Senna septemtrionalis aka Arsenic Bush

Senna septemtrionalis also goes by the name smooth senna. It belongs to Fabaceae, the pea family. Kew puts its native range from Mexico through Central America. Useful Tropical Plants says it reaches parts of South America too. These sources disagree on how far south its native range goes. Old names include Cassia septemtrionalis and Cassia laevigata. You may find these names in old garden books and plant records.

Smooth senna grows as a shrub that stays green year-round. At times, it grows as a tree instead. Some plants have more soft growth than woody stems. It can reach about 20 feet [6 metres], but often stays smaller. The sources give no clear height range for garden plants. Nor do they state how wide a full-grown plant gets. The species page shows its leaves, flowers, seedpods, and seeds. The captions show that ripe pods split and let seeds out. The text gives no leaf shape or flower colour for this species.

Kew links smooth senna with a hot, tropical climate with dry seasons. In the wild, it grows in damp thickets and hedges. It grows in moist forest and on waste ground too. It also grows where plants return after a site gets disturbed. Records place it at heights up to 7,546 feet [2,300 metres]. In gardens, it likes full sun and deep sandy loam. The soil should drain well and be no more than fairly rich. These soil needs still apply, though some wild sites are wet. The evidence gives no frost limit or hardiness zone.

Gardeners grow smooth senna for its looks and as a hedge. It has spread from gardens and become a weed in some places. This past spread matters when weighing its use near wild land. The sources name no pests or diseases for this species. Reports disagree on its links with soil bacteria. Thus, we cannot be sure that it can fix nitrogen. Growers can raise it from seed. Soaking seeds in warm water for 12 hours may help. Rubbing them with sand may help too. Both may make more seeds sprout and do so sooner.

People gather wild plants for local medicine and a coffee substitute. They use the seeds, not the leaves, for the drink. In Mexico, people have long used it to try to start menstrual flow. This use does not prove that it is safe or helps. Useful Tropical Plants lists no known hazards. That does not prove it is safe for people or pets.