Invasive Plant Species in Ontario: What to Avoid and What to Plant Instead
Gardening season is officially here, and as Ontarians return to their yards and gardens, it’s important to know which plants help your landscape—and which ones hurt it. Invasive plant species may look harmless, even attractive, but they can wreak havoc on your garden, choke out native species, and spread uncontrollably into natural areas. Before you dig in, here’s what you need to know.
What Is an Invasive Plant Species?
An invasive plant species is a non-native plant that spreads rapidly and aggressively, often without natural predators or controls in its new environment. These plants outcompete native flora for sunlight, water, and nutrients, reducing biodiversity and altering natural ecosystems. In Ontario, invasive species are not just a backyard problem—they can threaten wetlands, forests, farmland, and even water quality.
How Are Invasive Plants Different from Perennials and Groundcovers?
Many gardeners value perennials and groundcovers for their reliability and ability to fill space. But not all fast-growing or spreading plants are invasive. The key differences are:
- Perennials return year after year but typically stay within the area they’re planted, especially if they are native or well-adapted to local conditions.
- Groundcovers are low-growing plants that help prevent erosion and suppress weeds. Many are non-invasive and can be controlled with regular maintenance.
- Invasive plants, however, spread uncontrollably, often by underground rhizomes or prolific seed dispersal, and they escape garden boundaries into wild ecosystems.
The good news? For every invasive species, there are attractive, beneficial alternatives.
Top 10 Invasive Plant Species in Ontario—and What to Plant Instead
Here’s a list of some of the most problematic invasive plants in Ontario gardens, along with suggested alternatives that offer similar aesthetic value without the ecological risk.
Goutweed (Aegopodium podagraria) is a common groundcover that many gardeners plant for its ability to thrive in shade—but it comes at a cost. It spreads aggressively via underground runners and quickly smothers nearby plants, making it hard to control once established. A better choice for shady areas is Wild Ginger (Asarum canadense), a native plant that forms a lush carpet without overwhelming its neighbours.
Periwinkle (Vinca minor) may look charming with its glossy leaves and violet flowers, but it’s a serious threat to forest floors across Ontario. This plant escapes garden beds and forms dense mats that outcompete native species. It’s not harmful and can be contained using stone or other solid borders. Or you can try Foamflower (Tiarella cordifolia), a native groundcover that’s just as pretty and much more eco-friendly.
Japanese Knotweed (Fallopia japonica) is one of the most notoriously invasive plants in Ontario. It spreads rapidly, damages building foundations, and clogs waterways. Its resilience and deep root system make it incredibly difficult to remove. A safer alternative is False Solomon’s Seal (Maianthemum racemosum), a native perennial with a similar upright growth habit and graceful appearance.
English Ivy (Hedera helix) might look elegant climbing up walls and trees, but it deprives native plants of light and can cause significant damage to structures and ecosystems. It’s not dangerous and is a lovely addition to containers and isolated flowerbeds, so you don’t have to disregard it entirely. Another option is Virginia Creeper (Parthenocissus quinquefolia), a native vine that offers fast coverage and brilliant red fall colour without harming trees or buildings.
Norway Maple (Acer platanoides) was once a popular urban tree, but it has proven to be invasive. It outcompetes native maples, casts deep shade that limits undergrowth, and alters soil chemistry, which affects nearby plant life. The ideal substitute is Sugar Maple (Acer saccharum), an iconic Canadian species known for its vibrant fall foliage and ecological benefits.
Garlic Mustard (Alliaria petiolata) is a biennial herb that quickly invades forest understories, displacing native wildflowers and disrupting mycorrhizal fungi that many plants rely on. For a pollinator-friendly and non-invasive alternative, plant Golden Alexander (Zizia aurea), a native species with bright yellow flowers that support beneficial insects.
Dog-strangling Vine (Vincetoxicum spp.) lives up to its ominous name. It twines itself around shrubs and trees, smothering them and preventing natural regeneration in forests and fields. A beautiful and safer alternative is Virgin’s Bower (Clematis virginiana), a native vine with delicate white blooms that supports pollinators and wildlife.
Phragmites (European Common Reed) is a tall, dense grass that takes over wetlands, pushing out native plants like cattails and destroying critical wildlife habitats. In its place, consider Blue Flag Iris (Iris versicolor), a stunning native wetland plant that provides both beauty and biodiversity value.
Purple Loosestrife (Lythrum salicaria) may be attractive with its tall spikes of purple flowers, but it’s devastating to wetland habitats. It displaces native species and reduces food sources and shelter for birds and amphibians. A better option is Blazing Star (Liatris spicata), a native perennial that’s just as showy and is beloved by pollinators.
Tansy (Tanacetum vulgare) spreads readily in disturbed soils and is toxic to livestock. Though it’s sometimes planted for its ferny foliage and yellow button-like flowers, it’s best avoided. Choose Yarrow (Achillea millefolium) instead—a native lookalike with similar textures and blooms that supports pollinators and is much safer.
Giant Hogweed (Heracleum mantegazzianum) looks pretty when in bloom, but is a dangerous invasive species that poses a serious health risk. Its sap can cause severe skin irritation, burns, and even long-term scarring when exposed to sunlight. It also spreads aggressively in disturbed areas, quickly establishing large colonies. A safer, native alternative is Cow Parsnip (Heracleum maximum), which has a similar dramatic appearance but without the hazardous sap.
What You Can Do as a Gardener
- Avoid planting invasive noxious species, and monitor for them in your gardens.
- Contain species that spread aggressively by surrounding them with stone or another solid barrier.
- Choose native or non-invasive plants suited to your site conditions.
- Monitor your yard and naturalized areas for signs of invasive spread and act quickly to remove offenders.
- Support biodiversity by planting a variety of species that feed pollinators and birds.
By choosing the right plants, you can have a beautiful garden that supports Ontario’s natural ecosystems. If you’re unsure whether a plant is invasive, check with your local conservation authority or use online resources like Ontario Invasive Plant Council’s Grow Me Instead guide.
Need help planning a garden that’s both stunning and sustainable? Our landscape designers can help you make smart plant choices and create a space that thrives for years to come.
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While it’s not yet time to plant anything delicate or frost-sensitive, early spring is the perfect time to begin planning your landscaping project and preparing your yard. Here are some activities you can start with:
As the soil warms up and the last frost date approaches (typically in late April for Zone 6 and mid May for Zone 5), you can start thinking about planting. However, it’s important to wait until the soil is no longer too cold for seeds or young plants to thrive.
Once the danger of frost has passed and soil temperatures are consistently above 50°F, you can move forward with your full landscaping project. This is when most of the planting will take place. This period offers a long window for planting flowers, shrubs, trees, and vegetables. Here’s what you can do:
Choose the Right Plants
Not all snow or ice needs to be removed. In some cases, it’s better to leave it alone to avoid causing more harm than good.
Examine the Shape of Shrubs
Initial Watering: Immediately after planting or transplanting, water the tree thoroughly. This helps settle the soil and eliminate air pockets around the roots.
When to Fertilize: Avoid fertilizing immediately after planting. Wait until the tree shows signs of new growth, which indicates it has begun to establish its roots.
Staking: Young trees may need to be staked to protect them from strong winds, especially in exposed areas. Use flexible tree ties that allow some movement, promoting a stronger trunk.
Frost Protection: In areas prone to frost, new