Persicaria odorata (PROSEA)
Persicaria odorata (Lour.) Soják
- Protologue: Preslia (Prague) 46: 154 (1974).
- Family: Polygonaceae
- Chromosome number: 2n = unknown
- Polygonum odoratum Lour. (1790).
- Rau ram, Vietnamese coriander (En).
- Renouée odorante (Fr)
- Cambodia: chi krassang tomhom
- Laos: phăk ph'è:w
- Thailand: chanchom (Nakhon Ratchasima), phakphai (northern), homchan (Ayutthaya)
- Vietnam: rau răm
Origin and geographic distribution
Rau ram is native to and is cultivated in Indo-China. Since the 1960s its cultivation has spread with Vietnamese migrants, mainly to Australia, the Philippines and the United States.
The leaves of rau ram are used to flavour many Vietnamese dishes. The fresh leaves are eaten in salads and also with incubated duck eggs, while fresh or cooked leaves are used in various fish, shellfish (mussels, clams, oysters), turtle and frog dishes. It imparts a flavour reminiscent of lemon and coriander leaves with a slight radish-like pungent aftertaste. The flavour is destroyed by prolonged cooking.
A few shoots of rau ram and water dropwort (Oenanthe javanica (Blume) DC.) are often added when preparing cabbage preserved in brine (like sauerkraut). Although relished by the Vietnamese, the flavour of rau ram is not universally admired, though it is liked by some people who do not appreciate the taste of coriander leaves.
Medicinally the leaves are used as a diuretic, stomachic, febrifuge and anti-aphrodisiac. Externally the crushed leaves are applied against fever, vomiting, ringworm and phagedaena. Juice prepared from the crushed leaves is taken as an antidote against poisonous snake bite, and the bite is covered with the residue of the leaves. In Vietnam pregnant women avoid the use of rau ram, since fresh leaves seem to have abortifacient properties.
The nutritional value of rau ram is unknown. The leaves contain a yellowish essential oil, consisting mainly of alkane aldehydes. Drimane sesquiterpenoids, which are under investigation for their antifungal and anticarcinogenic properties, are present in some Persicaria and Polygonum species, but have not been found in P. odorata.
Rau ram leaf oil (from Australia) (Source: Hunter et al., 1997.)
- 44.1% dodecanal
- 27.7% decanal
- 10.9% 1-decanol
- 3.8% β-caryophyllene
- 2.6% 1-dodecanol
- 1.5% α-humulene
- 1.1% undecane
- 0.4% tetradecanal
- 0.3% 3-hexenol-1
- 0.3% 1-undecanol
- 0.2% caryophyllene oxide
- 0.2% 6-methyl-5-hepten-2-ol
- 0.1% 6-methyl-5-hepten-2-one
- 0.1% (E)-β-ocimene
- 0.1% undecanal
- trace nonanal
- trace (Z)-β-ocimene
- 93.4% total
Adulterations and substitutes
Besides coriander leaves, leaves of Eryngium foetidum L., known as sawtooth coriander or "Mexican coriander”, are used as a substitute.
- Short-lived perennial, fragrant herb, somewhat glandular in all parts. Stem ascending, 30-35 cm tall, 2-3 mm in diameter, red, grooved; base trailing and forming roots at all nodes, much thicker than upright part.
- Leaves alternate; ocrea membranous, short, up to a quarter of the length of the internode, loosely enveloping the stem, parallel veined, each vein culminating at apex in a long silky hair, with some glandular dots in horizontal lines; petiole attached to basal part of ocrea; blade entire, lanceolate to ovate-lanceolate, base attenuate, apex acuminate or obtuse, green, marked with red, margins and veins, especially the midrib, with appressed, fairly long hairs.
- Inflorescence an axillary, long, many-flowered, narrow spike, single or in pairs or in a small cluster; bracts long and funnel-shaped, with long hairs on margins.
- Flowers hermaphrodite; perianth pentamerous, white to purplish-pink, persistent in fruit; stamens 8; styles 3.
- Fruit triangular, 1.5 mm long, acuminate at both ends, smooth and shiny.
Growth and development
Rau ram is a tender perennial. In Vietnam and the Philippines flowering is profuse and starts in the first year. In Vietnam flowering occurs in August-September and fruiting in September-October. In the United States rau ram flowers only occasionally and only after vernalization. Harvesting can continue when plants are flowering. Under greenhouse conditions in the United States leaves can be harvested year-round.
