Delonix regia (flamboyant)

D. regia, the flamboyant or flame tree, is a very popular and most beautiful ornamental tree that has been very widely introduced to tropical countries around the world. Recently, however, it has been observed to be naturalizing in many countries, and has become invasive in Australia, and on Christmas Island and a number of Pacific islands. It has a tendency to form monocultures and prevent the regeneration of native species. It is possible that this species will exhibit invasive potential in other countries.

The genus Delonix belongs to the legume family (Fabaceae), subfamily Caesalpinioideae. It includes 12 species, 9 of which are native to Madagascar (Puy et al., 1995), the others native to East Africa, Arabia and parts of India (Menninger, 1962). Three varieties are recognized, the type var. regia, var. flavida Stehle and var. genuina Stehle (ILDIS, 2008). However, there appears to be disagreement as to the naming authority, with (Bojer ex Hook.) Raf. maintained in this datasheet (Missouri Botanical Garden, 2009), but also used are (Bojer) Raf. (USDA-ARS, 2009), and (Hook.) Raf. (ILDIS, 2008).

D. regia is a tall tree reaching a height of more than 15 m and a girth of 2 m under favourable conditions. The bole is short. The trunk is buttressed and the stem form above the buttress is generally normal in taper (Webb et al., 1984). The trees are almost evergreen, with broad-spreading, open, umbrella-shaped crowns (Randhawa, 1965). It is deciduous in localities which experience long pronounced dry seasons (Streets, 1962; Yusuf and Sheikh, 1986). The bark is grey or brown, smooth or slightly rough, and exfoliating (Gamble, 1902; Sheikh, 1993).The compound leaves of D. regia are bipinnate and feathery, up to 60 cm long, pinnae 11-18 pairs, petiole stout. The leaflets are in 20-30 pairs on each pinna, oblong, 7.5-10 mm long, 3.4-5 mm wide (Gamble, 1902; Randhawa, 1965). At the base of the leaf, two stipules occur which have long, narrow comb-like teeth (Luna, 1996). The inflorescence of D. regia is a lax terminal or axillary raceme. The flowers appear in corymbs along or at the end of branches and are large, 10 cm across and bright red. They vary considerably in intensity of colouring, ranging from orange-vermillion to deep scarlet. Most of the common names for D. regia are derived from the colour of its flowers. The pods are 5 cm broad and 30-60 cm long, ending in a beak when mature (Luna, 1996). They are green and flaccid when young and are compressed, firm and rather thick when mature. Seeds are large, yellowish, oblong, arranged at right angles to the length of pod and transversely mottled (Parker, 1956; Hutchinson, 1964; Ali, 1973). The seeds have a hard, bony testa.

D. regia is native to Madagascar but has been very widely planted in the tropics, though it is now very rare in its native range (Mabberley, 1997). In Madagascar, its latitudinal range is 12-25°S (Webb et al., 1984).

D. regia is widely planted in warm, moist sites in the tropics as an ornamental or avenue tree along roads, in homesteads, in parks and in gardens. It has been planted in the Indo-Pakistan subcontinent for more than 100 years and has also been introduced to many other countries in the Old and New World. The introductions to these countries have been largely successful, and it has become naturalized in those localities which do not experience frost during winter (Menninger, 1962; Streets, 1962; Webb et al., 1984).

It is likely that D. regia already exists in every tropical country of the world, and thus, being cosmopolitan, it cannot be introduced any further. Also, it registered the lowest score possible, of 1, in a weed risk assessment for the Pacific, indicating a very low risk.

The large and characteristic pods, 30-60 cm (1-2 ft) long, that are retained on the tree for long periods, make this species unmistakable for most of the year.

It is naturally found in wet tropical forests in Madagascar from sea level up to 2000 m. Where introduced and invasive, it is found in arid lowlands and moist uplands in the Galápagos Islands, low elevation and dry to mesic disturbed sites in Hawaii, infrequently naturalized from near sea level to about 500 m in Fiji, almost monospecific stands around parent trees within disturbed marginal rainforest and along roadsides, although slow to spread on Christmas Island, and has invaded coastal monsoon vine thickets that have been damaged by cyclones in the Northern Territory, Australia (PIER, 2009).

