The story of animal, insect and plant invaders is as old as the world itself. When humankind first began to roam, these beings tagged along. In strange new lands, the human instinct to collect unique specimens was born. So bits and pieces of non-natives came back with them – if indeed, these peoples ever returned to their points of origin.
From the tea-horse trade route in ancient Tibet to the Silk Road trade across Asia, Russia, Arabia and Africa into Europe, humans have moved about in search of economic opportunity. Certain plants and animals have been successfully raised far from their native regions when there were similarities in soil and climate: cotton, indigo, tobacco, rice, silk and wool come to mind along with all our domesticated animals. Other peoples moved to find religious freedom, escape tyrannical regimes, or just to have more living space. Whether intentionally or inadvertently, plants, animals and insects came with them.
By the eighteenth century, the quest began to find plants with beauty and ornament, hardiness, resiliance, disease resistance or unusual form or color. Collectors such as our founding fathers, George Washington and Thomas Jefferson, continually sought these qualities in their garden plants while keeping an eye out for species with economic or useful value to the new country. The Dutch collected bulbs from Turkey. The Victorians sent plant explorers around the world to bring back new curiosities. Such searching continues in the field of horticulture with modern day plant explorers like Dan Hinckley and Darrell Probst, as well as woody plant selector and hybridizer Dr. Michael Dirr.
Seeds have several ways to travel: they can float, stick, or blow. They can be eaten by animals like birds, pass through their digestive systems, and germinate in place. Bits of some roots or stems can grow readily if cut on purpose or acidentally. Now that we move about ever more widely in the 21st century, the movement of invaders has quickened. Natural controls in the land of origin, such as insect predators, host-specific funguses or herbivores, do not always accompany the invading plants. It may take eons for such controls to “catch up.” Meanwhile, our own native plants have evolved in place with their own control systems intact. And they may not be able to withstand the quickly moving, overpowering characteristics of many invaders.
Bringing the taste for non-native plants home to our own Northeastern United States:
• David Fairchild, plant explorer extraordinaire for the USDA in the early 1900s, had trouble propagating kudzu from cuttings but then had overwhelming success with seeds (he also collected what turned out to be many economically and culturally successful crops such as oranges and avocados)
• E. H. Wilson, Keeper of the Arnold Arboretum in Boston in the 1920s and 30s and a plant explorer in his own right, wrote about shrubs with berries for birds and recommended privet and buckthorn (as well as native viburnums and a host of “good” plants)
• State extension service agents wrote pamphlets in the 1950s through 70s advocating the use of many non-native exotic plants for erosion control and wildlife forage, including multiflora rose and autumn olive
• Most American yards today are 80% lawn and 20% non-native ornamental shrubs that provide absolutely no nutritional value to our native insects, birds and wildlife per the research of Douglas Tallamy, Professor of Entomology at the University of Delaware and best-selling author
• Lurking at the edges of these yards are a legion of non-native plant invaders that are spiraling out of control with the potential to destroy native ecosystems, change soil chemistry, biology, and structure while creating early season shade in monocultures that wipe out the native plant understory over time
• Invading continues: Asian Longhorned Beetle, Emerald Ash Borer, Hemlock Woolly Adelgid, Gypsy Moth, Colorado Potato Beetle, and numerous other insect pests have become scourges in our time and have damaged both native and non-native plants along with field crops
The field of conservation biology is a young science. The late Dr. Leslie Mehrhoff, member of the Department of Ecology and Evolutionary Biology at the University of Connecticut and Curator of its Herbarium, was one of the early researchers into the science of invasiveness. In 1999, a group of experts in horticulture, conservation, natural resources, agriculture, academic science and land management convened in Massachusetts under the aegis of the Executive Office of Environmental Affairs. They identified themselves as the Massachusetts Invasive Plant Advisory Group (MIPAG). This entity turned to Dr. Mehrhoff’s research as a starting point for their work on the question of identifying and listing invasive plants. A similar relationship developed in Connecticut between Dr. Mehrhoff and the Connecticut Invasive Plant Working Group, founded in 1997.
