness, both of which Master Gardeners can do, will ensure the long-term success of efforts at prevention. Human behavior is the primary cause of both intentional and unintentional invasive species spread, and well-publicized outreach and education is important to encourage the adoption of preventative measures by the public.

  
3. Management involves two approaches, "weed-based" and "site based". Weed-based management focuses on a particular species and is suitable for new invasions and small populations of species that have proven control methods. Some of these include hand weeding on a regular schedule, integrated pest management, and biological control. We do this on our own properties. Site-based management focuses on managing populations in specific areas where there is a feature we want to protect, for example, a rare or endangered species, protecting a large undisturbed forested area, historical and cultural sites, scenic sites and human health. Once a weed-based or site-based management approach is decided on, an "integrated vegetation management" is recommended by MIPAG. The large scale effort required to manage an established area is impossible due to time, funds, staff and effective and practical management methods. Integrated vegetation management incorporates an ecosystem based on a strategy with the long term goal of reducing levels of invasive species and preventing new invasions. Some of the techniques used in combination include habitat manipulation, mechanical removal, chemical control and cultural practices in addition to the biological control and manual controls done for weed-based management. We can also identify and assist native species with increased abilities to compete with and live alongside the invasive species. This may result in a new mixed community of native and non-native species, but desirable ecosystem functions and properties will then be possible.


Gardeners need to be alert and find out what plants are invasive in their region. What is not invasive in the Midwest, may very well be invasive here in Massachusetts and elsewhere in New England. Everyone needs to be responsible and be involved by knowing which plants are harmful, utilizing native plants in their landscape, educating others, and by doing their part in controlling invasive plants in their area.

 

Rose hips of multifloral rose
Multifloral rose
 
Honeysuckle
Honeysuckle
Bittersweet Bittersweet

 
The populations of many native plants have been greatly reduced by the destruction of many millions of acres of natural habitat due to human encroachment. Invasions of non-native plants are the second greatest threat to native species after direct habitat destruction. Clearly, invasive plants are not "just weeds".  
 
Further reading:  
 
Making Room for Native Plants and Wildlife - A guide to Invasive Species in the Mill River Watershed. A guide by the New England Wild Flower Society. 
 
Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens - Douglas W. Tallamy 
 
Resources: 
 
Common Invasive Plants of Massachusetts - Ted Elliman, New England Wild Flower Society, 2008 
 
Lady Bird Johnson Wildlife Center 
 
Invasive Plants by Todd Meier- Fine Gardening issue 65 
 
Strategic Recommendations for Managing Invasive Plants in Massachusetts - Massachusetts Invasive Plant Advisory Group Final Report February 28, 2005 
 
Guidance for the Effective Management of Invasive Plants - Massachusetts Invasive Plant Advisory Group (MIPAG) Version 2 December 2012 
 
Common Weeds of the Yard and Garden a guidebook - Utah State University cooperative extension Brenda Jarvis Lowry, et al 
 
Allelopathy: How Plants Suppress Other Plants - University of Florida IFAS Extension James J. Ferguson, et al 
 
Invasive Plants University of California IPM - Published 11/07 
 
Weeds Gone Wild - htt://WWW.nps.gov/p[lants/alien Plant Conservation Alliance, Alien Plant Working Group 
 
Invasive Plants - Mass Audubon: www.massaudubon.org/learn/nature/invasiveplants 
 
Attack of the Alien Invaders Invasive Species, Invasive Plants, Invasive Animals - National Geographic 
 
Massachusetts Prohibited Plant List:httpL//www.mass.gov/eea/agencies/agr/farm-products/plants/massachusetts-prohibited-plant-list.html 
 
Massachusetts State Noxious Weeds List USDA PLANTS http://plants.usds.gov/java/noxious?rptType=State&statefips=25 
 
Massachusetts Invasive Plant Advisory Group - htp://www.massnrc.org/mipag/speciesreviewed_category.htm 
 
Massachusetts Invasive Plans Advisory Group - htt://www.massnrc.org/mipag/notmeetingcriteria.htm 
 
Massachusetts Invasive Plant Advisory Group - http://www.massnrc.org/mipag/index.htm

 
Photos by Larri Cochran

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