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Controlling Invasive Plants

Vegetation is easier to control at the newly rehabilitated pond at the Frédéric Back Tree Pavilion. Photo credit: Espace pour la vie (Claude Lafond)
Pond at the Frédéric Back Tree Pavilion surrounded by greenery
  • Pond at the Frédéric Back Tree Pavilion surrounded by greenery
  • One of the rehabilitated banks of the pond at the Frédéric Back Tree Pavilion
  • Aquatic plants on one of the banks of the pond at the Frédéric Back Tree Pavilion
  • Waterfall and stones at the Jardin botanique de Montréal

The ambitious project to rehabilitate the pond at the Frédéric Back Tree Pavilion provided an opportunity to explore different techniques for getting rid of undesirable plants. Developed in 2021, this is the second phytotechnology station at the Jardin botanique after the filtering marshes at the Aquatic Garden.

The station has been designed to control the growth of invasive plant species, while at the same time providing an environment that suits a wide range of native species. The resulting natural environment requires only minimum maintenance.

Limiting plant growth and spread

Undesirable plants controlled by a transformed pond

When the pond was rehabilitated, its perimeter was expanded and the bottom was dug out to a depth of approximately 2 m, deep enough to ensure cooler water, which helps limit the proliferation of algae and invasive plants.

Steeper banks and a gravel beach with a gentle slope have been created to reduce the surface area of the ground that can be colonized by unwanted plants. The banks have been buttressed by gabion baskets—rock-filled steel-mesh cages—placed 35 cm below the surface. In addition, the bottom of the transformed pond is covered with an impermeable membrane and a layer of sand and pebbles.

Well-vegetated banks

Planting the banks densely with native species—grasses, perennials, shrubs—prevents invasive species from gaining a foothold.

A bit of shade, please

Another way to curb the growth of aquatic and semiaquatic invasive plants is to deprive certain parts of a landscape of sun. This has been done through judicious planting of trees and shrubs on the south banks of the pond.

Floating plant islands also provide shade, which curtails the growth of aquatic plants and algae, while the roots of the island plants can capture nutrients available in the water. Plant rafts, secured firmly to the south and southwest banks, serve the same function. Besides improving water quality, these features increase plant diversity and create new habitats for wildlife.

Redesigned water circulation

An oxygenating waterfall

A waterfall built on the north side of the pond is in operation from May to November and plays an aesthetic and functional role. Some of the water from the waterfall flows through filtering sand trays, which retain soluble phosphorus. This both oxygenates and purifies the water.

A closed loop

The project’s main upgrade is the creation of a closed-loop water circulation system. Water coming out of the pond via the spillway now flows in a completely redeveloped stream to a reservoir located 400 m downstream at the entrance to the Shade Garden.

The new, considerably larger reservoir has pumps that return the water to the pond, above the waterfall—where the cycle starts again. Thanks to a supply of fresh water from a nearby artesian well, the pond’s water is renewed completely once every two months.

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