Not a herb that I've experienced here in Europe, but I may well have eaten it in Vietnam, where it is commonly used.
Rice paddy herb plants in flower
Variegated cultivar of rice paddy herb
Rice paddy herb leaves
Several species of limnophila are found in still waters in South-East Asia and some of these are common aquarium plants in the West. In Vietnam, rice paddy herb is mostly cultivated in flooded rice fields, hence the name. The related species l. rugosa has anise-scented leaves and is sometimes used for culinary purposes in Java. Because of the plant's dependence on high temperatures and high humidity, rice paddy herb cultivation is a challenge outside the tropics.
Scrophulariaceae (figwort family).
This herb has a unique citrus flavour that has been variously described as "sharp lemon", "sweet cumin" and "the air after a thunderstorm in summer".
The genus name limnophila is formed from the Greek limnē "pond" and philos "friend" and refers to the plant's natural habitat. Whereas other members of the genus only grow submersed, species l. aromatica can be cultivated both above and below the water surface. Species name aromatica derives from Latin aroma, from Greek arōma "aromatic".
In several Eastern Asian languages there are very close ties between the names of rice paddy herb and perilla (in some cases they are identical). Chinese zi su, Korean soyop and Japanese shiso usually mean "perilla" but can also mean rice paddy herb, especially if expanded by an element meaning "grass" or "herb" as in zi su cao, soyop-pul and shiso-kusa.
The English name "rice paddy herb" refers to the growth of the plant in flooded rice fields, paddy meaning "wet rice field" (derived from Malay padi "uncooked rice"). English common name "finger grass" is misleading, however, because this term also designates the quite different plant eleusine indica "yard grass", which has flowers in finger-like spikes.
Rice paddy herb is one of the many culinary herbs used only or predominantly in Vietnamese cuisine (other examples include chameleon plant, Vietnamese coriander and long coriander). The flavour of rice paddy herb is as much unique as it is pleasant and the plant deserves wider recognition and usage. It appears occasionally in Cambodian and Thai recipes but nowhere else.
The intense, almost sparkling lemon odour of this plant harmonises perfectly with freshwater fish. In Southern Vietnam, its use is almost mandatory for spicy (sweet-sour-hot) fish soups (canh chua), in which the herb is not cooked but served raw and coarsely chopped as part of the herb garnish that uniquely identifies Southern Vietnamese food. Canh chua is a mild version of the fiery Thai soup, tom yam. Cambodian cuisine has an almost identical soup called samlor machu trey and this is perfectly complemented by rice paddy herb.
The flavour of rice paddy herb goes particularly well with mild Vietnamese curries, e.g. ca ri ga "chicken curry". As with Thai curries, Vietnamese curries are based on coconut milk, but they are lightly flavoured with a dash of chilli, lemon grass and (as a French heritage) curry powder. The intensive red-orange colour often comes from annatto oil. If rice paddy herb is not available, lemon- or anise-flavoured basil or coriander leaves make good alternatives.
Most Vietnamese cookbooks omit reference to this plant (substituting basil, mint or coriander) as rice paddy herb is difficult to cultivate (and hence difficult to obtain) outside of South-East Asia. One successful approach to cultivation in the West involves placing fresh stems in water, covered with a plastic bag, until they develop roots. The shoots are then planted in a tall, transparent container filled with a mixture of soil and small, porous grains of burned clay. After a few days kept warm and humid the plants grow strongly and they tolerate (even appreciate) full sunlight.