Friday, September 7, 2012

Invasive Plants: Bittersweet, Knotweed, Garlic Mustard...

One plant species a year is written about as "Invasive Species of the Year." Water Chestnut for 2015, Tree-Of-Heaven 2013; Garlic Mustard 2012; Japanese Knotweed 2011; Oriental Bittersweet 2010. All five columns are consolidated below. Click on any photo to enlarge. 

Water Chestnut 

Water chestnut, an invasive water plant, has a nature akin to lily pads on steroids, growing rapidly in nutrient-rich fresh water ponds, lakes and slow-flowing rivers. Unchecked, it will almost completely cover water surfaces, making boating, swimming and fishing impossible. The dense floating mat of overlapping leaves also blocks sunlight penetration, causing oxygen deprivation lethal to fish and other animal life. In addition to this ecological horror story, the large, sharply pointed seeds, which mature in early August, fall to the bottom, and can cause painful wounds if stepped on.

Seed sinks to bottom, then following spring, sends one stem
to the surface, which forms a spreading rosette of leaves up
to a foot across. Seeds form underneath. Click to enlarge.
This species, Trapa natans, is not to be confused with the edible water chestnut common to Chinese cuisine. The plant was initially brought to the Harvard University Botanic Garden, possibly from southeastern Europe or western Asia. In the 1870s staff gardener Louis Guerineau took it upon himself to throw seeds into Fresh Pond and other Cambridge waterways. This came to the attention of botanist George E. Davenport, who decided to bring seeds and live plants to his friend Minor Pratt, in Concord. He and Pratt seeded a pond near the Sudbury River, and he suspected Pratt conducted additional distributions. Thus, Cambridge was point zero and Concord the plus one. Current distribution ranges from Canada to Maryland, and westward into New York and Pennsylvania.

As early as 1879 there was a concern voiced by botanist Charles S. Sargent, Director of Boston's Arnold Arboretum, that this non-native species threatened to become a nuisance, based on dense growths reported in Cambridge. Davenport fessed up in the Bulletin of the Torrey Botanical Club, Vol. 6: "I have several times had plants of Trapa natans that were collected in the vicinity of Boston, during the present year, brought to me for identification, and I have entertained no doubt as to the manner of its introduction into waters outside Cambridge Botanic Garden. But that so fine a plant as this, with its handsome leafy rosettes and edible nuts, which would, if common, be as attractive to boys as hickory nuts now are, can ever become a 'nuisance' I can scarcely believe."

Volunteers in canoes pull the water chestnut rosettes by hand,
then bring the baskets ashore to be disposed of. Hot, wet work.
This past Saturday a doughty band of about 16 volunteers, organized by OARS (Organization for the Assabet Sudbury & Concord Rivers), launched canoes onto the Assabet River from the property of Bob Collings, in Stow, to put in three hours pulling water chestnut plants. I was there as a first-time participant. What this involved was paddling upstream about one-third of a mile. Two occupants per canoe would steer into an area with plants to pull them by hand, each yank resulting in a dripping, muddy mess dropped into laundry baskets in the middle of the canoe. After a half-hour of this, the laden canoes would be paddled back to the launch site, the baskets lugged ashore to a compost pile, the canoes bailed out, the process repeated. Messy, messy, messy! The harvest was sixty full laundry baskets.  

Years of these visits, conducted every July before the nuts mature and fall to the bottom, have done a great job of eradicating the plants from long stretches of the Assabet River and reducing density in the still impacted parts. Surveillance visits are repeated each year, because while most seeds sprout next spring, some are still viable as much as 8-10 years later.      

Worst case: without the past, present and future efforts of volunteers from non-profit organizations the Assabet River upstream of the Ben Smith Dam could have become blanketed shore to shore with water chestnut. A few rosettes would have broken loose from anchoring stems, floated down the canal, and ending up infesting Maynard's mill pond.

To get an idea of how bad it can get, Vermont spends over half a million dollars a year hiring companies with mechanical harvesters to manage the worst parts of Lake Champlain, plus paying dozens of people to do hand-pulling in less-infested waters on the big lake and elsewhere. The 2013 report described 1,200 tons collected by the harvesters and more than 21 tons by hand.

Locally, mechanical harvesters have been needed on badly impacted parts of the Sudbury River. Heavily infested areas can also be treated with chemical herbicides, but these are non-selective, killing all plants. Researchers are looking into biological controls (plant diseases or insects from parts of the world where water chestnut originated), but are wary about introducing anything that is not species-specific.


Ailanthus altissima is the Latinate name for tree-of-heaven, a tree native to China, and thus invasive in the United States. The common name refers to its extraordinary growth rate. Ditto the Latinate name: Ailanthus derives from an Asian word for sky-reaching-tree while altissima has Latin roots in altus, for high or highest.

