European Buckthorn (Rhamnus cathartica)

European Buckthorn was initially introduced to Ontario in the late 1800s. Used as a hedgerow species to block wind and reduce erosion, it quickly spread throughout the province. European Buckthorn spreads quickly through their berries, which are eaten by birds and mammals. These berries have a laxative effect, which helps in their spread. European Buckthorn seedlings (along with many other invasive species) can often be found sprouting under perching sites popular with songbirds, such as large conifer trees.  

This species ranges in size from shrub to small tree, with thorn-like tips on stems and black berries in dense clusters along stems. Like most invasive species, Buckthorn leafs out very early in spring, and retains its leaves late into the fall. 

When European Buckthorn takes over an area, it changes the nitrogen composition of the soil to restrict the growth of other species. This effect persists even after all Buckthorn in the area is removed. Like all invasive species, European Buckthorn reduces species diversity in any area it occupies and facilitates the spread of other invasive species.

Garlic Mustard (Alliaria petiolata)

Garlic Mustard is an herbaceous plant in the mustard family. It emits a strong garlic smell when crushed. This plant was initially introduced as an herbal medicine plant in the late 1800s, but quickly spread out of control. Garlic Mustard seeds can be dropped directly from mature plants, or spread by humans, pets, and wildlife. Garlic Mustard self-pollinates, so a single plant can start an entire population. A single plant can produce upwards of 150 seed pods with 22 seeds per pod, and a population can produce over 100,000 seeds per square meter. Seeds usually remain dormant for a year and can remain viable in the soil for up to five years.

Garlic Mustard is biennial, meaning it has a two-stage life cycle. It begins life as a basal rosette that over-winters before growing into its second stage the following May, a tall flowering stalk that produces seeds in July and August. If this stalk is damaged, the plant can grow a new one as late as August.

This plant is unappetizing to native herbivore species, which puts more predation pressure on the native plant species herbivores prefer eating. Garlic Mustard can also grow well in undisturbed soil, which makes it a major threat in established forest communities. This plant competes with native spring ephemerals, a group of plants that already faces many challenges. The highly nutritious leaves of Garlic Mustard stimulate decomposition, which puts many native forest species (such as salamanders) at risk by reducing leaf litter depth. Finally, Garlic Mustard can prevent forest recovery by outcompeting native tree seedlings.

Invasive Honeysuckles (Lonicera spp.)

A number of non-native honeysuckle species and hybrids were introduced to Ontario as ornamental shrubs as far back as the 18th century. By 1920, these plants had escaped captivity and were widely abundant by the 1960s. These honeysuckle species reproduce primarily through seeds, which are eaten by birds and mammals and dispersed in their droppings. Invasive honeysuckles produce lots of berries, which remain on the plant well into the winter. 

These honeysuckle species outcompete native shrubs by growing faster and larger (giving them more access to light and nutrients) and through their high seed production. They also grow well in disturbed habitat where native species struggle to get established. Through their rapid growth they form dense thickets, reducing species diversity and damaging ecosystems. They can grow both in forest edge and interior forest habitat, putting them in direct competition with many native species (including species at risk).

Japanese Barberry (Berberis thunbergii)

Japanese Barberry is a common ornamental shrub that was introduced to North America in the late 1800s and has remained popular in the horticulture trade. This plant spreads primarily through its bright red berries, which are eaten by birds and dispersed in their droppings. Additionally, broken branches and stems can grow roots and establish as a new plant if not properly disposed of, so yard waste dumping is another common way this plant spreads. 

Japanese Barberry forms dense thickets and grow well in forest edge and forest interior habitat. The sharp thorns of this shrub make it unappetizing for most herbivores, which helps them outcompete native species. Recent research also suggests that tick populations are higher in areas where Japanese Barberry is prevalent, and that ticks in these areas are more likely to carry Lyme disease-causing bacteria.

More information on the link between barberry and ticks

Multiflora Rose (Rosa multiflora)

Multiflora Rose is a shrubby rose species that grows in long, arching canes. Originally introduced to North America to prevent soil erosion and as an ornamental species, it quickly spread out of control. This species matures quickly and can reproduce both from seed and vegetatively. When a Multiflora Rose cane touches the ground, it can grow roots from the contact point and sprout a new plant. This species can be distinguished from other rose species by its fringed stipules (the flared area at the base of each leaf), and by its greater number of thorns, greater number of flowers, and larger size. 

Multiflora Rose easily outcompetes native species in edge habitat and open areas through its quick growth and reproduction. The most common ways it eradicates other species is by shading them out or suffocating them under its dense foliage. When this species has taken over an area, it is often one of the only plant species growing there or is mixed with other invasive species.