Mean and Green – Alien plants Part 2: Green, and really mean


Last month, in Part 1 of this two part series, I described a number of plants that are non-native to Lanark County. These plants were intentionally brought here by European settlers who valued them for their culinary or medicinal properties, but they are now considered nuisance weeds by most gardeners. They are not a threat to native flora and fauna, and do actually have medicinal and culinary uses. So very unlike the plants presented in this month’s column….

Hallowe’en has come and gone, but be prepared for a harrowing account of invasive plants gone berserk! Choking forests, fields and wetlands! Poisoning people! Wrecking buildings!

 Japanese knotweed (Fallopia japonica, aka Reynoutria japonica, aka Polygonum cuspidatum) first appeared in the U.S. prior to 1890, when it was imported from Asia as an ornamental plant and for erosion control. When a weed goes by three different Latin aliases, doesn’t that already arouse suspicions? Here’s what Wikipedia has to say about it: “Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo…. While stems may reach a maximum height of 3–4 m (10-12 ft) each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The leaves are broad and oval…. The flowers are small, cream or white, produced in late summer and early autumn….The invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35 °C (−31 °F) and can extend 7 metres (23 ft) horizontally and 3 metres (9.8 ft) deep, making removal by excavation extremely difficult. The plant is also resilient to cutting, vigorously re-sprouting from the roots…. It can be found in 39 of the 50 United States and in 6 provinces in Canada.” Several plants have been spotted in the region between Kingston and Ottawa, and there are reports that it has reached northern Ontario. It has a USDA hardiness zone rating of 4 to 8, so its spread is assured as global warming increases. (According to the Ottawa Horticultural Society, Ottawa has a USDA hardiness rating of 4. Lanark County’s rating is the same.) The Nature Conservancy recommends the following control mechanisms: Manual/mechanical control methods such as mowing, trimming, digging and pulling may work if you are persistent over a period of years. The objective is to starve the root system. Keep stems that you pull, cut or mow out of the compost pile and well away from any nearby body of water. You don’t want the plant to spread to a new location. Herbicide applications may be appropriate for large infestations. Use only an herbicide approved for riparian use and take precautions to minimize drift.

Garlic mustard (Alliaria petiolata) is another holy terror. It’s a biennial with kidney-shaped, toothed, green leaves and flower stalks terminating in clusters of small white flowers, that in its second year attains a height between 30 and 130 cm. Young garlic mustard leaves release a strong garlic odour when crushed. Native to Europe, Asia, and northwestern Africa, it was brought to North America as a culinary and medicinal herb in the 1860s. It is shade-tolerant and has now displaced native vegetation to become the dominant under-story plant in woodlands and floodplains, where it’s difficult to eradicate.

Garlic mustard has a vast arsenal of weapons at its disposal. First of all, disguise. Its appearance helps it to hide among the many white-flowered plants that populate our woodlands. Second, poison. The toxic chemicals it releases into the soil impede the growth of mycorrhizal fungi that most native plants, including trees, need to thrive. (These fungi improve the ability of plants to absorb water and mineral nutrients from the soil.) Third, a high rate of reproduction and mobility. Plants that survive the winter produce hundreds of seeds in their second year, which are easily spread by people and animals on shoelaces, hooves, paws, fur and clothing. Dense stands produce more than 60,000 seeds per square metre. Fourth, hardiness. Garlic mustard seeds can remain in the soil for up to 5 years and still be viable. This plant can grow anywhere: in sun, in shade, in forests and forest edges, on riverbanks and roadsides within USDA hardiness zones 3 to 8. It is found in all eastern and mid-western U.S. states, and is established in southern and eastern Ontario as far north as Sault Ste. Marie, and parts of Quebec. Plus, it’s self-fertile, so it doesn’t need cross-pollination. Fifth, no natural enemies. To top it all, potential predators such as deer dislike garlic mustard, and resort to consuming native plants instead. Talk about devious!

The Ontario Ministry of Food, Agriculture and Rural Affairs (OMAFRA) recommends the following controls: watch for any invaders, and destroy them immediately. Young plants can be pulled out easily, but new sprouts will grow if the plant breaks off. With established plants, prevent the seed from shedding. You can burn the plants or apply chemicals, but this risks damaging adjacent desirable plants. The Landowner’s Guide to Controlling Invasive Woodland Plants (website address: is a good source of information. You can report sightings of garlic mustard or other invasive species in the wild at You can also contact the Invading Species Hotline at 1-800-563-7711.

Giant hogweed (Heracleum mantegazzianum) (USDA rating 3-9) started out as a garden ornamental from southwest Asia, but has escaped to the wild in North America and has a scattered distribution across southern and central Ontario. It can spread readily and grows along roadsides, ditches and streams. It invades old fields and native habitats such as open woodlands. Under ideal conditions it grows up to 5.5 metres tall! Giant hogweed blooms once in its 2- to 5-year life, producing white flower clusters that can form a flower-head almost 1 metre wide. Each plant can produce up to 120,000 winged seeds, which can travel long distances via water and wind. Its seeds are viable in the soil for up to 15 years. There is concern that it can shade out native plants. The clear watery sap of this plant contains toxins that can cause severe dermatitis in people, so stay clear of it! It goes without saying that you should not deliberately plant giant hogweed in your garden. If you find it on your property, report it to the Invading Species Hotline (see contact information above). It is recommended that you hire a professional exterminator to get rid of it.