As with many invasive plant species the initial introductions were deliberate. By the 1840s these trees were being sold commercially for garden plantings. One route was China to England, thence to the eastern United States. Tree-of-heaven was a popular city planting because it thrived in poor soil and is resistant to drought and pollution. In California, immigrant Chinese workers at mines and railroads brought tree-of-heaven with them for its traditional medicinal purposes - the bark used to make an astringent tea.

Trees are either male or female. Both have flowers, but only the female trees create seed clusters. A mature tree can produce more than half a million seeds in a single season. These disperse by wind, and are rarely eaten by birds, mammals or insects. Deer will not eat the leaves nor nibble on the bark of saplings. Trees - both male and female - also are producers of new shoots from a far-reaching root system, so what starts as one tree can easily become a thicket.

This species does not play well with others. As with garlic mustard, this plant produces chemicals which inhibit the growth of other plants, a trait that is referred to as being allelopathic. Combine that with prolific seed production, a growth rate much faster than any native tree, plus resistance to pollution and drought, and this tree is a nuisance in urban and semi-urban environments unless vigorous combated.
Young trees have smooth bark
Tree-of-heaven smells bad. One disparaging nickname is 'stink tree.' Anyone who has tried to pull up seedlings or cut sprouts knows this tree has an offensive odor - sometimes described as having overtones of rancid or burnt peanut butter.

Tree-of-heaven is difficult to kill. Cutting results in new growth from the stumps that can exceed ten feet in the first year. Cutting will also stimulate a massive production of shoots from the surviving roots as far away as ninety feet from the original trunk. Any site where a mature tree was cut down will require follow-up cutting of new shoots several times a year for at least five years.

Systemic herbicides that kill roots (for example, triclopyr and glyphosate) currently provide the best chemical control for tree-of-heaven. These can be sprayed on shoots, or holes can be drilled in the stumps of freshly cut trees and the high concentration herbicide products applied directly. Both application methods can cause collateral damage to nearby plants. 


No, "Garlic Mustard" it is not some vile genetic modification experiment escaped from the laboratories of agri-business. Rather, garlic mustard (Alliaria petiolata) is a European plant naturalized to New England and other parts of the United States more than 100 years ago. It is in the mustard family, but the leaves of this sub-species have a mild garlicky smell when torn or crushed.

Garlic mustard is one of the few woodland plants flowering in early May. The plant has a two-year life cycle: close to the ground the first year, than taller and with flower stalks topped with small white flowers the second year. The flower stalks are one to two feet tall, capped in bouquets of four-petaled white flowers in the shape of a cross. The plant prefers full or partial shade to full sun.

Garlic mustard: second year plants 1 to 3 feet tall
The problem with garlic mustard is that it is displacing native woodland species. Invasives, by their nature, do not play well with others. Once transported these species find themselves in a new land not populated by their natural enemies. This plant in particular not only physically displaces native species but is suspected of waging chemical warfare. According to a Michigan State University website "Several compounds isolated from garlic mustard were shown to depress growth of both grasses and herbs in laboratory experiments. Researchers concluded that release of these compounds from garlic mustard root systems might account for its dominance in forest ecosystems. Others have suggested that such compounds might also disrupt mutually beneficial relationships between plant roots and certain fungi in the soil, known as mycorrhizal associations. These fungi are used by most North American forest ground-layer plants and are critical for nutrient and water uptake in many trees."

While immigrants to America get most of our attention, American species can be invasive after crossing in the other direction. Poison ivy now exists in the wild in the United Kingdom and Australia because it was planted as a garden ornamental. Bullfrogs are a spreading menace to native amphibian species worldwide. Grey squirrels are displacing native red squirrels in ItalyIreland and England; in the last an "Eat the enemy" campaign has received lots of media attention.

Cross-shaped flowers appear in May
Garlic mustard is relatively easy to pull up, roots included. Hand pulling, however, is very labor intensive, and can result in soil disturbance, damaging desirable species and putting soil at risk for erosion. An alternative is to cut the plant as close to the soil surface as possible, either with pruning shears or a weed wacker. Pulled plants should not be composted, especially those in seed, as the seeds are resistant to temperatures reached in compost piles and remain viable for years.


 Japanese knotweed (Polygonum cuspidatum), also known as Japanese bamboo, is a winter-hardy transplant from Asia. It is water- and sun-loving, and so can be found alongside marshy areas and roadside drainage ditches. Knotweed’s hollow, smooth stems and rapid growth gives it a superficial appearance akin to bamboo, hence the common name, but the species are not related.

