Lythrum

1. Flowers in axillary clusters of 2–several, the ± ovate-acuminate bracts and floral tube pubescent; petals ca. 7–11 mm long.

L. salicaria

1. Flowers solitary in the axils of linear-lanceolate leaves or bracts, these and the floral tube glabrous; petals ca. 2–6.5 (–7) mm long.

2. Petals ca. 4–6.5 (–7) mm long; flowers borne in the axils of small linear bracts on the distal half of the plant, which bears larger leaves below.

L. alatum

2. Petals ca. 2–3 mm long; flowers borne all (or almost all) the way to the base of the plant in axils of small ± linear leaves.

L. hyssopifolia

All species found in Lythrum

Lythrum alatumWINGED LOOSESTRIFE 
Lythrum hyssopifoliaHYSSOP LOOSESTRIFE 
Lythrum salicariaPURPLE LOOSESTRIFE 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 10, 2017. http://www.michiganflora.net/genus.aspx?id=Lythrum.