Garlic Mustard (Alliaria petiolata)
Imported as an edible green by European settlers, this biennial shade-tolerant species dominates the ground cover of deciduous forests and disturbed areas such as flood plains, stream banks and roadsides. Since its introduction to a few locations in North America in the 1800s garlic mustard spread gradually, but has recently undergone a series of population explosions over the past 20 to 30 years. It is now widespread across northeastern North America (south of the Canadian Shield) and has also been found to the west in Vancouver and California. Garlic mustard is found in many forests and disturbed sites throughout the Credit River watershed.
In its first year garlic mustard is often easily-overlooked, growing as a relatively small basal rosette (circular arrangement) of leaves that may be mistaken for native species such as violets. It can be easily distinguished by the garlic odour it gives off when crushed.
By its second year garlic mustard grows a flowering stalk up to one-metre in height, with clusters of four-petaled white flowers in early spring. Each plant can produce hundreds of seeds which remain viable in the soil for up to five years. Seeds can be spread by flowing water as well as by birds, animals, and people.
Invasion of forests usually begins along the forest edge, and spreads along watercourses and trails. An aggressive competitor, garlic mustard can change soil chemistry, making it unfavourable for many native plants. Its early spring growth can ‘crowd out’ species such as trilliums, violets, wild ginger and many tree seedlings. Therefore, it dominates the undergrowth, often forming dense areas with a single species in invaded sites.
Garlic mustard invasions may lead to reduced biodiversity and loss of forest productivity and growth. They may also rob insects and other wildlife of suitable food or habitat by reducing or eliminating the required native plant cover. Some fauna may disappear from an area after garlic mustard starts to grow, and it has the potential to dramatically alter the long-term composition and structure of forests and natural areas. Once garlic mustard has become established at a site, control methods need to be repeated every year until the existing seed bank is exhausted or the population may re-establish itself and spread. Early detection and removal of garlic mustard before the first generation of seeds is produced is the best way to prevent its growth.