Ontario’s Invasive Species Act Explained: Prohibited Plants Under O. Reg. 354/16

Ontario’s landscapes contain some of the most diverse freshwater and terrestrial ecosystems in North America. Lakes, wetlands, forests, and grasslands support thousands of native species that depend on one another to maintain healthy ecological balance. When non native species enter these environments and begin to spread uncontrollably, that balance can be disrupted very quickly.

For many years invasive plants and animals entered Ontario unintentionally. Some were introduced through the aquarium and water garden trade. Others arrived as ornamental plants for landscaping. A few spread through agriculture, shipping, or international trade. At the time, most introductions were not done with harmful intent. Many species were brought in simply because they were attractive, hardy, or useful for aquariums and ponds.

The problem emerged when certain species escaped cultivation and began spreading into natural environments. Without natural predators or environmental controls, these plants were able to reproduce rapidly and outcompete native species. In wetlands and lakes, some aquatic plants formed thick mats that blocked sunlight and reduced oxygen in the water. On land, aggressive vines and grasses began replacing entire plant communities.

Once an invasive species becomes established, removing it is extremely difficult and often very expensive. In some cases complete eradication becomes impossible. Governments and conservation organizations found themselves spending large amounts of money trying to control species that could have been prevented from spreading in the first place.

To address this growing ecological threat, Ontario introduced the Invasive Species Act in 2015, with regulations such as O. Reg. 354/16 identifying species that pose the highest risk to the province. The purpose of the law is prevention. By stopping certain species from being imported, sold, transported, or cultivated, the province reduces the chances that they will escape into natural ecosystems.

The Act also establishes clear responsibilities for individuals, businesses, and industries. While most early introductions occurred before people fully understood the ecological consequences, continued distribution of high risk species today could lead to severe environmental damage. For this reason the legislation includes significant penalties.

Under the Act, individuals who intentionally violate the regulations can face fines of up to $250,000 and up to one year of imprisonment. These penalties are not designed simply as punishment. Their purpose is to discourage the deliberate movement or release of species known to cause ecological harm.

In other words, the law reflects an important shift in environmental awareness. Plants that were once sold freely in garden centres or aquarium shops are now recognized as major threats to biodiversity. By regulating these species, Ontario aims to protect native ecosystems, safeguard wildlife habitat, and reduce the long term costs of invasive species management.

Understanding why these plants are prohibited helps gardeners, landowners, and hobbyists make informed choices. Many of the species listed in the regulation were originally introduced for ornamental beauty or aquarium use, but their impact on natural ecosystems proved far greater than anyone expected.

Prohibited Aquatic Plants

Aquatic ecosystems are particularly vulnerable to invasive plants. Many of these species reproduce rapidly, grow in dense mats, and spread through fragments that move easily between water bodies.

Brazilian Elodea (Egeria densa)

Brazilian elodea is a submerged aquatic plant commonly sold in the aquarium trade. In natural waterways it grows extremely quickly, forming dense underwater forests.

These thick growths block sunlight, reduce oxygen levels in the water, and interfere with fish habitat. Dense infestations can also clog water intake systems, irrigation channels, and boat motors.

Fragments of the plant can easily break off and establish new colonies, which allows it to spread rapidly between lakes and rivers.

Hydrilla (Hydrilla verticillata)

Hydrilla is considered one of the most aggressive aquatic invasive plants in the world.

It grows beneath the surface of the water, forming dense mats that can completely dominate a water body. Hydrilla spreads through multiple mechanisms including fragments, underground tubers, and specialized buds.

These growths alter water chemistry, block light to native plants, and make waterways difficult to navigate. Hydrilla infestations have caused severe ecological and economic damage in many regions.

European Water Chestnut (Trapa natans)

European water chestnut is a floating aquatic plant that forms large rosettes on the surface of lakes and slow moving rivers.

These plants grow in thick floating mats that prevent sunlight from reaching underwater vegetation. This leads to the collapse of native aquatic plant communities and reduces habitat for fish and aquatic insects.

