The herb known as Vietnamese coriander and sometimes used in pho is rau ram, scientifically called Persicaria odorata. Its Vietnamese name is also written rau răm, and it is sold under several regional names, including Vietnamese mint and laksa leaf.
Rau ram has narrow, pointed green leaves often marked with a dark or burgundy patch. Its flavor is peppery, citrusy, and coriander-like, but it is not botanically related to ordinary coriander or cilantro. Persicaria odorata belongs to the knotweed family, Polygonaceae, whereas cilantro belongs to the carrot family, Apiaceae.
The herb is more strongly associated with salads, spring rolls, seafood dishes, and other Vietnamese preparations than with every traditional bowl of pho. Some cooks add it to pho as a fresh garnish, particularly in regional or modern versions, which explains the wording “sometimes used.” Thai basil, cilantro, and sawtooth coriander are separate herbs commonly encountered alongside pho.
Rau ram grows as a perennial in warm, damp climates and is also important in Malaysian, Thai, Cambodian, and Laotian cooking. Its sharp flavor is best appreciated fresh; drying substantially changes its aroma and texture.