Decumbent to erect with many stems arising from the central rootstalk. Inflorescence is an oblong spike of small white flowers - spike measured at 4 cm long, flower at 3 mm long. Leaves are all cauliine, alternate, entire, lanceolate in shape. Leaf highlighted in photo above was 4 cm long and 9 mm wide. Stems sparsely hairy. Leaf margins, top and bottom have tiny hairs.
Habitat is openings in woods and semi-wooded prairie. Fairly common.
Height listed in Budd's Flora to 50 cm. We measured plants to 33 cm tall.
This plant might be mistaken for Polygala alba which has a similar growth habit and inflorescence. However that plant has linear leaves, whereas Polygala senega has lanceolate leaves. Budd's Flora and Taxonomic Reminder for Recognizing Saskatchewan Plants also both list as a distinguishing characteristic the shape of the inflorescence - oblong for P. senega, cylindrical for P. alba. I haven't found this quite as helpful because I've seen oblong spikes on both plant species.
The above photos were taken June 19th and July 3rd, edge of mixed woods, boreal forest, about 400 km north of Regina, Saskatchewan.