Most non-native plants behave nicely; others are domineering. Here’s what you need to know to keep things in check

“I don’t like those trees,” I said to my long-suffering spouse as we stood in the front yard contemplating the to-do list. “They’re going to come down as soon as I can get others growing up under them.”

He was puzzled. “But they’re maple trees, and you love maples.”

From garden to garbage

Don’t dump garden waste or unwanted aquarium plants in natural areas. “One of the biggest issues we face is how to best advise people about disposing invasive plants,” says Marika Godwin, who works with the Invasive Species Alliance of Nova Scotia. "The industry consensus about appropriate disposal of a plant like giant hogweed, for example, is to double-bag it in black garbage bags and leave it in the sun to rot, then dispose of it in regular garbage collection."

He’s right; I do love maples—most of them, but not Norway maples (Acer platanoides). These were planted by a previous owner. Norway maples, including the red-leafed and variegated varieties, have been a popular landscape tree for many years—you will frequently see them in subdivisions and public spaces. They are well adapted to most growing conditions, which is why they’re common. But they aren’t overly attractive trees, getting tar spot fungus on the foliage most years, and having a nondescript, yellow colour in autumn. They produce vast quantities of winged seeds, which drift on the wind and disperse over wide areas. As they mature, they create a dense canopy, and outcompete many native tree and shrub species. They’ve become problematic in some woodlands, including parts of the Acadian forest regions throughout Atlantic Canada.

Norway maple, purple loosestrife, multiflora rose, Himalayan balsam, Japanese knotweed, garlic mustard, giant hogweed and, of course, goutweed—these are all plants that are not native to our region, or alien, and have a tendency to become invasive, problematic species. Some started life as bullies in our gardens, outgrowing their spot in the border and overcrowding better-behaved plants. Those are annoying enough. But these species can become especially problematic when they leap the garden fence, so to speak, and take hold in the wild, displacing native plants. Often, an invasive non-native species has no local natural predators to help keep it in check, so it spreads vigourously, outcompeting native species that play important roles in local ecosystems. Some species, such as garlic mustard, grow very quickly and in vast numbers due to copious seed production, shading out slower or lower-growing species, and competing with them for nutrients, moisture and space.

Granted, some of these plants provide nourishment for beneficial creatures, but may be considered “bad plants” by farmers and others. As with anything to do with complex ecosystems, issues around invasive plants don’t have cut and dried answers.

Respect your alders

Sometimes we need to rethink what we consider to be nuisance plants—we may not realize just how important a role a plant plays in nature. Consider the humble alder, a family of shrubs and trees, many of them native species; they have been cursed by landowners for generations. Alders grow quickly, can form many-stemmed trunks, and are not popular when they appear in pasture or farmland. Yet these plants provide many ecological benefits, including providing food for birds and other wildlife, and helping to stabilize slopes and banks near watercourses with their tough, water-tolerant roots.

They create their own fertilizer through nitrogen fixation—bacteria that live in nodules in their roots convert atmospheric nitrogen into nitrates, which is used by plants. Alders also act as water purifiers, helping to remove excessive fertilizers and other potential pollutants from ground water. Research is being conducted on the usefulness of alders in purifying wetlands, as well as their use as a biomass fuel.

While you may not want to grow a crop of them, spare a few kind thoughts for these hardworking shrubs the next time you see them near a pond or ditch.

Personally, I’m pretty forgiving when it comes to many plants that are considered to be “bad.” My opinion is similar to that of Dr. Nick Hill of Fern Hill Institute for Plant Conservation, in South Berwick, NS. Nick says the people who often overreact to plants, considering them to be problematic, have come from mainstream agriculture, “where ‘crop-anything’ is good, anything else is bad, and we don’t think about ecological and evolutionary relationships.”

He encourages public education, especially to emphasize that calling all alien plants bad is propaganda, which can lead to poor land management. “In most cases, introduced species reflect our footprint of disturbance, and they are usually doing us a favour by preventing erosion and taking up nutrients that otherwise would end up in a watercourse,” he says.

Nevertheless, as Melanie Priesnitz, horticulturist at the Harriet Irving Botanical Gardens in Wolfville, NS, says, “Ecosystems need to work in balance in order to be healthy, and this is where invasive aliens are a problem... They throw off the natural balance of local ecosystems, and have been identified as the second most significant threat to biodiversity—habitat loss being the first.”

While we may grumble about the weather in Atlantic Canada, our cool climate actually helps deter some of the invasive plants found elsewhere in North America. Plants that we grow as annuals or short-lived perennials, such as Chinese forget-me-not (Cynoglossum amabile) and butterfly bush (Buddleia), are problematic and in some cases banned. Kudzu, the bane of southern American homeowners, doesn’t grow here because our winters are too cold. However, global warming is a reality, and a warmer climate may allow for the spread of invasive plants, insects and other organisms.

Controlling the spread of invasive plants can be highly challenging because of their very nature as prolific seed producers or rampant rhizome growers. The spread of pretty multiflora roses, on the other hand, has been accelerated by humanity’s love for rose gardening—the species is typically used as rootstock to which fancy hybrid roses are grafted. Often the hybrid will die and the rootstock starts to sprout in its place; it flourishes, producing lots of flowers and in turn, rosehips. In June and early July, you can see hundreds of shrubs, with their cascading branches covered with petite flowers along roadsides, in fields, at the edge of woodlands and in yards and gardens. The flowers are beloved by pollinators including native bees and honeybees, while the hips that form in autumn are greedily eaten by birds and other wildlife, which of course helps to spread the seeds—and they can last up to 20 years in the soil—so more plants germinate.

Marika Godwin works with the Invasive Species Alliance of Nova Scotia (ISANS), an organization with representation from a number of government departments, universities and non-profit agencies. She points out that a native plant species evolves in association with other native species in its ecosystem, and while an invasive species may provide food and shelter—as in the case of multiflora roses—it is not necessarily of the same quality as native species.

“For example, people see redwing blackbirds sitting on phragmites (the common rush, an invasive grass species) around the marsh in Annapolis Royal and assume it’s providing a roosting spot and possibly food, which may be true,” she says. However, its seed is not very nutritious, and its dense monocultures prevent the growth of other, native wetland species, limiting the wetland’s use as a shelter for wildlife.

Godwin says while ISANS acts as a coordinating body and has not been actively involved in the direct management of any invasive species, the agency does offer information and recommendations to help prevent the spread of invasive species (see “From Garden to Garbage,” top left).

While it may seem like the countryside could become overrun with alien plants, research is being done in Canada on how to effectively control or utilize some species. Researchers at the Nova Scotia Agricultural College are working on control of Japanese knotweed (Fallopia japonica or Polygonum cuspidatum), a highly aggressive perennial found throughout Atlantic Canada, while other researchers with the Organic Agriculture Centre of Canada are testing the effectiveness of using the common rush to purify water from greenhouse production.

Ducks Unlimited has been involved in biological control of purple loosestrife in wetlands, using two species of leaf-eating beetles that eat only the invasive plant.

What about those Norway maples in our yard? Next spring one is being cut down, and the other two will get a pruning, but have a stay of execution for a couple more years while the underplanted young trees get established. Meanwhile, this year I planted three native sugar maples, two native red maples, several pines, a yellow birch and assorted native shrubs around our property, to add to the natural ecosystem. And my long-suffering spouse cut down a young Norway maple he found growing on his woodlot.

If I find the definitive cure for the spread of goutweed, I should be able to retire from writing for a living.