Any naturalization project is susceptible to invasive species. Disturbing the landscape, even to plant native species and to create a more natural habitat, will provide the conditions that allow invasives to become established. If you are involved in this sort of work, please learn about the plants that are invasive in your area and try to control them from the beginning. Once they become established, control is much more difficult.

To find out more about invasive plant species and which ones might be a problem in your area, see Environment Canada’s, Invasive plants of natural habitats in Canada

Common and Glossy Buckthorn (Rhamnus cathartica and R.  frangula)

Common or European Buckthorn and Glossy Buckthorn are highly invasive shrubs (or small trees). They bloom in June, then produce blue-black berries, which are attractive to birds. Unfortunately, they are cathartic; seeds are excreted around favourite perching trees and soon a thicket of small buckthorn saplings surrounds these trees.

In some areas of the Ottawa region, including the Green Belt, buckthorn is becoming the dominant understorey species.

The most effective way to remove buckthorn is to uproot it. Luckily, the roots are very shallow. Small saplings can be pulled out by hand, especially in our woodlot where the soil is loose. For larger saplings, we use a Weed Wrench, which works on trees up to about 5 cm in diameter.

In grassy areas, pulling is much more difficult as buckthorn roots will grow into and under the dense layer of grass roots. One of our volunteers developed an effective method to overcome this problem. Using an 24-cm camp saw pushed down into the dirt, cut all around the tree about 30 cm from the trunk. Trees up to 6 cm in diameter can then be pulled with the weed wrench. Roots left in the ground do not resprout.

Buckthorns can grow to the size of an apple tree and have multiple trunks. These large trees are very difficult to remove, especially when habitat preservation is the goal.

Weed Wrench

Buckthorn trunk “girdled”; photo D. Gordon E. Robertson

In the past, we have also tried “girdling” trunks (i.e., cutting away the living layer of the bark all the way around). This can be effective as it causes less damage to the surrounding area than removing the tree. However, multistemmed trees are difficult to girdle. Resprouting below the girdled section is prolific.

We are now resigned to cutting large trees – usually in stages. Cut branches first. On the ground, cut them into smaller sections and pile them together. Finally, cut the main trunk (using a chain saw if possible). We cut the trunk as close as possible to the ground and cover the cut area with heavy plastic sheeting – the kind you use as pond liner. We cover the cut trunk plus about 15 centimetres all around. This method has been very successful in preventing resprouting from the cut trunk as it completely excludes light.

If trunks are not removed or covered, resprouting the following year can be prolific. In theory, follow-up cutting for several years should eventually kill the roots.

In buckthorn, male and female reproductive parts are on different trees. Where an infestation is heavy, it’s best to tackle the female trees first; they are the ones that produce berries, which spread the species.

More information

Common Buckthorn, berries; photo Malcolm Leith

Common Buckthorn, leaf underside

Glossy Buckthorn, leaves and berries; photo Malcolm Leith

Glossy Buckthorn, leaf underside

European Frog-bit (Hydrocharis morsus-ranae)

Frog-bit is an aquatic plant that floats on the surface of ponds or other still water. Left unchecked it can build up a thick layer covering all open water.

The plant reproduces mainly vegetatively. In late fall, it produces “turions,” which sink to the bottom of the pond where they overwinter.

At the FWG, we have been removing Frog-bit manually. We use an inflatable boat or wade into the water and scoop up handfuls of the floating leaves. As this is an aquatic plant, they can be left to compost in any dry area on the shore.

One method that is particularly effective is using a rake to pull frog-bit into a sort of raft that can then be floated over to the shore and deposited far enough from the water to keep the plants from regrowing

European Frog-bit


Flowering Rush; photo D. Gordon E. Robertson

Raft to carry rushes to shore for removal

Flowering Rush (Butomus umbellatus)

Flowering Rush is native to Eurasia. In about 1998, one or two plants appeared at the edge of our Amphibian Pond. Within 2-3 years, the plant had spread throughout the pond and was covering most of the open water. In 2002, we began a concerted effort to pull the plants out.

At the FWG, Flowering Rush is late growing. The plants are just reaching maturity at the end of June, which is about the time that our Red-winged Blackbirds have fledged.

We wade into the water and gently pry up the roots of the Flowering Rush plants, sometimes using a garden “claw” to help loosen them from the bottom of the pond. Where they are growing among cattails, removal is more difficult.
We’ve found that putting uprooted plants into the inflatable boat saves us some effort as it can be filled, then floated over to the edge of the pond and emptied. We generally pile the weeds in one designated area; on dry land, there is no risk of them regrowing and they decompose quickly. We’ve also found that the uprooted plants will float on the water surface. Many can be placed together to form a “raft” then floated over to the edge of the pond and deposited far enough from the edge to ensure that the plants will dry out.

In 2010, muskrats moved into our pond and started “helping” by eating flowering rush plants, especially the roots. By 2011, most of the Flowering Rush, along with quite a few cattails had disappeared. Unfortunately, few cattails means few Red-winged Blackbird nests.


Purple Loosestrife (Lythrum salicaria)

Purple Loosestrife is beautiful. I’ve actually seen it for sale at a garden centre. BUT there is no doubt that it is a threat to our already threatened wetlands. Allowed to flourish, it will quickly fill in a wet area

Fortunately, loosestrife is not widespread at the FWG. We have a small patch at the edge of our amphibian pond, and individual plants have popped up in our old field and adjacent habitats. (It doesn’t seem to need a wetland to grow in.)

We were also lucky to obtain some of the beetles that were imported to control loosestrife — Golden Loosestrife Beetle (Galerucella pusilla, see photo below) — and have introduced them onto our plants. These beetles are very effective. They don’t kill the plants, but do enough damage to prevent most of them from blooming.



Purple Loosestrife

Golden Loosestrife Beetle; photo Christine Hanrahan


Lesser Burdock

Lesser Burdock (Arctium minus)

Lesser Burdock or Common Burdock is a Eurasian species that can seriously disrupt native ecosystems. Although found primarily on disturbed sites, it will spread to natural areas from nearby roadsides, rail tracks, abandoned fields, and other similar areas. Its large leaves can shade out and prevent other plants from growing, while its prodigious seed production helps it readily colonize a site.

Comfrey (Symphytum officinale)

It is native to Europe and it is known elsewhere, including North America, as an introduced species and sometimes a weed. The flowers are mostly visited by bumblebees.

Comfrey; photo Christine Hanrahan

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This page was revised 14 February 2018
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