This little weedy plant keys out to Lythrum hyssopifolia L. (Hyssop Loosestrife). It was found on the South Tract on Telegraph Road near the old Beltsville Airport in a wet area next to the road. The Hyssop Loosestrife is in the same genus as the more notoriously invasive Purple Loosestrife (Lythrum salicaria L.)
The Hyssop Loosestrife is a biennial or an annual growing between 10 to 60 cm (4 to 24 inches) high. The flowers are borne in the leaf axils.
It is native to Europe, but has become naturalized on the East and West Coasts of the USA, and in Australia. It prefers temporary wet marshy habitats, like where it was found on the refuge. This is a new record for the refuge.
According to the International Union for Conservation of Nature (IUCN), this species is classed as Endangered (A2c) in the UK and Critically Endangered in Switzerland and is under regional protection in France: in Alsace and Rhône-Alpes.