Finger grass is a Southeast Asian herb which is most commonly used in Vietnamese cuisine, although it also appears in cuisine from other nations. The flavor of finger grass is a bit difficult to describe: it has a strong lemony note, and a hint of cumin, but it also tastes faintly soapy. This herb is available from Vietnamese markets, sometimes in both fresh and dried forms, and it can also be grown at home, for people who live in a warm climate.
This herb goes by a number of alternate names. Formally, it is known as Limnophilia aromatica, but it is also called rice paddy herb, ngo om, and shui fu rong, among other things. Many of the Asian names for this plant reference the fact that it prefers to grow in very moist environments, like rice paddies and streams, and it is often found near lakes and rivers. In addition to requiring moisture, finger grass needs hot weather and high humidity.
The plant has fleshy stems, small pointed green leaves which look sort of like thyme, and delicate lavender-colored flowers. All parts of the plant are usable, although the stems need to be minced finely before being added to a dish. Finger grass is a key ingredient in the traditional sour fish soup of Vietnam, and it is also used in pho, a popular noodle dish.
When a recipe calls for finger grass, shui fu rong, rice paddy herb, kayang, ngo om, rau om, or shiso-kusa, and you can't obtain this herb, you can try adding a little bit of cumin to the dish, along with a hint of lemon. Although this will not precisely replicate the flavor, it will approximate it, getting you close to the flavor the recipe is going for.
If you want to try growing finger grass, you can sometimes find starts at Vietnamese markets. The best way to grow this plant if you don't have a greenhouse is in a pot covered in a plastic bag with a few holes for ventilation. The bag creates a miniature greenhouse, keeping the plant humid and warm, and the pot makes it easy to move the finger grass around to find the best spot to grow it. These plants tend to prefer partial sun, as they will roast in plastic in full sun. You can also try growing finger grass in clear plastic soda bottles, which will create humidity and look a little neater than plastic bags draped over pots.