Common Name: Siamese Senna

General Information

Senna siamea is a medium-size, evergreen tree with a crown that is usually dense and rounded when young, later becoming irregular and spreading with drooping branches[]. It grows up to 18 metres tall, with a short, straight bole that is up to 30cm in diameter[].
This is one of 33 species named as being a suitable Hongmu timber, used for producing high quality Chinese furniture following traditions from the Ming and Quing dynasty, which makes it particularly valuable[]. The tree is also particularly valued for its high quality firewood, and it is widely cultivated in the tropics both for this and for its many uses in agroforestry systems[]. It is also grown as an ornamental and frequently planted along the sides of roads[].
Senna siamea has been introduced and grown for timber, fuel wood or for ornamental value. Its use for reforestation activities is also growing. Overall, the species is widespread and its population is not considered to be in decline or threatened. The plant is classified as 'Least Concern' in the IUCN Red List of Threatened Species(2018)[].

Known Hazards

The sawdust may cause some irritation to the nose, throat and eyes[].
The wood sometimes produces a yellow powder that may cause irritation to the skin[].

Botanical References

Range

Southeast Asia - Myanmar, Thailand, Malaysia, Cambodia, Laos, Vietnam.

Habitat

Various types of forests at low elevations[]. Secondary forest formations on the plains[].

Properties

Weed Potential	Yes
Conservation Status	Least Concern
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Tree
Height	15.00 m
Growth Rate	Fast
Cultivation Status	Cultivated, Ornamental, Wild

Cultivation Details

The tree will grow in a range of climatic conditions, but is particularly suited to the lowland tropics with a monsoon climate, where it can succeed at elevations up to 1,300 metres[]. It grows in areas where the mean annual rainfall ranges from 500 - 2,800 mm with an optimum of about 1,000 mm[]. The maximum length of the dry period should not exceed 4 - 8 months[]. Under semi-arid conditions (rainfall of 500 - 700 mm), it will only grow when its roots have access to groundwater[]. It requires a mean minimum temperature of 20°c, ranging from 14 - 28°c, and a mean maximum temperature of 31°c, ranging from 24 - 36°c[]. It is susceptible to cold and frost[].
Requires a sunny position[]. It grows best on deep, well-drained, fertile soils, but will succeed on degraded, lateritic soils provided drainage is not impeded[]. It grows poorly on infertile, poorly drained podzolic soils[]. It is not tolerant of salinity, but is reasonably tolerant of acid soil conditions[]. It prefers a pH in the range 5.5 - 7.5[].
Often grown as an ornament, the plant has escaped from cultivation in the Americas and has become abundantly naturalized in some areas[]. In Australia the species is considered invasive and it has the potential to become as prolific in other tropical locations[].
Trees grow fast even in comparatively infertile soils[].
Seedling trees start flowering and fruiting at the age of 2 - 3 years[]. Once established, they flower precociously and abundantly throughout the year[].
Planting density varies according to use. In fuel wood plantations, spacing ranges from 1 x 1 metre to 1x 3 metres. In hedges used for alley cropping or as a shelterbelt, spacing between plants in the row should be 25 - 50 cm[].
Trees respond well to coppicing[]. For the production of fuel wood and charcoal, plantations are generally pollarded or regenerated by coppice leaving 2-3 shoots/stump after 1 year. It has been reported that sapwood should be removed as soon as possible after felling to prevent insect attack of the heartwood[].
The root system consists of a few thick roots, growing to considerable depth, and a dense mat of rootlets in the top 10 - 20 cm of soil, which may reach a distance of 7 metres from the stem in 1 year and eventually a distance up to 15 metres[].
Although many species within the family Fabaceae have a symbiotic relationship with soil bacteria, this species is said to be devoid of such a relationship and therefore does not fix atmospheric nitrogen[].

Edible Uses

The young fruits and leaves are eaten as a vegetable. During preparation the cooking liquid is replaced 3 times to remove toxins[].

The flowers and young fruits are used in curries[].

Medicinal

In traditional medicine, the fruit is used to charm away intestinal worms and to prevent convulsions in children[].

The heartwood is said to be a laxative, and a decoction is used against scabies[].

Agroforestry Uses:

The tree is grown to provide shade along roads and in cocoa, coffee and tea plantations. It is also planted as a dense windbreak and shelterbelt[, ]. It is pruned into hedgerows and used as a live fence around food crops[]. When used as a hedgerow, it effectively increases topsoil infiltration, reducing runoff and combating soil erosion[].
The leaves are used as green manure, and a well-grown tree can yield 500 kg/year of fresh leaves. Senna siamea forms ecto-mycorrhizae and provides very useful mulch, especially in alley-cropping systems[].
It is used extensively for rehabilitation of degraded land, for example, to re-vegetate aluminium mine tailings[].
Although not a nitrogen-fixing tree, it has been increasingly used in alley cropping systems, largely because of its coppicing ability and high biomass production[].
In India, it is used as a host for sandalwood (Santalum spp.), a parasitic tree producing the well-known aromatic wood[].

Other Uses

All parts of the plant can be used for tanning. The concentrations of tannin vary slightly from 17% in the leaves to 9% in the bark and 7% in the fruits[].

The heartwood is black-brown with paler streaks, sharply demarcated from the 6cm wide band of pale sapwood[]. The grain is interlocked and occasionally straight; the texture is slightly coarse but even[]. The wood is medium-weight to heavy, hard to very hard, resistant to termites, strong and durable[]. It is difficult to work, with a tendency to pick up in planing and it takes a high polish[]. The dark heartwood, which is often nicely figured, is used for joinery, cabinet making, inlaying, handles, sticks and other decorative uses[]. The wood has also been used for poles, posts, bridges, mine poles and beams[].
The dense wood makes good fuel, although it produces some smoke when burning[]. The energy value of the wood is 22 400 kJ/kg[]. The wood was formerly preferred for locomotive engines[]. Its charcoal is also of excellent quality[].

Propagation

Seed - requires pre-treatment to soften the hard seedcoat and allow the ingress of water[]. This can be done by soaking the seed in a small amount of nearly boiling water (which cools down quickly and does not cook the seed) and then soaking the seed for 12 - 24 hours in warm water. Alternatively, a small area of the seed coat can be abraded, being careful not to damage the embryo[]. Germination of treated seed is about 90% within 60 days. Germination of untreated seeds is about 75% in 4 - 29 days[]. The seed is usually sown in situ[]. Seeds should be sown in areas with full sunlight, as the slightest shade reduces germination considerably[]. Early seedling growth can be quite slow, reaching only 29 cm after 8 weeks of planting[].
Storage behaviour is orthodox. Viability can be maintained for 3 years in hermetic storage at room temperature with 11-15% mc. There are 35 000-45 000 seeds/kg[].

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