Pests

D. regia plants are susceptible to attack from termites and shoot borers (Salman et al., 1987; Webb et al., 1984). Pteroma plagiophleps is a serious pest of avenue plantings of this species. A severe outbreak of the bostrichid Sinoxylon anale was observed on D. regia in Israel in 1984 (Argaman, 1987). Similarly, larvae of the noctuid Pericyma cruegeri have been observed causing severe defoliation of trees in Nagaland, India (Reddy et al., 1985).

Acanthopsyche reimeri is a bagworm of tropical Africa. Its larvae are polyphagous, feeding on the leaves of various dicotyledonous trees, principally open-grown trees and ornamentals. It has caused severe defoliation of D. regia in Kenya (Gardner 1957).

Anoplocnemis curvipes is a bowlegged bug and is widely distributed in tropical Africa. Both the adults and nymphs of this species are polyphagous, feeding on the sap of many agricultural and garden plants (Sweeney 1961). In Malawi it has been recorded on D. regia, and is a pest of some importance as heavily infested shoots become disfigured and increment is considerably reduced; it has even been known to kill 1-year-old plants.

Leptostylus praemorsus has been recorded in Antigua, Bermuda, Dominica and St. Lucia. A longhorn beetle, known principally as a pest of citrus, it also infests other trees, both dicotyledons and conifers, including D. regia (Duffy, 1960; Parker, 1945).

Orthezia insignis is also very widely distributed in the tropics, subtropics and warmer parts of temperate zones. In Malawi, it is frequently injurious to D. regia and other trees, principally ornamentals, and often kills seedlings or even fully-grown trees if heavily infested.

Oxyrhachis latipes is a tree-hopper, which feeds on the sap of D. regia in Malawi. Records of infestation are few and it is apparently unimportant. Injury from Schedorhinotermes lamanianus has been recorded on D. regia by Gardner (1957). The beetle and larvae of Poecilips sierraleonensis can bore into the pods of D. regia and damage the seed (Luna, 1996).

Diseases

A Ganoderma sp. has been observed attacking seedlings of D. regia in Australia (Hood et al., 1996). Root rot is caused by Fusarium oxysporum in the northern Guinea region of Nigeria (Gbadegesin, 1993). Root and butt-rot disease is characterized by affected parts slowly enlarging and development of a thick, dark brown mycelial sheath around the bases of infected trees. Wilting and discoloration of the leaves and development of brown mycelial mats on roots and basal stems, followed by death of D. regia plants, has been reported by Chang (1992).

A fungus, Pleiochaeta setosa, has been noticed on D. regia in India. This attacks the cotyledons of germinating seedlings and the leaves of young seedlings, causing shrivelling, leaf death and leaf shedding. The seedlings however, do not die.

The well known root rot fungus Armillaria mellea has a worldwide distribution and extensive host range, including D. regia (Spaulding 1961). Thick, white mycelia form a felty sheet between bark layers and also between the dead bark and underlying wood.

Sphaerostilbe repens, known as stinking root disease, affects D. regia. Infection is by waterborne spores through root contact. It produces dark brown or reddish rhizomorphs beneath the root bark. The inner surface of the root is bleached and a strong odour is produced due to the combined activity of fungus and bacteria (Browne, 1968).

