Aliens Invade Alberta
Alien invasions are not confined to science fiction, they've happened in Alberta too! If you are reading this, you are an example of a successful invasive alien species, one that spread from Africa to invade the rest of the globe and change it irrevocably. The distinction between alien or exotic species and native species is artificial. Go back in time just a short interval, 10,000 years, and few of our native species were here --- they're all invasive aliens! At earlier, preglacial, times the flora and climate here were very different from what they are now. Go forward a short time, and new species will arrive, with or without human assistance. If the US had acted on the slogan "54o 40' or fight" and won, Iris missouriensis would be an exotic in relation to what would have been left of Alberta. Instead, it's a rare native, because the 49th parallel slices through the northern extremity of it's current range.
Now that we've disrupted most of what we think of as natural landscape, we need to think carefully about how we go about conserving what is left. That can't be done by stopping the clock and bring evolution and geologic change to a halt. The assemblages of species that we think of as natural ecosystems and use to classify landscapes are artifacts of our time. They have changed in the past, and will change in the future, and there's no reason why they should not.
To be sure, human intervention has altered the process of change, and made it possible for many species to establish here that might not ever have arrived, had we not been the vector of their introduction. This may justify corrective action, but it will be difficult to distinguish between changes that we have wrought and can reverse, and changes that would have happened anyway, or that are harmless.
The options available to us for controlling plants are not very selective. So, before going out on the warpath to eradicate an invasive alien, think carefully about why you want to, and what damage to other species might occur as a result of your efforts.
The Alberta Native Plant Council offers this Wiki to help users identify exotic plants, and find out more about them. This is a work in progress, not the final word about invasive non-native vs. native plants. At the outset, it is based on the Rogue's Gallery originally posted as a PDF on the ANPC Website. Illustrations and new links have been added, and obsolete links revised. Many of the linked sites include photographs and other species-specific information. Check them out while you can --- the target sites may disappear at web-speed. We hope that new and more comprehensive information will come to light to be included here. If you have reliable knowledge about any invasive alien, or merely exotic species naturalized in Alberta, please offer to contribute.
Rather than produce a document that is authoritative and beyond reproach, we err by inclusion. As a result, it is likely that many of the listed species, although exotic and recently arrived, are not actually invasive. Always bear in mind that the distinction between "native" and "alien" is not clear-cut. The sources consulted are not fully concordant. Their diversity of opinion survives in this compilation.
That some alien species are invasive seems distressingly evident. However, most exotic species are not threats. Although world trade and transport have increased the rate at which plants or propagules are introduced to new places, there has always been exchange. In a study of transoceanic plant colonizations in the Southern hemisphere, researchers have found that plants tend to colonize environments that are similar to the environment they evolved in, and that successful introductions to contrasting environments are relatively rare. Over 11,000 species were studied. Among these species 225.7 transoceanic colonizations have occurred (with uncertainty resulting in the fraction). 186.1 colonizations had similar source and destination environments (bog to bog, arid to arid, forest to forest, alpine to alpine, etc.). When a new species appears, it is thus likely that similarity of Alberta environments to it's home environment will be a good predictor of future spread. (Crisp et al, "Phylogenetic biome conservation on a global scale", Nature, Vol 458, p. 754, doi:10:1038/nature07764). 
Species that are highly invasive tend to be fast-growing species that are adapted to rich resources, but also subject in their original environments to competition, predation, and disease, factors which they escape when introduced to otherwise similar environments. (Blumenthal et al, "Synergy between pathogen release and resource availability in plant invasion") 
The majority of exotic plants are passengers, not drivers of change. When present in natural plant communities, they are more often a symptom, not the cause of disruption of the assemblage of species that existed there prior to their arrival. Anthropogenic disturbance and changes in land use are more often the drivers of recent change.