MIPAG released a document in 2005 listing plants classifed by scientific criteria of Invasive, Like Invasive, Potentially Invasive, or Evaluated but not meeting Criteria of Invasiveness. This list was last updated in 2016 and is found at https://www.massnrc.org/mipag/speciesreviewed_alpha.htm, and the Connecticut list can be viewed at https://www.invasive.org/species/list.cfm?id=66. Another valuable resource is the website of the Invasive Plant Atlas of New England, https://www.eddmaps.org/ipane/, continually updated by trained volunteer field spotters and professionals. There is a smart phone app to report sightings of invasives directly from the field. Lists include plants, insects, diseases and wildlife.
Also in 1999, a group of NOFA members from Massachusetts and Connecticut involved in the landscape business banded together to meet monthly under the leadership of Dr. Kimberly Stoner. Our goal was to write standards for the organic care of landscapes, modeled after what existed for organic farmers. We would do that and more, going on to develop an annual accreditation course to train land care professionals and presenting a series of public outreach lectures to raise awareness of this alternative to traditional chemical- and pesticide-based landscaping. I was fortunate to be a part of the group at the same time that I began my own organic landscape gardening company.
We were able to adopt much of the organic farming standards. However, we noticed that there was no mention of invasive plants and how to deal with them. Donald Bishop, one of our number, was charged with researching and writing this section. He was at the time a member of the new MIPAG entity and owner of an organic land care business. With each of the five succeeding editions of the Standards for Organic Land Care, this chapter has had an update. Now entitled “Native, Exotic and Invasive Plants,” this section of the Standards is worth reading (www.organiclandcare.net).
In the nearly 20 years since the writing of the Standards, I notice that clients are now much more aware of which plants are invasive and which are non-invasive or native. There are a few people who request that we leave the invasive plant in place, saying “at least it’s green” or “it screens me from my neighbor.” We work carefully to educate these clients, dropping nuggets of wisdom and leading by example. In time, we will replace those outlying invasive plants with a native alternative.
Most states have watch lists or lists of prohibited plants that are published online for reference. Burning bush, red barberry and Norway maples are no longer propagated and sold by nurseries, at least in this area, but may be available on the Internet from other states. Buyer beware! However, these and other invasive species still grow in many yards.
It is up to us individually to create what Doug Tallamy is calling “Homegrown National Park” of at least 80% native plants in our own backyards, nourishing the caterpillars and the birds that consume them. This web of life is fragile and oh-so important to our own lives.
Recommended Reading List:
Fairchild, David, The World Was My Garden: Travels of a Plant Explorer. New York: Charles Scribner’s Sons, 1943.
Grimshaw, John. The Gardener’s Atlas: The Origins, Discovery and Cultivation of the World’s Most Popular Garden Plants. London: Quarto Publishing, 1998.
Shepherd, Sue, Seeds of Fortune: A Gardening Dynasty. New York: Bloomsbury, 2003.
Spongberg, Stephen, A Reunion of Trees: The Discovery of Exotic Plants and Their Introduction Into North American and European Landscapes. Cambridge: Harvard University Press, 1990.
Tallamy, Douglas, Bringing Nature Home. Portland, Oregon: Timber Press, 2007.
Tallamy, Douglas, Nature’s Best Hope. Portland, Oregon: Timber Press, 2020.
Todd, Kim, Tinkering with Eden: A Natural History of Exotics in America. New York: W. W. Norton & Co., 2001.
Wilson, E.H., Aristocrats of the Garden. New York: Doubleday, Page and Company, 1917.
Wulf, Andrea, Founding Gardeners. New York: Alfred A. Knopf, 2011.
Wulf, Andrea, Brother Gardeners: Botany, Empire and the Birth of an Obsession. London: William Heinemann, 2008.
Priscilla Hutt Williams is Founder and President of Pumpkin Brook Organic Gardening in Townsend, Massachusetts and a co-author of the first edition of Standards for Organic Land Care: Practices for the Design and Maintenance of Ecological Landscapes, 2001.