D. regia plants are susceptible to attack from termites and shoot borers (Webb et al., 1984; Salman et al., 1987). Pteroma plagiophleps is a serious pest of avenue plantings of this species. A severe outbreak of the bostrichid Sinoxylon anale was observed on D. regia in Israel in 1984 (Argaman, 1987). Similarly, larvae of the noctuid Pericyma cruegeri have been observed causing severe defoliation of trees in Nagaland, India (Reddy et al., 1985). Acanthopsyche reimeri is a bagworm of tropical Africa, with polyphagous larvae feeding on the leaves of various dicotyledonous trees, principally open-grown trees and ornamentals. It has caused severe defoliation of D. regia in Kenya (Gardner, 1957). Anoplocnemis curvipes is a bowlegged bug and is widely distributed in tropical Africa, with both the adults and nymphs being polyphagous, feeding on the sap of many agricultural and garden plants (Sweeney, 1961). In Malawi it has been recorded on D. regia, and is a pest of some importance as heavily infested shoots become disfigured and increment is considerably reduced; it has even been known to kill 1-year-old plants. Leptostylus praemorsus has been recorded in Antigua, Bermuda, Dominica and St. Lucia. A longhorn beetle, known principally as a pest of citrus, it also infests other trees, both dicotyledons and conifers, including D. regia (Parker, 1945; Duffy, 1960). Orthezia insignis is also very widely distributed in the tropics, subtropics and warmer parts of temperate zones. In Malawi, it is frequently injurious to D. regia and other trees, principally ornamentals, and often kills seedlings or even fully-grown trees if heavily infested. Oxyrhachis latipes is a tree-hopper, which feeds on the sap of D. regia in Malawi, though records of infestation are few and it is apparently unimportant. Injury from Schedorhinotermes lamanianus has been recorded on D. regia by Gardner (1957). The beetle and larvae of Poecilips sierraleonensis can bore into the pods of D. regia and damage the seed (Luna, 1996). During January 2001, a large scale outbreak of the giant looper, Boarmia selenaria [Ascotis selenaria], was observed on several trees of D. regia and Prosopis juliflora in Coimbatore, Tamil Nadu, India, causing severe defoliation (Rabindra et al., 2003).   Ganoderma tropicum was isolated from a D. regia tree in India, being parasitic to the tree and quite aggressive, killing the tree within 1-5 years (Aryantha et al., 2001), and an unidentified Ganoderma sp. has been observed attacking seedlings of D. regia in Australia (Hood et al., 1996). Root rot is caused by Fusarium oxysporum in the northern Guinea region of Nigeria (Gbadegesin, 1993), with root and butt-rot disease characterized by affected parts slowly enlarging and development of a thick, dark brown mycelial sheath around the bases of infected trees. Wilting and discoloration of the leaves and development of brown mycelial mats on roots and basal stems, followed by death of D. regia plants, has been reported by Chang (1992). A fungus, Pleiochaeta setosa, has been noticed on D. regia in India. This attacks the cotyledons of germinating seedlings and the leaves of young seedlings, causing shrivelling, leaf death and leaf shedding, but not mortality. The well known root rot fungus Armillaria mellea has a worldwide distribution and extensive host range, including D. regia (Spaulding, 1961). Thick, white mycelia form a felty sheet between bark layers and also between the dead bark and underlying wood. Sphaerostilbe repens, known as stinking root disease, affects D. regia. Infection is by waterborne spores through root contact, it produces dark brown or reddish rhizomorphs beneath the root bark, the inner surface of the root is bleached and a strong odour is produced due to the combined activity of fungus and bacteria (Browne, 1968).

D. regia is a beautiful tree when in flower and is generally grown as an ornamental in parks and gardens, as an avenue or roadsides tree, and also in residential and school compounds for shade and shelter (Webb et al., 1984). It can also be planted as a multipurpose tree on eroded sites for erosion control, and for soil rehabilitation and improvement through atmospheric nitrogen fixation. In alley cropping studies in the uplands of Sierra Leone, D. regia trees were very effective in conserving soil moisture and reducing soil temperature (Karim, 1987). D. regia is also planted in tea plantations to provide shade.   D. regia wood is whitish-grey, straight- and loose-grained, light to medium density (440 kg/cubic metre), weak, soft and durable, but it polishes well and is used for making small implements, such as cutlery and toys (Gamble, 1902; Streets, 1962; Watt, 1972; Sheikh, 1993). Branches and stemwood can be used as fuelwood.   D. regia bark produces large amounts of a granular, yellowish- or reddish-brown gum, soluble in water forming a thick opalescent mucilage and containing a large quantity of calcium oxalate (Watt, 1972; Luna, 1996). The seeds can be made into necklaces; and they contain a gum which can be used in the textile and food industries. Immature pods are edible and have good potential as a dietary protein source for humans and livestock (Webb et al., 1984; Grant et al., 1995). The leaves (with 39.5% protein) provide nutritious fodder and browse for livestock. In the Virgin Islands, the annual dry matter yield of forage from D. regia has been estimated as 13.45 t/ha and protein as 1.45 t/ha (Oakes and Skov, 1962). The aqueous extracts of D. regia contain allelopathic compounds, including phenolic acids, alkaloids and flavonoids; these can be used as natural herbicides and pesticides to increase the productivity of agricultural crops (Chou et al., 1995). An extract of D. regia leaves has been found to disrupt insect growth and development (Saxena and Yadav, 1986).

