Plant: Shrub or tree up to 18 m tall if not checked, branching strongly when cut.
Stems: Young stems are soft and green; older stems and the trunk have greyish bark.
Leaves: Feathery leaves (bipinnate), comprising 4 – 9 main branches, each with 11 – 22 pairs of small, dark blue-green “leaflets”
Flowers: Small and white, clustered into a round head (c.f. wattle flowers) mostly 2 – 3cm diameter.
Pods: Brown and flexible; usually about 15cm long and 2cm across, carrying up to 20 or more seeds.
Seeds: Flat, brown and shiny, 6 -10 mm long.
Usually grown in single rows (or double rows 70 – 90 cm apart), with 4 – 10m between rows; inter-row may be left as native pasture or sown to improved pasture. Inter-row width is increased as average rainfall declines.
Leucaena is mostly sown in subhumid areas (600 – 800mm annual average rainfall), but will grow well under much higher rainfall conditions.
It prefers fertile, well-drained, neutral to alkaline soils, but will grow on acid soils if aluminium levels are low.
Leaves are “burnt” by light frosts, and although stems are killed back to ground level by very heavy frosts, mature plants mostly recover.
• brigalow scrub soils: Floren bluegrass, buffel grass, creepingbluegrass, green panic, rhodes grass.
• heavier clay soils: Floren bluegrass, Bambatsi panic, purple pigeon grass,
• more humid areas: Floren bluegrass, humidicola, pangola grass, sabi grass, signal grass.
Legumes: burgundy bean, butterfly pea, caatinga stylo, desmanthus.
Sowing/planting rates as single species
It is normally sown at 1.5 to 3 kg/ha, depending on row spacing. This is roughly equivalent to a seed every 5cm. Seed should be scarified before inoculating and sowing, to reduce the high level of hard seed.
Sowing/planting rates in mixtures
Other species should be sown once the leucaena plants are well established. Careful cultivation or chemicals are used to control weeds after sowing.
Leucaena should be sown as early as possible in the growing season, because it is very slow to establish.
It requires special inoculum, either CB 3060 or CB 3126.
If soil analysis indicates available soil phosphorus to be below about 10 ppm, phosphorus fertiliser should be used at sowing, either broadcast, or more efficiently banded under the seed. Since leucaena is sensitive to sulphur deficiency, it is wise to use superphosphate (10% P, 10% S) if soil sulphur levels are low or unknown. Lime banded below the row is beneficial on acid soils.
Since leucaena is mostly grown in fertile soils, maintenance fertiliser is generally not necessary.
Plants should be grazed lightly when they reach about 1.5m tall to promote branching. It is best to restrict more mature plants to a maximum height of around 2m, using a combination of rotational and continuous grazing. It may be necessary to slash at 1m height if trees “get away”.
Ability to spread
Leucaena seed is readily moved by surface flow of water, or by passing through cattle. However, it normally does not spread much under grazing as cattle relish young seedlings. Some thickening up of grazed stands has occurred where leucaena is left ungrazed during the growing season for provision of autumn feed. Advice on minimising spread is incorporated into a Code of Practice, developed by the Leucaena Network, and in a Queensland Government Policy (see “Further Information”)
Leucaena produces large amounts of hard seed that can be moved by water or livestock. Dense thickets can form if the seed reaches ungrazed areas. The weed threat has been reduced somewhat by the appearance of flower-eating caterpillars and bruchid beetles. However, the best risk-minimisation approach is to abide by the above-mentioned Code of Practice.
The most important pest of leucaena is the psyllid, a small aphid-like sucking insect that attacks in large numbers, significantly reducing productivity (see “Cultivars”). Seed production can be reduced by the flower-eating larvae of a moth and by three species of seed-eating bruchid beetles.
There are no serious diseases of this species in Australia.
Leucaena is susceptible to triclopyr (240 g/L) +picloram (120 g/L) e.g. Access® as a basal bark treatment, or triclopyr (300 g/L) and picloram (120 g/L) e.g. Grazon DS® as a foliar spray.
Feeding value of leaves and young stems of leucaena is roughly equivalent to that of lucerne.
Steers on rain-grown leucaena plus grass can gain about 260 – 280 kg liveweight a year at a stocking rate of a beast to 1.5 to 2 ha. Under irrigation, and with a higher stocking rate (1 beast/0.8 ha) liveweight gain/ha/year can be doubled.
The alkaloid, mimosine, is high in young vigorous growth of leucaena. It can cause hair loss in non-ruminants, and reduced productivity in ruminants if leucaena comprises more than about 30% of the animal’s diet. In such cases, it is best to drench 10 – 20% of the herd with rumen fluid containing the detoxifying bacterium (Synergistes jonesii). The bacteria then spread quickly to other animals in the herd. A few inoculated cattle should be retained in the leucaena paddock to pass the bacterium on to new stock entering the paddock.