Calotropis procera (Ait.) Ait. f. giant milkweed
Calotropis procera (Ait.) Ait. f. giant milkweed ASCLEPIADACEAE Synonym: Asclepias procera Ait. General Description.—Giant milkweed is also known as sodom apple, calotrope, French cotton, small crown flower (English), algodón de seda, bomba (Spanish), cotton-france, arbre de soie, and bois canon (French) (Howard 1989, Liogier 1995, Neal 1965, Parrotta 2001). This plant is a soft-wooded, evergreen, perennial shrub. It has one or a few stems, few branches, and relatively few leaves, mostly concentrated near the growing tip. The bark is corky, furrowed, and light gray. A copious white sap flows whenever stems or leaves are cut. Giant milkweed has a very deep, stout taproot with few or no near-surface lateral roots. Giant milkweed roots were found to have few branches and reach depths of 1.7 to 3.0 m in Indian sandy desert soils (Sharma 1968). The opposite leaves are oblongobovate to nearly orbicular, short-pointed to blunt at the apex and have very short petioles below a nearly clasping, heart-shaped base. The leaf blades are light to dark green with nearly white veins. They are 7 to 18 cm long and 5 to 13 cm broad, slightly leathery, and have a fine coat of soft hairs that rub off. The flower clusters are umbelliform cymes that grow at or near the ends of twigs. The flowers are shallowly campanulate with five sepals that are 4 to 5 mm long, fleshy and variable in color from white to pink, often spotted or tinged with purple. The fruits are inflated, obliquely ovoid follicles that split and invert when mature to release flat, brown seeds with a tuft of white hairs at one end (Howard 1989, Liogier 1995, Little and others 1974). Range.—Giant milkweed is native to West Africa as far south as Angola, North and East Africa, Madagascar, the Arabian Peninsula, southern Asia, and Indochina to Malaysia (Rahman and Wilcock 1991). The species is now naturalized in Australia, many Pacific islands, Mexico, Central and South America, and the Caribbean islands. Ecology.—Giant milkweed favors open habitat with little competition. This condition is most completely met in overgrazed pastures and rangeland. Other common habitats are beachfront dunes, roadsides, and disturbed urban lots. The species grows in dry habitat (150 to 1000 mm precipitation) and sometimes in excessively drained soils in areas with as much as 2000 mm of annual precipitation. Giant milkweed may be found in areas up to 1,000 m in elevation in India (Parrotta 2001). It roots very deeply and rarely grows in soils that are shallow over unfractured rock. Soils of all textures and derived from most parent materials are tolerated, as well as soils with high sodium saturation. Beachfront salt spray is not detrimental. Competition with tall weeds, brush, and especially grass weakens existing plants, and being overtopped and shaded by trees soon eliminates them. During droughts in Puerto Rico, giant milkweed is attacked by the orange aphid, Aphis nerii Boyer de Fonscolombe, which causes defoliation, death of branches, and aborted fruits (Little and others 1974). Reproduction.—Flowering and fruiting takes place throughout the year (Little and others 1974). Hundreds to thousands of seeds may be produced per plant each year. The seeds in a Puerto Rican collection averaged 0.0095 + 0.0027 g/seed or about 100,000 seeds/kg (author’s observation). Eighty-nine percent germination took place in potting mix between 7 and 64 days after sowing. Half the seed weight was found in the wing (silk). The seeds are dispersed by wind and may fly for several hundred yards in gentle breezes. Seedlings may arise in abundance after rainy periods, but only a few survive the first season. Using the reserves in its large taproot, giant milkweed can resprout year after year when burned or cut. Growth and Management.—Giant milkweed usually reaches heights of about 2 m, but may occasionally reach 5 m in height and stem diameters of 25 cm (Little and others 1974). Growth is about 1 m the first year after sprouting in Puerto Rico. Senescence of individual stems takes place in about 5 years, but plants often resprout afterwards. Giant milkweed does not form dense stands, normally occurring as scattered individuals. It can be a serious weed in pastures, overgrazed rangelands, and poorly managed hay fields. Although it probably is not possible to eliminate existing stands through management without ceasing grazing and harvest, it may be possible to prevent their establishment. Some form of chemical control would seem to be the only practical option for eliminating existing stands but no specific guidelines are available. Establishing giant milkweed has been advocated for environmental protection and as a nurse crop for more valuable species (Campolucci and Paolini 1990). This can be done easily by planting containerized seedlings or rooted cuttings. Benefits.—Giant milkweed tissues, especially the root bark, are used to treat a variety of illness including leprosy, fever, menorrhagia, malaria, and snake bite (Parrotta 2001). The latex is toxic and can cause blisters and rash in sensitive persons. The plant is occasionally grown as an ornamental in dry or coastal areas because it is handsome, of a convenient size, and is easy to propagate and manage. It is recommended as a host plant for butterflies (Mikula 2001). In the past, the silky hairs were used to stuff pillows (Little and others 1974). Giant milkweed was tested as a host for sandalwood, Santalum album L., a partial root parasite. It resulted in greater growth of sandalwood than all other species tested (Shinde and others 1993). Extracts, chopped leaves, and latex have shown great promise as nematicides, in vitro and in vivo (Anver and Alam 1992, Charu and Trivedi 1997). Sheep, goats, and camels will eat the leaves of giant milkweed during droughts, but consumption is low. If the leaves are chopped and mixed with other feed, consumption greatly increases with no ill effects. (Abbas and others 1992, Nehra and others 1987). Shade-dried giant milkweed leaves contained 94.6 percent dry matter, 20.9 percent ash, 19.6 percent crude protein, 2.2 percent fat, 43.6 percent acid detergent fiber, and 19.5 percent neutral detergent fiber (Abbas and others 1992). Although it is lightweight, the wood is used in impoverished desert areas for a cooking fuel (Varshney and Bhoi 1988).