D. regia is also an aggressive colonizer, occupying blank areas to the exclusion of other species, and it can form dense canopies that can exclude native species. Seedlings germinate thickly under parent trees and rapidly form monospecific stands which compete strongly with other plants, possibly assisted by allelopathic exudations (PIER, 2009). Due to its spreading root system, other plants are killed through competition, thus rendering the surrounding ground bare. There are also impacts on soil fertility, with a 13% increase in soil N was observed with the application of D. regia prunings to a calcareous soil after five years, also higher organic C and higher mineralization and turnover rates as compared to the control (Isaac et al., 2003).

Larger trees can be cut and the stumps treated with glyphosate to prevent resprouting. Systemic herbicides such as glyphosate can be used, either as a foliar or basal spray on smaller plants in full leaf, or on saplings and trees either as cut stump treatments, stem injections or basal bark treatments (PIER, 2009).

D. regia is a light-demander and under shady conditions it grows slowly. It is almost evergreen and is only briefly deciduous during the dry season. It has an extensive superficial root system, which renders it vulnerable to windthrow during storms (Menninger, 1962). Because of its spreading root system, other plants are killed through competition, thus rendering the surrounding ground bare. It is naturally regenerated by seed. D. regia seed is able to germinate at a wide range of soil pH values (4.9-10.6), but take a long time to germinate and may lie buried in the soil for 2-3 years without germinating. Young plants of D. regia are fire-tender. It does not withstand frost or winter cold at any stage of seedling, sapling or tree growth. It grows quickly, reaching a height of up to 8 m in three years. It tolerates severe pruning (Streets, 1962) and salt winds (Menninger, 1962). It is a nitrogen-fixing species and the roots have mycorrhizas.

D. regia is usually grown from seed. There are 1600-3700 seeds per kilogram, with about 10.5% moisture content. The seed can be stored in a clean dry store for up to 4-5 years without losing viability (Webb et al., 1984). Seed pre-treatment is necessary to hasten and improve germination (Luna, 1996). The seed is treated with sulfuric acid for about three hours, soaked in hot water for 24 hours or mechanically scarified before sowing to hasten germination (Webb et al., 1984; Singh, 1989). Soaking in hot water at 90°C for 10 seconds, followed by soaking for 24 hours under controlled conditions of 28.2°C and 83% relative humidity, gave 80% germination (Millat, 1989). Scarification and soaking in 400 ppm gibberellic acid for 48 hours also improved germination (Toaima et al., 1993). Seeds are sown in nursery beds, pots or polythene bags without shade. Plants ready for field planting are obtained in 4 months. These may be allowed to grow for up to 10 months for the production of stump plants, with 5 cm shoot and 25 cm root portions. Plants raised from seed may have flowers of different colours. Therefore, vegetative propagation by stem cuttings can be advisable, especially for propagating trees with scarlet flowers. Under ordinary conditions, rooting of cuttings is poor, but this can be enhanced by application of growth substances, such as IBA (Bhattacharjee and Balakrishna, 1983). Rooting of cuttings can also be increased by mist propagation techniques in a chamber. D. regia has been successfully micropropagated from a number of different explants on suitable media.In India, the best time for planting seedlings in the field is at the onset of the monsoon. Seedling survival is about 20-30% after the first growing season.Young plants need proection from frost, drought and damage from livestock. Pruning the crown results in improved growth (Singh, 1989). Due to a shallow root system, trees are vulnerable to windthrow; therefore, soil is put around the base of the trees to improve root system anchorage (Singh, 1989).

D. regia is mostly planted as an ornamental in rows along avenues and singly, or as groups of trees, in homesteads, gardens and parks.

D. regia trees raised from seed exhibit variable flower colour. A golden-yellow flowered form has been described as var. flavida Stehle (Kunkel, 1978).

D. regia trees are liable to be uprooted during storms due to their superficial root system. They are frost-tender. The young plants are fire-sensitive, and are also browsed by cattle and goats. D. regia trees are susceptible to termite attack and are sensitive to urban air pollutants including ozone, nitrogen dioxide and sulfur dioxide (Verma, 1989; Pandey and Agrawal, 1994). Trees raised from seed do not breed true to type as far as flower colour is concerned.D. regia is also an aggressive colonizer, occupying blank areas to the exclusion of other species.