On the face of it the pattern of flower color in dame’s rocket (Hesperis matronalis), an introduced invasive that is blooming along road sides and stream courses over much of northern United States and Canada right now, would like to be a text book case of incomplete dominance. There are purple flowers, white flowers and pink flowers. It looks like if white flower is fertilized with pollen from purple plant, it will produce a lot of pink flowers.
In central New York state it is uncommon to see large stands of dame’s rocket that are only one color. They tend to be found in adjacent clumps of single colored flowers, as if they had spread vegetatively from an original plant that grew from a seed.
None of this turns out not to be the case. To work backwards from the most recent assumption, according to a paper in the Canadian Journal of Plant Science (Francis et al., 2008), Hesperis matronalis reproduces only from seed, not vegetatively. Furthermore the populations of central New York are not particularly typical in the pattern of flower color.
Francis et al. (2008) note that some Ontario populations include populations of plants with flowers with patterns of white, purple, and pink (co-dominance), populations of white and purple flowers plants (complete dominance), and some of the prairie provinces only purple flowered plants were found.
An actual look at H. matronalis genetics (according to Francis et al., 2008) reveals that the species has both diploid and polyploid populations and that number of chromosomes (n) varies from 7 to 28, and that flower color seems to be controlled by several different alleles.
The plant is abundant in most states, but in Canada seems to occur most commonly in the southern Ontario and the St. Lawrence River valley. By all accounts it has been in this hemisphere since it was introduced deliberately in the 17th century as an ornamental. It is still sold as a perennial in American nurseries in some states.
Interestingly it does not invade agricultural fields like a lot of “weeds” of European origin, but instead tends to grow along roadsides and woodland edges. In addition to merely crowding out native species and reducing biodiversity, H. matronalis also harbors several viruses that are contagious among the Cruciferae (also called Brassicaceae) taxa, which include important food plants like cabbage, broccoli, cauliflower (all derived from Brassica oleracea), turnips (B. rapa), rapeseed (B. napus; from which canola oil is derived), and radishes (Rhapanus sativus).
H. matronalis produces abundant seed. After it germinates it will produce a ground-hugging rosette for the first year of its existence, and then in the second year begin to grow taller. The leaves are lanceolate (long and thin; tapered at both ends) and hairy on both sides. Leaves closer to the ground attach to the stem by short petioles, but the leaves get smaller and the petioles get shorter as the plant grows taller.
The leaves are arranged alternately, which is one characteristic that can be used to distinguish from the native phlox (e.g., Phlox paniculata), which has similar looking pink, purple or white flowers, but they are five petaled, while like all members of the mustard (Brassicaceae) family, Hesperis flowers are four-petaled.
Because of its tendency to take over and produce if not a monoculture, then something close to it, horticulturalists recommend that dame’s rocket be controlled by physically removing the entire plant before it sets its seed. This can be done with a dandelion digger and the whole plant should be bagged and removed from your property. This will at least keep its numbers down. Eradicating it entirely is nowhere mentioned as practical, and some sources, particularly more southern ones, where the plant is more challenged by climate, do not seem as concerned as northern temperate region authorities.