One of the main characteristics of a plant for a given territory is indigeneity. A plant for a given territory is either native or introduced, the definition of a native plant is : a species is defined as indigenous to a region if its presence in this region is the result of natural processes without human intervention.
A plant that does not meet this definition is, in turn, qualified as introduced. But this distinction - native or introduced- is artificial and often misleading. Misleading because, plant, described as native, does not mean that the species necessarily outcome of the place where it is classified as so. In fact the plant may have appeared there by simply increasing its territory by reproduction or its seeds were brought by a weather event or by an animal (other than humans).
For its part, the concept of introduced is artificial and is determined by men. Indeed it's impossible to know all human populations that occupied a territory, through the history of mankind. Therefore we fix a date, a time, beyond which a new species brought by humans in a given territory, is described as "introduced"
In Europe, it's a puzzle as to establish such a date, 1914 was selected for the Paris Basin, but this date can vary greatly depending on location and legislation, to 6000 years before Christ and even before in the absolute. We need only to imagine all the population migrations, including the establishment of the Roman Empire, to give us an idea of the difficulty of identifying plants introduced or not by man over a territory as Europe.
In Quebec and in the rest of North America, this limit corresponds to the establishment of the first European settlements near the XVII century (1600 and older) But this does not mean that there has been no another introduction in Quebec before this, for example by Basque fishermen around the XIV centuries or by the Vikings around the year 1000. So, while useful be aware that the indigeneity of plants (native or introduced) is not an absolute concept.
One of the first " modern introduced" in Quebec was the Common plantain
(Plantago major) that served the early settlers as a drugs and for food. Native did not appreciated the plant which damaged the ecosystem close to European establishment, they have nicknamed this plant "Whiteman's-foot" or "White man's-footprint"