The PEI Invasive Species Council wants help monitoring the spread of a number of invasive plants.
There are four lists of different priority.

Garlic mustard and about a dozen other species are the top priority of plants that have already become established on PEI. Garlic mustard can cover the understory of a woodland and crowd out native species:

List of Top Priority Species

Plants that are higher than top priority are ones that have not been seen on the island but would become a serious issue if they were to become established, like kudzu, a vine that can completely overwhelm old buildings and live trees:

List of Possible Future Invasives

You should report the above plants whenever you see them.
More common plants on the next list include wild cucumber, virginia creeper and glossy buckthorn. The PEIISC is only interested in sightings of these in "natural areas".
Glossy buckthorn leaves and berries:

List of Common Plants

One final list is made up of flowers or ground cover from home gardens. Don't report them if you spot one in somebody's backyard, but report them in ditches or field edges or anywhere else.
Small Balsam:

List of Garden Plants

Many of these observations will not be viewable by the public because of privacy concerns, but we appreciate you sharing any sightings of these plants.

New, March 2018: There is a dedicated invasive species page that lists all the checklists and sightings.