The plant also produces hard, spiny seeds that accumulate in sediments and remain viable for many years.

Yellow Floating Heart (Nymphoides peltata)

Yellow floating heart resembles native water lilies but spreads aggressively across the surface of ponds and lakes.

Its floating leaves form dense patches that shade out native plants and disrupt aquatic ecosystems. Once established, the plant spreads quickly through fragments and root systems.

Because it can resemble ornamental water garden plants, it has historically been introduced through backyard ponds.

Water Soldier (Stratiotes aloides)

Water soldier is a floating aquatic plant that moves up and down through the water column during the growing season.

It forms dense colonies that crowd out native vegetation and alter aquatic habitats. Its stiff, serrated leaves can also create physical hazards for swimmers and wildlife.

Once introduced, water soldier spreads quickly through vegetative reproduction.

Oxygen Weed (Lagarosiphon major)

Despite its name, oxygen weed can actually disrupt aquatic ecosystems.

This submerged plant forms thick underwater growth that outcompetes native species and interferes with water flow. It spreads primarily through plant fragments, which can easily attach to boats, fishing gear, or aquatic equipment.

Once established, it becomes extremely difficult to remove.

Giant Salvinia (Salvinia molesta)

Giant salvinia is a floating fern that grows rapidly across the surface of water bodies.

Under favorable conditions it can double in size within days. The plant forms thick floating mats that block sunlight, reduce oxygen in the water, and severely damage fish and aquatic ecosystems.

Because it spreads through tiny fragments, even a small introduction can lead to a large infestation.

Prohibited Terrestrial Plants

While many invasive plants spread in forests or fields, some species also threaten agriculture and native wildlife habitat.

Pale Swallow Wort/Dog Strangling Vine (Vincetoxicum rossicum)

Pale swallow wort is a climbing perennial vine that spreads aggressively through both seeds and underground roots.

It invades forests, meadows, and urban green spaces, forming dense stands that replace native vegetation. The vine also interferes with wildlife. Monarch butterflies sometimes lay eggs on swallow wort, but the caterpillars cannot survive on the plant.

This disrupts important ecological relationships and contributes to the decline of native pollinators.

Black Swallow Wort (Vincetoxicum nigrum)

Closely related to pale swallow wort, black swallow wort behaves in a similar way.

The vine spreads rapidly and can dominate large areas of habitat. Its ability to produce abundant wind dispersed seeds allows it to expand quickly into new areas.

Once established, swallow wort species are extremely difficult to eradicate.

Prohibited Invasive Grass

Invasive Common Reed (Phragmites australis subsp. australis)

Invasive phragmites is a tall perennial grass that can grow over four meters high. It spreads aggressively through underground rhizomes and seeds.

The plant forms dense monocultures that replace wetlands, shorelines, and marsh habitats. These thick stands block sunlight, alter water flow, and reduce habitat for birds, amphibians, and aquatic species.

Native wetlands that once supported diverse plant life can become dominated by a single species of reed.

Protecting Ontario's Landscapes

Ontario’s natural environments are shaped by complex relationships between plants, wildlife, water systems, and soil communities. When new species enter these systems and spread beyond control, those relationships can change in ways that are difficult to reverse. Protecting these environments requires attention not only from scientists and government agencies, but also from the people who interact with plants every day.

Gardeners, aquarium hobbyists, landscapers, and outdoor enthusiasts all play a role in preventing the spread of invasive species. Being mindful of what plants are grown, transported, or disposed of helps reduce the chance that harmful species will enter wetlands, forests, or waterways.

Learning to recognize regulated plants and understanding the rules surrounding them also helps avoid unintentional violations. Many people encounter these species without realizing they are prohibited, particularly when plants resemble common ornamentals or aquarium vegetation.

The most effective protection for Ontario’s ecosystems is awareness. When people understand how invasive species spread and why certain plants are restricted, it becomes much easier to make responsible choices that support biodiversity and healthy landscapes.