New stems grow from the roots each year, quickly reaching heights eight feet or higher. The main-trunk stems are more than one inch in diameter. The plants branch into narrower stems that are horizontal or drooping towards the ground, with large leaves to either side of the stems. Fall finds these branches topped with wispy white flowers. At first frost, the leaves die, but dead stems remain standing, orange-brown in color, all winter and into next spring. An example of a colony of these plants can be seen at the northern edge of Tobin Park, which is behind the Blue Coyote restaurant. 
Japanese knotweed clusters spread from roots

As with many invasive species, Japanese knotweed was first introduced to the United States towards the tail end of the nineteenth century as an ornamental plant. Its dense growth crowds out native species while providing little in the way of sustenance or shelter for native animals.

Knotweed is extremely difficult to eradicate. Cutting stimulates new growth. The root system is very broad and very deep. After any attempt to remove it by digging it out, even small remnant sections of roots can start new plants. According to at least one report, even cut stem pieces can form roots, so cut material should not be added to piles of plant material intended for composting and reuse in gardens. 

Success in removing knotweed usually involves a multi-pronged approach involving cutting, digging, herbicides and covering the afflicted areas with tarps for the entire growing season. Experimental testing is underway with biological management using either insects or leaf fungus disease apparently specific for knotweed.

A little-known fact – knotweed roots are used as the source material for the popular dietary supplement ingredient resveratrol. Thus, while resveratrol is widely known to be found in red wines and (mistakenly) attributed all the health benefits of red wine, what is sold in most dietary supplements is not grape-derived. Resveratrol, whether grape, knotweed, or synthetic, does not yet have any proven health benefits in humans. Researchers are still in the preliminary stages of figuring out safety. There is evidence resveratrol prolongs lifespan – in mice.


Volunteer cutting large bittersweet vines on Summer Hill
Oriental bittersweet is a slow-motion disaster for the trees of New England. This ornamental plant “gone wild” was introduced to the  U.S. in the mid 1800’s and widely naturalized by the early 1900’s. When fully grown it can overtop sixty foot tall trees, breaking branches with its weight, and finally, killing the trees. Examples of its extent and damage can be seen in Maynard along Rockland Avenue and from the footpaths on Summer Hill, and in Stow along Pompisitticutt Street.

Infestations of bittersweet are easiest to see in the winter, when the red berries are a colorful haze across bare treetops. In spring the over-wintered berries are consumed by birds, contributing to the spread.

Bittersweet likes full sunlight. It tends to grow fastest on trees bordering roads and open spaces. Growth rates are 5-10 feet/year. Stems up to an inch or so in diameter are smooth, with increasing roughness as mature stems thicken to 3 to 4 inches in diameter. These vines are easy to differentiate from other vines. Poison ivy clings to the bark of the trees with thousands of fuzzy rootlets and rarely exceeds 15-20 feet in height. Wild grape vines have a brown, flaking bark.

Wreath, showing bittersweet berries
Combating bittersweet is both a private and public virtue. Property owners can begin by policing their own property. Vines should be cut as close to the ground as possible and also as high up as it is easy to reach. The gap makes it harder for the new shoots to reach the old vine ends and get back into the trees. In time, what is left up in the tree will rot and fall. For those who would complain that the vines will just grow back, so does grass – but you commit to cutting that twice a month. Hardware stores should have products that can be applied with a brush across the cut stems to block regrowth. Small vines can be pulled up roots and all. The roots are identified by a reddish-orange color. Just take care that you are not pulling up poison ivy, which can also be a ground based, woody-stemmed plant.

Volunteer-minded individuals could approach the Conservation Commissions in Maynard and Stow for guidance on addressing our bittersweet problems. Much as there are organizations committed to cleaning the Assabet River and picking up trash, towns need a coordinated effort to save the public woods. The Friends of the Assabet River National Wildlife Refuge periodically schedule work events to combat invasive plant species in the Refuge. Details are on the website (

Invasive species are noteworthy based on how much they disrupt the ecological balance of their new land. Various world lists of the one hundred worst invasive species are not limited to plants. Insects populate the lists, as do mollusks, crabs, birds, reptiles (Burmese pythons in Florida!), and even a smattering of mammals such as pigs, goats, cats and rats. Our international travels and trade continue to promote accidental or intentional introduction of species. An outbreak of Asian longhorned beetles is being combated near Worcester. In 2009 an outbreak of zebra mussels was found in Laurel Lake in the Berkshires, and a “late blight’ fungal infection damaged much of the tomato plants bought for home vegetable gardens.

1 comment:

  1. You can approach japanese knotweed in a lot of ways, if you're going to approach it yourself, then the thing you really need is tenacity to make sure you treat it year after year.