Trees, up to 40 m high.  Stem erect; bark smooth, scaly.  Leaves simple, alternate, aromatic.

Many species were introduced into North and South America and almost all of them are used in traditional medicine.  Furthermore, hybridization has occurred between the different species which makes identification difficult.  Therefore, no detailed description is provided for a species.

Annual herb ranging from erect to creeping habit.  Leaves opposite, rough on both surfaces, sessile, elliptic-lanceolate, 8 cm x 2 cm, margin serrulate. Inflorescences axillary on long stalks; flower-heads 1 cm across, florets white; achenes black, wrinkled, without pappus appendages.