Story and photos by Diana Westland, Mississauga Master Gardener
February 20-26, 2023 was Invasive Species Awareness Week, with many opportunities for the community to participate in seminars and workshops related to identifying and managing invasive plant species. Harmful invasive species rarely provide equivalent habitat or food sources to native wildlife. They reproduce aggressively, are frequently the first to grow in spring robbing native plants of necessary warmth and sunshine, and many stay green through the winter making them easier to identify.
I am fortunate to live within easy walking distance of a beautiful green space created around the Lisgar Creek. This is a small greenway 5 km long, stretching from north of Derry Rd between 9th and 10th Line, terminating at the Osprey Marsh storm water containment lagoon near Britannia Road and Ninth Line. The Lisgar Creek is a tributary in the 16 Mile Creek watershed. The water levels vary from flooding in spring to bare trickles in the fall. This stormwater drainage area has been gradually naturalized over the years through the efforts of the city conservation authorities, and community volunteers. Unfortunately, invasive plant species have crept in uninvited and are gradually taking hold. Seeds have infiltrated the area, by wind, or by opportunistic seeds carried by animals and people from other areas. The saddest reality is that some are preventable garden escapees, illustrating the dangers of growing invasive plants in our gardens.
The first unwelcome visitor is of course the European common reed or Phragmites australis. We have all seen them taking over low areas, road ditches, and wetlands as we drive our cars around the city. A small stand of these reeds was the first thing to greet me as I turned into the walking trail . They are slowly increasing in the areas previously occupied by native cattails Typha latifolia. These invasive reeds will eventually smother the cattails and other native wet land species.
The reeds can grow in dense stands up to 5 m tall, robbing the native plants of sunlight and nutrients, eliminating all biodiversity (Phragmites fact sheet | ontario.ca). They can spread by seeds, frequently carried by vectors such as animals, people, vehicles working in the stands, or by wind and water. They can also spread by rhizomes in the soil. They can be differentiated from less aggressive native phragmites by reddish rather than tan stems, yellow-green rather than blue-green leaves, and sparser seed heads (Picture right, source from NCC: Where We Work - Ontario - Fighting phragmites — Ontario’s worst invasive species (natureconservancy.ca)). The loss of native plants such as cattails spells trouble for wetland species such as turtles, frogs, and birds. Cutting down the reeds will allow sunlight to reach the native plants in spring and prevent production of new seeds provided the native vegetation is still in place to shade any reed seeds on the soil. Smothering the area may be needed once reeds take over an area. The local watershed conservation authority should be consulted where action is needed to ensure the best approach is taken.
As I continued walking south, I found garlic mustard Alliaria petiolate as a ground cover under trees. This plant spreads quickly by seed and is considered very harmful as it releases chemicals that alter soil chemistry. The altered soil will discourage native plants from growing, a feature called allelopathy (Invasive Species - Credit Valley Conservation (cvc.ca)).
This plant is a member of the Brassicaceae family, like common vegetables such as broccoli or cabbage. It is a biennial plant whose leaves smell like garlic when crushed. In the first year it produces a distinctive basal rosette, while in the second year white flowers with four petals and seeds pods on tall stems appear. The best control, therefore, is to remove the plants' first-year rosettes to prevent more seeds being created. Again, ensure the bare soil is covered by mulch or desirable plant species to prevent garlic mustard seeds in the soil from germinating. If left unchecked, these plants replace native woodland groundcover and ephemerals such as trilliums Trillium sp, trout lily Erythronium americanum, and white wood aster Eurybia divaricata. Garlic mustard also interferes with the growth of soil fungi that nourish forest root systems (Garlic mustard | ontario.ca)
Further down the trail, the garlic mustard ground cover was quickly followed by periwinkle Vinca Minor. This plant, unfortunately, was probably a garden escapee, as they are still being sold in some nurseries. Anybody who has a shady garden and is not aware of the problem loves these plants, as they are survivors that produce beautiful blue flowers in late spring.
However once periwinkle reaches a naturalized area, it takes over the natural biodiversity. Propagation is from pieces of rhizome carried in leaf piles or in waste piles, as these pieces will produce new plants. For that reason, any garden waste that comes from periwinkle area should not be composted or dumped in green areas, but instead should go into waste.
When I arrived at the bridge crossing, I noticed a clump of English ivy Hedera helix, another garden escapee. It had taken over the ground under the trees and was using rootlets to climb the trees. If left unchecked, it has the same effect as the periwinkle, choking out food and habitat necessary to our insects and wildlife, creating forest monocultures. This plant is also found in garden centers.
The final invasive I found was located near the property fence on the north side of the Osprey Marsh area: oriental bittersweet Celastrus orbiculatus. This plant is still found in some nurseries. The red-yellow seeds are attractive, and these are frequently used in flower arrangements which then are composted or discarded with viable seeds still in place. This is a woody vine that twines itself around native shrubs and trees, eventually killing them by smothering them. The weight of the vine can result in trees uprooting. It spreads through rhizomes and birds will eat the seeds dropping them far from the mother plant.
The native American bittersweet Celastrus scandens is differentiated by its leaves which are more oblong than rounded. Flowers emerge at ends of stems rather than leaf axils, and the seed covering is more orange than yellow (Differentiating Round Leaf and American bittersweets | Minnesota Department of Agriculture (state.mn.us). Repeatedly cutting the vines at the base of the tree or shrub in spring and cutting any new surface shoots through the summer will help weaken the root system over time and prevent further seeds from developing. The process of hand removing the thick woody vine may damage the host tree so should be avoided. With the base cut, it will eventually wither. The young soft vines can be removed if careful.
We all love our green spaces in the city. There are so many organizations (see source references) working to identify and mitigate the harm of invasive spaces. As gardeners, let us do what we can to not add to the problem. We should be careful of what we grow, be able to recognize species that need to be removed or controlled, and try to participate in public events that help to restore areas taken over by invasives. We are both the vectors to invasive species spread and the solution.