Indigofera spicata
(creeping indigo)

I. spicata is native to Africa, Madagascar and throughout South and Southeast Asia, and was introduced to the Americas in tropical areas as a cover crop (Duke, 1981; Sunarno, 1997). I. spicata may potentially threaten native flora, as it will send out trailing stems reaching up to 3 m long, producing numerous adventitious roots at the nodes and smothering other weeds (Morton, 1989; Sunarno, 1997). 

I. spicata received a score of 6 in a risk assessment prepared for Australia, indicating its potential invasiveness but recommending further research (PIER, 2014). It is listed in the Global Compendium of Weeds as an ‘agricultural weed’, ‘naturalised’ and ‘environmental weed’ (Randall, 2012). I. spicata is invasive to many parts of Asia and the Pacific, including French Polynesia (Marquesas Islands, Society Islands., Austral Islands), the Cook Islands (Mangaia), Micronesia (Pohnpei and Nauru), New Caledonia, Hawaii, Singapore, Japan, and Taiwan (Wagner et al, 1999; Chong et al, 2009; PIER, 2014; Flora of Taiwan Editorial Committee, 2014; Goka, 2014). Other tropical places where it is a known weed include Australia, Puerto Rico and adjacent islands, the United States (southern Florida), French West Indies (Guadeloupe), and Mexico (Villasenor and Espinosa-Garcia, 2004; Randall, 2012).

Several disagreements in distribution for I. spicata were found between sources. In Java, the species is reported as native according to ILDIS (2014), but Morton (1989) reports that it was introduced to Java in 1923, with Sampson (1928) reporting novel cultivation efforts in Java in 1925 and 1926. On the French Polynesian island of Tahiti, the species is reported as native by USDA-ARS (2013), but it is an introduced invasive species according to PIER (2014). It is listed as native in the Philippines by some sources (USDA-ARS, 2013; ILDIS, 2014), but this is apparently a mistake, as it was reported by Merrill (1919) as introduced to the country by 1918, with Morton (1989) reporting its introduction from Java to the Philippines as a cover crop in 1927. It is also mistakenly listed as native and endemic to Brazil (Forzza et al, 2012). Some of these discrepancies may be due to the varied name use of I. spicata when describing I. hendecaphylla,I. spicata, or both.

The risk of introduction for I. spicata is moderate but not insignificant, as the species’ adaptability to various soil types and usefulness in erosion control, crop cover and green manures has led to its widespread cultivation and use across tropical regions around the world (Morton, 1989; USDA-ARS, 2013). The species spreads by seeds, which are numerous, viable, and easily dispersed by human, animal and bird movement, as well as by cuttings, which have been known to spread by commercial soil transfer and machinery such as lawn mowing and landscaping equipment (Morton, 1989). 

I. spicata was included in a 2008 checklist of the world’s poisonous plants and has been researched for its hepatoxic components that have caused abortion in cattle and are reportedly toxic to ponies, sheep, rabbits and other grazers (Hegarty and Pound, 1968; Verdcourt and Trump, 1969; Duke, 1981; Wagstaff, 2008; Ossedryver et al, 2013). In some neglected pastures, it can constitute far more than 50% of the forage, posing risk to grazing animals (Morton, 1989). 

I. spicata has moderate weed potential in New South Wales, Australia (Hosking et al., 2011). It was given a score of 6 for Australia (Further Research Needed) in a risk assessment of its potential invasiveness (PIER, 2014).

I. spicata is generally found in disturbed grasslands, cultivated areas and waste places at altitudes between 0 and 2700 m, and can tolerate acidic and phosphorus-deficient soil (Duke, 1981). In Puerto Rico and the Virgin Islands, it is found in trails, roadsides, gardens and lawns (Mas and Lugo-Torres, 2013). In Nicaragua the species is occasional on dry slopes, grasslands and shrublands at altitudes of 130-1100 m (Missouri Botanical Garden, 2014). In Colombia, it is found at altitudes of 2000-2500 m, while in Bolivia at lower altitudes of 500-1000 m (Missouri Botanical Garden, 2014). It is found in Atlantic rainforests of Brazil (Forzza et al., 2012). In China, the species is found in open ground and moist, sunny trailsides at 800-1100 m altitudes (Flora of China Editorial Committee, 2014). In Hawaii, it was introduced as a pasture legume and is now naturalized in low elevation, dry, disturbed areas (Wagner et al., 1999).  In Madagascar, it is present in open, disturbed areas such as along roads, tracks and around villages, evergreen, humid forests at low (0-800 m) and lower montane (1800-2000 m) altitudes, as well as in deciduous and seasonally dry, western forests and coastal areas from 0-800 m (Puy et al., 2002).

