References

For Physostegia

Cantino, P.D. 1982. A monograph of the genus Physostegia (Labiatae). Contr. Gray Herb. 211.

Harley et al. 2004. In Kadereit, J.W. 2004. The families and genera of vascular plants. VII. Flowering plants – Dicotyledons – Lamiales (except Acanthaceae including Avicenniaceae). Springer, Berlin. 478 pp.