Friday, September 7, 2012

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

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


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.