Hesperis matronalis

Warning: this is a very invasive plant in southern Ontario Hesperis matronalis, known in English as "dame's rocket", "damask violet", "dame’s violet", "dames-wort", "dame’s gillyflower", "night-scented gillyflower", "queen’s gillyflower", "rogue’s gillyflower", "summer lilac", "sweet rocket", "mother-of-the-evening" or "winter gillyflower", and in French as julienne des dames is a biennial or short-lived perennial native to Eurasia. It can be distinguished from similar-looking phloxes by the four petals (phlox has 5 petals).

This is a very pretty flower, and it is easy to see why some gardeners allow it or even encourage it in their gardens. Unfortunately, here in southern Ontario dame's rocket is seriously invasive. According to Invasive Exotic Species Ranking for Southern Ontario, dames rocket is a Category 1 invasive (the worst category, like the dreaded garlic mustard). They describe Category 1 species as follows:

Aggressive invasive exotic species that can dominate a site to exclude all other species and remain dominant on the site indefinitely. These are a threat to natural areas wherever they occur because they can reproduce by means that allow them to move long distances. Many of these are dispersed by birds, wind, water, or vegetative reproduction.

These are the top priority for control, but control may be difficult. Eradication may be the only option for long-term success.

Similar-looking alternatives for Toronto gardeners include Phlox pilosa (native) or Phlox paniculata (not native, but not invasive and widely available).