Leaf arrangement is variable, so don't rely strictly on whether the leaves are opposite or alternate, but also use the flower characters in the key. When measuring the length of the stigmas, include the basal portion that is decurrent onto the ovary, and be aware that the stigma does not reach its full length until mid to late August. By then the ovary will be rather plump and most of the anthers will have fallen from the stamens. Reference: Gillett (1959).
This Genus’s Species in New England:
Visit this genus in the Dichotomous Key