Other botanical information
In popular horticultural handbooks confusion between Persicaria odorata and P. hydropiper (L.) Spach (synonym: Polygonum hydropiper L.) is common. The synonym Polygonum odoratum is occasionally confused with Polygonatum odoratum (Mill.) Druce (Liliaceae).
Rau ram requires warm and humid growing conditions. Light frost is probably tolerated. It grows best under partial shade, but full sunlight is tolerated if ample moisture is available. In Vietnam it can be grown and harvested year-round, but it grows best in spring. In cold and dry winters in northern Vietnam it may wither away. Fertile soils with adequate soil moisture are essential for optimal production. Under drier conditions, the stem base becomes woody and the leaves turn yellowish.
Propagation and planting
Rau ram is usually and easily propagated by stem cuttings with 4-6 internodes (8-10 cm long) taken from the top of mature stems. These are planted obliquely 5-6 cm apart with a row spacing of 10-15 cm in raised beds of light, well-manured soil and are watered well. Under warm and humid conditions cuttings start rooting after 3-5 days and growing after about a week. They are planted out in the field at a spacing of about 20 cm × 20 cm. In lowland Vietnam cuttings are sometimes planted directly in well-prepared and manured muddy rice fields.
Seed is difficult to obtain in the United States and Australia.
Rau ram can be rejuvenated by cutting back the stems.
Diseases and pests
Occasional damage to rau ram from diseases and pests is reported from Vietnam, but the causal agents are not known. In the United States fungal diseases and slugs sometimes affect rau ram. Providing more sunlight can reduce damage. Caterpillars are the main problem in the Philippines.
The first harvest of rau ram is taken when plants are nearly 2 months old; subsequent harvests are every 12-15 days in Vietnam, and every 3-4 weeks in the United States. In home gardens leaves are picked when required. In commercial plantings whole tops are harvested.
Under very intensive cultivation in small market gardens yields of fresh tops of rau ram may reach 1.3 t/ha per harvest or about 15 t/ha per year.
Handling after harvest
Leaves of rau ram should be kept cool and moist to maintain their fresh appearance.
Genetic resources and breeding
It is unlikely that germplasm collections of rau ram are being maintained and there are no breeding programmes.
Migration of Vietnamese people has greatly increased interest in Vietnamese cuisine in many countries, which will be a strong incentive to increase production of rau ram. As people become more familiar with its flavour, rau ram may become an alternative for coriander leaves, as many who do not like coriander find the taste of rau ram more acceptable.
- Courchet, L., 1910. Polygonaceae. In: Lecompte, H. (General editor): Flore générale de l'Indochine [General flora of Indo-China]. Vol. 5. Masson, Paris, France. p. 29.
- Do Tat Loi, 1995. Nhung cay thuoc va vi thuoc Viet Nam [Medicinal plants and products of Vietnam]. Science and Technics Publishing House, Hanoi, Vietnam. pp. 689-690.
- Kuebel, K.R. & Tucker, A.O., 1988. Vietnamese culinary herbs in the United States. Economic Botany 42: 413-419.
- Nguyen Thi Do, 1994. Polygonum odoratum Lour. - Rau ram. Journal of Biology 16(4): 44-45.
- Nguyen Van Duong, 1993. Medicinal plants of Vietnam, Cambodia and Laos. Mekong Printing, Santa Ana, California, United States. pp. 343-344.
- Potter, T.L., Fagerson, I.S. & Craker, L.E., 1993. Composition of Vietnamese coriander leaf oil. Acta Horticulturae 344: 305-311.
- Small, E., 1997. Culinary herbs. National Research Council of Canada. NRC Research Press, Ottawa, Canada. pp. 491-494.
- Soják, J., 1974. Bemerkungen zur Gattung Truellum Houtt. (Polygonaceae) [Remarks on the genus Truellum Houtt. (Polygonaceae)]. Preslia (Prague) 46: 139-156.
- Vo Van Chi, 1997. Rau ram. In: Dictionary of medicinal plants. Medicinal Publisher, Hanoi, Vietnam. pp. 960-961.
Sources of illustrations
Do Tat Loi, 1991. Nhung cay thuoc va vi thuoc Viet Nam [Medicinal plants and products of Vietnam]. Science and Technics Publishing House, Hanoi, Vietnam. Fig. 425, p. 611. Redrawn and adapted by P. Verheij-Hayes.
- L.P.A. Oyen & Dzuong Duc Huyen