Dog-strangling vine (Vincetoxicum rossicum) (USDA rating 4-9), is aptly named. Although it doesn’t strangle dogs (so far as I know), it does strangle just about everything that doesn’t move, and can trip unwary cyclists and pedestrians as it sprawls across pathways, lying in wait. This common name refers to two almost identical plants native to Eurasia – black swallowwort and pale swallowwort – that were introduced to the northeastern United States in the mid-1800s as a garden plant. Its leaves are oval with pointed tips, and its pink to dark-purple star-shaped flowers have 5 tiny petals. The plant produces bean-shaped seed pods that, like the milkweed to which it is related, release white feathery seeds.

This vine can grow up to 2 metres high in sun and light shade, and rapidly wraps itself around anything in its path, resulting in the strangulation of plants and small trees. Dog-strangling vine can produce up to 2,400 seeds per square metre, which are easily spread by the wind. As if that wasn’t enough, it can also grow from root fragments. This vine has taken over ravines, hillsides, stream banks and roadsides, and has been found in grasslands and forests. This plant was first observed in Ontario in the late 1800s, and since then has spread within Ontario (It is well-established in the Ottawa area) and to southern Quebec.

Not only does dog-strangling vine crowd out and overpower native plants and young trees, it threatens the monarch butterflies, which mistake it for the monarch caterpillar’s host plant, milkweed. Because the caterpillars can’t eat the vine, they die. Also, as with garlic mustard, this plant is not eaten by deer and other browsing animals, which puts greater grazing pressure on more palatable native plants.

In addition to reporting this plant to the Invading Species Hotline (see contact information above) if you see it, and getting rid of it should you find it on your property, make sure you don’t inadvertently spread the seeds. When enjoying the great outdoors, stay on trails and keep your pets on a leash.

European buckthorn (Rhamnus cathartica) and glossy buckthorn (Frangula alnus
) are both small shrubs or trees (up to 6 metres tall) native to Eurasia. They first landed on North American shores in the 1880s as a garden plant, and were widely planted as a windbreak in agricultural fields. They are now rampaging throughout northeastern and north central United States, and in Canada, from Saskatchewan to Nova Scotia. The common buckthorn has deeply-veined, oval, toothed leaves about 5 centimetres long, small pale green bell-shaped flowers, and red fruit which later turns black. The glossy buckthorn is similar in appearance, but its leaves are shinier and lightly-veined with smooth edges.

As with the above-mentioned invasive plants, these non-native buckthorns have a whole repertoire of weapons to dominate the North American landscape. They are hardy from USDA hardiness zone 3 to zone 9, will grow anywhere (swamps, bogs, fens, marshes, river banks, pond edges, mature forests, farm fields, hydro corridors, and roadsides), are hard to kill, have no natural predators, and spread quickly. In the case of buckthorn, the fruits are relished by birds and mammals which eat them (apparently the fruit is not particulary nutritious and in fact has a cathartic effect on these animals), then deposit the seeds far and wide in their droppings. Once they’ve gained a foothold in a new location using their extensive, spreading root system, they then form dense thickets, crowding out native shrubs and herbs, often eliminating them completely. Buckthorn can also alter the nitrogen levels in soil, which improves the conditions for its own growth and slows down the growth of native species. Farmers dislike buckthorn because it can host oat rust and soybean aphids, which damage their crops.

If you find glossy or European buckthorn on your property (they have certainly been trying to gain a foothold on my own property!), try to pull them out if the plants are small enough (some people use a weed wrench). If they’re any bigger, you may have to resort to cutting them as

low to the ground as possible. This is an ongoing job. Whatever you do, don’t let them flower.

The kudzu vine (Pueraria montana var. lobata) is another potential threat to our area as the climate continues to warm. Native to eastern Asia, it was intentionally planted in the southeastern United States in 1876 as an ornamental, then planted extensively for erosion control and livestock feed. It also has several culinary and medicinal uses, and people also use it to make paper, lotions, soaps and baskets. Any benefits kudzu offers, are vastly outweighed by its destructiveness in warmer climates. Kudzu, with a USDA hardiness zone rating of 5-10, is now found throughout the southern U.S. as far north as Ohio and Connecticut. In the U.S. the vine has been spreading at the rate of 150,000 acres (61,000 ha) annually! Many other countries report similar concerns. Kudzu was found in Leamington, Ontario in 2009. This comment on Dave’s Garden/PlantFiles website, posted on June 26, 2005 by sky4va, who lives in USDA zone 6, says it all:

I moved onto 2 acres a year ago that had previously been pasture land with a fencerow at the back of the property. It took me months to clear out all the blackberry brambles and multiflora roses, all the while being eaten up by poison oak. Then I came upon a horrible vine that was strangling every sapling in its way. That was my first close-up view of Kudzu. It doesn’t matter how cold it gets in the winter; it WILL come back and with a vengeance. This past winter we had several 0 degree nights, with the temps running in the 20s (Fahrenheit) for long periods. I skipped one week earlier this month not going to the back to tend the vines and they had covered 20, yes 20!, feet in that time, crawling to the tops of all the young red maple saplings. It took me hours just to clean this new mess up. DON’T EVER PLANT KUDZU if you don’t already have it! I think it’s one of the worst plants God ever put on this earth! Just my opinion, but you will regret it. It is a total menace here in southwest Virginia.

Yet other highly invasive species are: Amur, Morrow, and Tartarian honeysuckles, Norway maple, and wild chervil. Space limitations prevent me describing them at length, but you can find out more about them from the Ontario Woodlot Association website, as well as OMAFRA

Who knew that plants could wreak such havoc on our natural environment? Let’s all do what we can to prevent their spread and eradicate them.