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How what you plant can have unexpected results—at home and beyond

Horticulture bullies can be introduced, or native. They might be fine in one environment or set of conditions, and not in others. Like many things, the topic is complicated.

Many of the plants that are considered invasive are species that were introduced to North America, and they have no natural predators to keep them in check. Some were brought to this continent centuries ago, whether accidentally or on purpose, by settlers and explorers. Japanese knotweed, for example, was often planted for use as a windbreak, which only worked in the spring and summer months, knotweed being a herbaceous perennial. Other species have arrived more recently, and there may be plants in our gardens even now with invasive tendencies that haven’t yet manifested.

The gardening world brims with many opinions; some gardeners take an extreme view regarding what they plant, choosing only plants that are designated as native. Several questions arise from the native-only stance: How many years or centuries does a plant have to have existed in a region to be considered native? What if it’s native in another part of North America, but was brought here generations ago? And, while there are many wonderful native plants for use in gardens and landscapes, not all natives are well-behaved or disease- and pest-resistant.

The bottom line is that gardeners and stewards of landscapes have to decide for themselves not only what we want to plant, but also what we don’t want to plant in our gardens.

Purple loosestrife, a bane of wetlands. Although pollinators like the showy spires, the plant outcompetes native species.

Dig for dirt before you buy

What can we as gardeners do to curb the spread of invasive plants from taking over our gardens—and beyond? Melanie Priesnitz of Harriet Irving Botanical Gardens, in Wolfville, NS, says the most important thing gardeners can do is research a plant before taking it home.

“Unfortunately, some nurseries still sell forms of invasive species,” she says, including purple loosestrife and goutweed. “While it may seem harmless at the time, as you feel you will be able to control these plants in your own garden, it’s really important to think beyond your garden.” She suggests that gardeners imagine the implications if the seeds of some plant were to escape into the wild and become the next dandelion in terms of its spread. In order to preserve biodiversity, and to help restore areas such as the unique Acadian forest, we have to work together to preserve native species and be cautious about what plants we introduce or help spread.

Many garden cultivars are hybrids, so either they don’t set seed at all, or else seedlings from their seeds don’t resemble the parent plants. But other garden plants spread by rhizomes, underground jointed stems that can produce many new plants in short order. Sometimes, this trait can be highly useful, especially if you want to create a groundcover in areas that are difficult to mow, such as hillsides, or under trees where many plants are reluctant to take hold and grow. If the rhizomes are shallow-growing, as with creeping sedums and bugleweed, they can be relatively easy to control, but if they grow deeply in the ground, the plant can spread and pop up somewhere beyond your garden.

This happens to many people who have “caught” goutweed from a neighbour’s uncontrolled patch and, as we’ve heard many times, the best cure for goutweed is to sell your house and move to a goutweed-free one!

Free plants—with a cost

Dr. Bob Pett, with the Environmental Services Section of the Nova Scotia Department of Transportation and Infrastructure Renewal, has been working with the Invasive Species Alliance of Nova Scotia (ISANS)—an organization that facilitates networking among invasive plant researchers and decision-makers—on controlling the highly invasive garlic mustard that has appeared in parts of the Annapolis Valley. He urges gardeners to take great care about accepting soil and free plants for their gardens.

“Your new resource/treasure may contain plant propagules (seeds, roots, and rhizomes) of invasive species like knotweed, goutweed, garlic mustard and Himalayan balsam,” Bob says. The seeds of many species can remain viable for years, germinating when they have the optimal conditions of light and moisture that they need.

How to curb problem plants

Bob reminds people not to compost their invasive plants or send them to municipal composting depots. The composting process won’t necessarily kill off the seeds or rhizomes, and then these plants end up being spread around the countryside.

So what do we do when faced with trying to dispose of invasive plants? Organic materials are banned from landfills in many municipalities, which doesn’t leave a lot of options for people trying to dispose of dug-up or cut-down plants in a responsible manner. Some people burn this plant material along with any diseased plants they need to dispose of. Others recommend burying plant material at least a metre deep. Still others put theirs in black garbage bags and leave the bags in the sun to rot, which will quickly decompose the material; they then dispose of it in the regular garbage collection. With large infestations of something like giant hogweed, goutweed, or Japanese knotweed, this sort of disposal might be more problematic. Obviously, there is still much work that needs to be done on handling invasive plants.

