Limnophila aromatica (synonym: Limnophila chinensis var. aromatica; also called rice paddy herb) is a tropical flowering plant in the plantain family, Plantaginaceae. It is native to Southeast Asia, where it flourishes in hot temperatures and grows most often in watery environments, particularly in flooded rice fields. It is called ngò om or ngổ in Vietnam and used as an herb and also cultivated for use as an aquarium plant. The plant was introduced to North America in the 1970s due to Vietnamese immigration following the Vietnam War. It is called "roum om" (រំអម) in Khmer or Phnom Penh dialect "ma om" (ម្អម). It is used in all traditional Cambodian soup dishes. It can grow in flooded rice paddies during wet season but it grows best on drained but still wet sandy soil of harvested rice paddies for a few months after the rainy season ended. After the rain stops at the end of monsoon season in Cambodia, on the right soil, the herb grows everywhere like wildfire. It dies out soon after it flowers. Rural Cambodians often harvest them and put them on the roof of their houses to dry for later use.
L. aromatica has a flavor and aroma reminiscent of both lemon and cumin. It is used most often in Vietnamese cuisine, where it is called ngò om. It is an ingredient in canh chua, a sweet and sour seafood soup which also includes tamarind,not to be confused with ngò gai which is also added as an accompaniment to the noodle soup called phở. In Thai cuisine it is known as phak kayang and is also used to make om.
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- Rice Paddy Herb (Limnophila aromatica [Lomk.] Merril) page from Gernot Katzer's Spice Pages