Genetics

The chromosome count for I. spicata is 2n=16, 32 (Duke, 1981; Missouri Botanical Garden, 2014). 

Reproductive Biology

The following is taken from Sunarno, 1997: The species can be propagated by seed and by stem cuttings. Seed is very hard and germinates poorly without scarification. Mechanical scarification and treatment with sulphuric acid for 40-60 minutes can increase the germination rate to about 80%. To obtain a good distribution of the seed it is mixed with sand or filtered dry soil at a ratio of seed to sand of 1:4 before sowing. If planted in rows 60 cm apart the seed rate is about 3.3 kg/ha. Cuttings are used when seed is difficult to obtain. For large-scale plantings, they should be raised in nurseries. Cuttings of about 20 cm long are planted at a spacing of 60 cm x 60 cm with 5 cuttings per hole. Once established, [I. spicata] is self-sowing. 

Growth and Development

Seedlings of I. spicata develop a strong taproot, which serves to loosen the soil. If cuttings are used for planting, the plant remains very low and cover rarely exceeds 12 cm in height. A fair cover can be established in 4-6 months and a continuous, even cover in a year from planting. The plants send out trailing stems which, under favourable conditions, may attain a length of 2-3 m and may produce roots from the nodes. Plants grow taller as they mature, and after two years are normally 30-40 cm tall. Vigorous regrowth occurs at the start of the rainy season (Morton, 1989; Sunarno, 1997). 

Associations

Like several other members of the Indigofera genus, I. spicata has been cultivated as green manure due to its capability of nitrogen-fixing symbiosis with rhizobia and bradyrhizobia (Sprent et al, 1987; Sunarno, 1997). 

Environmental Requirements

I. spicata may compete with and dominate the native flora, as it is well adapted to a wide range of soil types. The species thrives best on clay soils, but can tolerate various soil types including limestone, sandy, nutrient-poor and phosphorus-deficient soils as well as moderately acidic (pH 5.0-7.7) soils, and can grow at altitudes up to 2700 m (Duke, 1981; Puy et al, 2002; for altitudes in various countries, see Habitat section). It can reportedly tolerate an annual rainfall of 600-1500 mm, but may be found in wetter locations receiving up to 4000 mm annual rainfall and can grow following the onset of rain (FAO, 1977; Duke, 1981; Sunarno, 1997). In cultivation it is fairly resistant to drought, salt, and heavy rain (Morton, 1989). When grown under heavy shade, for example in rubber plantations, growth is poor (Sunarno, 1997). Regarding climate tolerances, the species ranges from Warm Temperate Moist through Tropical Moist Forest Life Zone (Duke, 1981). [Rainfall, temperature and pH levels in tables are from Duke, 1981].

While in 1981 I. spicata was reported to be “remarkably free of pests and disease” (Duke, 1981), newer research has shown two natural enemies from the plant and animal kingdom. Spodoptera littoralis (cotton leafworm), a parasitic insect, targets I. spicata as one of its host plants (CABI, 2013). I. spicata is also a host plant for the parasitic plant Striga gesnerioides (cowpea witchweed) which occurs in western Africa. 

In Sri Lanka, the only important pest of I. spicata is the caterpillar Dichomeris ianthes, which can sometimes totally defoliate the cover crop, although I. spicata usually recovers very quickly (Sunarno, 1997). In Peninsular Malaysia, the plant is a host of the larvae of a small moth, probably Lamprosema diemenalis, but again, the damage is not permanent (Sunarno, 1997).