Traditional herbicides are often ineffective on invasive plants given they grow and spread by underground rhizomes. Installing a root barrier around a garden bed can be effective, but if you have a large area to cover it can be expensive and time-consuming to put in, and will have to be dug down at least a foot in order to curb the majority of rhizomes spreading from an area.

When bad plants are good

Garden thugs can have their place in a garden, providing you plant them with awareness, knowing they will spread. I have one area of our garden that is particularly wet, and many plants won’t grow there. In fact, several normally invasive species have faltered and died in that part of the garden because they prefer drier growing conditions. What grows there now are some perennials that do extend their reach over time—several types of Maltese cross (Lychnis), spiderwort (Tradescantia), meadowsweet (Filipendula) and yellow loosestrife (Lysimachia) grow very well there, but are also easy to keep from spreading elsewhere in the garden.

As gardeners, sometimes we find ourselves in a quandary when our sensibilities clash with those of agriculture or government. Take the common milkweed (Asclepias syriaca). This plant is listed as a noxious weed in Nova Scotia even though it is native to eastern North America. Farmers dislike it because it can outcompete many crops, and it spreads quickly. However, common milkweed and related Asclepias species are the only food for the larval stage of Monarch butterflies, populations of which are declining due to habitat loss. Gardeners can plant other species of milkweed that aren’t listed as noxious, but those of us who are concerned about Monarchs tend to think all milkweeds are important to grow.

There are no real easy answers when it comes to dealing with invasive plants, other than to stay informed, and exercise caution in your garden. Always, however, just say no to goutweed. In case I hadn’t mentioned that earlier.

7 potential space invaders

While many vigorous garden plants can have their place in a border or bed, others tend to take over. Here are a few to watch for.

  1. Bugleweed (Ajuga) is a low-growing mint relative, with round or oval leaves in a host of different colours. Very useful as a groundcover but hard to eradicate if it gets into your lawn. Some report bugleweed will curb the spread of goutweed, so I’m planting some in my goutweed patch. Nice in containers, if you are concerned about its roaming skills.
  2. Chinese or Japanese lanterns (Physalis alkekengi) are beloved by floral decorators, who enjoy the brilliant orange, papery seedpods. You will often see these plants flourishing around old farmsteads, left undisturbed. They spread by runners, but the ones we planted in a small raised border stayed within their border—until my long-suffering spouse left boat-building supplies on them for weeks, and smothered them out.
  3. Goutweed (Aegopodium podegraria), also known as ground elder, bishop’s weed and snow-on-the-mountain, spreads by rhizomes and seed. Commonly sold as a decorative groundcover for its attractive variegated foliage, and Queen Anne’s lace-like flowers; it becomes especially invasive when it reverts to all green foliage. Cut it back to the ground regularly (don’t allow it to flower and set seed.)
  4. Mints (Mentha) of many types spread by rhizomes, and once an aggressive mint gets into a garden—or a lawn—you have enough mint to make juleps for an entire nation.
  5. Teasel (Dipsacus fullonum) is a strikingly handsome plant with huge, spiky cones of seeds that stand on tall stems right through winter. There can be dozens of seedheads on each plant, yielding thousands of seeds; some will be eaten by songbirds, but many will germinate as seedlings the following year. If you do enjoy this plant in your garden, mulch very heavily around stems and be quick to remove seedlings in spring.
  6. Variegated ribbon grass (Phalaris arundinacea), also known as gardener’s garters and strawberries and cream grass, is a spreading grass that can become problematic in light soils. When purchasing ornamental perennial grasses, make sure to ask about their growth and spreading habits, unless you intend to use one as a groundcover or to help prevent erosion.
  7. Yellow loosestrife (Lysimachia ciliata) ‘Firecracker’ and several other species of Lysimachia aren’t related to purple loosestrife (Lythrum salicaria). While several of the yellow-flowered lysimachias are clump formers that spread relatively slowly, ‘Firecracker’ spreads by rhizomes and forms a thick mat of runners and rootlets making it very hard to remove from clay soil. Goosenecked loosestrife (Lysimachia clethroides) is often listed as an invasive plant, but in three instances it died in my wet clay soil. It’s apt to run quite rampant in well-drained soil; on the plus side it’s a good plant for pollinators.
This is the second (and last) part of a series on invasive plants. Are there landscaping topics you’d like Jodi to write about? Please e-mail your ideas to This email address is being protected from spambots. You need JavaScript enabled to view it..

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