Animal feed, fodder, forage

• Fodder/animal feed

Environmental

• Erosion control or dune stabilization
• Soil improvement

Materials

• Dyestuffs
• Green manure

I. spicata Forssk. and I. hendecaphylla Jacq. were long considered to be synonymous. In 1993 Puy et al. reported I. spicata and I. hendecaphylla to be two separate species. According to Puy et al., I. hendecaphylla is more widespread throughout the Old World tropics and subtropics to the Pacific Islands, whereas I. spicata refers to the more restricted species, confined to Africa, Yemen, Madagascar and the Mascarenes, extending into drier areas where I. hendecaphylla does not occur (Puy et al, 1993). The main morphological differences are:

• The staminal sheath of I. hendecaphylla is 4-5 mm long and distinctly exceeding the calyx, whereas the staminal sheath of I. spicata is 3-3.5 mm long, not or hardly exceeding the calyx in I. spicata.
• The leaves of I. hendecaphylla have 9-11 narrowly oblong-elliptic leaflets each, rarely any fewer, and glabrous or sparsely strigose on the upper surface. The leaflets of I. spicata are 5-8 in number, obovate and cuneate in the basal half, with the upper surface strigose and rarely subglabrous. 

However, considerable variation occurs in both species and within species varieties, and exceptions to these differences often occur (Puy et al., 1993; JSTOR, 2013). Previous literature in which both species are referred to as I. spicata has caused confusion regarding species distribution, invasiveness, and toxicity.I. spicata has been separated from I. hendecaphylla based on the work of Puy et al. and from the examination of collections deposited at the US National Herbarium. Both species are found in the West Indies; according to the collections so far analysed by the Herbarium, I. hendecaphylla is only found in some islands of the Lesser Antilles, while I. spicata is present in Jamaica, Puerto Rico, St. Thomas, and some of the Lesser Antilles. 

According to the Brisbane City Council (2013), in Australia I.spicata is considered similar to I. linnaei and I. linifolia, but both of these other species have much smaller pods (less than 7 mm long). I. spicata is also very similar to I. circinella. The main differences are:

• I. spicata has mostly creeping (prostrate) stems, and its stems and leaves are covered in close-lying hairs (appressed pubescent). Its cylindrical fruit are straight and borne in a backwards facing position (reflexed).
• I.circinella has mostly creeping (prostrate) stems, and its stems and leaves are hairless (glabrous) or covered in close-lying hairs (appressed pubescent). Its cylindrical fruit are strongly curved or coiled. Hairy indigo (Indigofera hirsuta) has spreading or upright (erect) stems, and its stems and leaves are covered in spreading hairs (hirsute). Its cylindrical fruit are straight and borne in a backwards facing position (reflexed) (Brisbane City Council, 2013). 

I. spicata has also been sometimes confused with Indigofera miniata Ortega (Missouri Botanical Garden, 2014).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

I. spicata responds to herbicide treatment, which has been found to shrink the taproot so that it can be extracted more easily (Morton, 1989). An evaluation of the species for live mulch reported that it appeared to be suitable as a cover crop to be killed with herbicide for minimum or zero tillage production of arable crops (FAO 1977). In eastern Africa, repeated application of 2,4-D and MCPA are considered effective control measures (Nyamjom and Konyango, 2008). Similarly in Brisbane, Australia, herbicide application via foliar spray is recommended for weed control (Brisbane City Council, 2013).

Further research is needed regarding the toxicity of I. spicata, particularly on the variation of the toxic compound indospicine within different varieties. Clarification between I. spicata and I. hendecaphylla is needed for the body of literature existing prior to the separation of I. spicata into the two species by Puy et al. (1993), as this is essential for understanding the distribution and invasiveness of the plant within different regions of the world. Likewise more research is needed on potential impacts of the species’ introduction, as well as detection, prevention and control measures to ensure it does not reach invasive status in the many places it has already naturalized.

10/3/2014 Original text by:

Marianne Jennifer Datiles, Department of Botany-Smithsonian NMNH, Washington DC, USA

Pedro Acevedo-Rodríguez, Department of Botany-Smithsonian NMNH, Washington DC, USA