Common name: Jerusalem thorn

Jerusalem thorn is an ornamental plant native to dry and sub-humid areas of the Americas, extending from the southern United States southward through Central America to South America.

It is commonly a multi-trunked shrub 3 to 5 m (10 to 16 ft) tall, though occasionally grows into a tree with a single-trunk and up to 8 m (26 ft) tall.

The branches come off the trunk in an upright, almost vertical direction, then arch outward and with the ends bending toward the ground, forming a rounded, weeping crown. Sharp thorns arm the branchlets.

Leaves long and pencil-like, with tiny leaflets, arranged sparsely along the length. These grow in drooping strands, emphasising the plant's weeping appearance.

It is an opportunistic flowering species, flowering only when conditions are favourable rather than following any seasonal pattern. The flowers are yellow on long stalks and are especially showy when the plant is in full bloom. These are followed by long thin, brown seedpods enclosing small, oval-shaped seed.

Use

Jerusalem thorn was at one tome widely cultivated as an ornamental for its showy flowers, foliage and its ability to withstand dry conditions. However, its cultivation is now discouraged in areas outside of its natural range. This is due to its ability to escape cultivation and form dense thickets, which can crowd-out natural vegetation and disrupt ecosystems.

It is valued in dry climate gardens for its attractive, weeping form, showy flowers and its usefulness as a barrier plant. Its high tolerance to drought and extensive root system also makes it a good candidate in serving to minimise soil erosion in dry areas, particularly where seasonal or flash flooding occurs.

The flowers produce nectar that is dilute and not very attractive to honeybees, which limits its usefulness as a honey plant. The small amount of honey produced is amber coloured and has a fair flavour.

The seedpods are eagerly browsed by livestock, particularly sheep and goats.

The wood is medium-weight, with a density averaging out at around 600 kgs per cubic meter (37 lbs per cubic ft), but only comes in small diameter sizes. It is mostly cut into pieces for firewood and for making charcoal.

Climate

Grows naturally in dry to  moderately humid subtropical and tropical climates, generally in areas with annual lows of 10 to 25°C, annual highs of 22 to 36°C, annual rainfall of 300 to 1500 mm and a dry season of 3 to 8 months, extending to 12 months with irrigation or groundwater.

Growing

New plants are usually grown from seed, but vegetative propagation also gives good results.

Performs best on free-draining loam, sand and limestone soils of a slightly acid to alkaline nature, generally with a pH of 6.0 to 8.5, and on sites with full sun exposure. It has good tolerance to drought, saline and limestone soil conditions but poor tolerance to slow-draining or waterlogged soils.

Problem features

The dried seedpods float and are easily dispersed by flowing water, sometimes far distances from the parent plant. The seed are also unwittingly dispersed in mud that clings to the soles of shoes and animal fur. Each plant produces thousands of seed, which can remain dormant for several years and germinate when conditions are favourable.

It has been introduced into many countries where it has easily escaped cultivation and is now naturalised. It is assessed as a high weed risk species for Hawaii by the Hawaii Pacific Weed Risk Assessment (HPWRA) project and is classed as a weed of national significance in Australia. 

The branches are armed with sharp thorns that can inflict injury on the unwary.

Books

• Adams, C. D. 1972, Flowering plants of Jamaica, University of the West Indies, Mona, Greater Kingston

• Allen, O. N. & Allen, E. K. 1981, The Leguminosae : a source book of characteristics, uses, and nodulation, University of Wisconsin Press, Madison, Wisconsin

• Australia. Dept. of the Environment and Heritage & Cooperative Research Centre for Australian Weed Management 2003, Weeds of national significance : weed management guide, CRC for Australian Weed Management and the Commonwealth Department of the Environment and heritage, [Osmond, S. Aust]

• Barwick, M., et al. 2004, Tropical & subtropical trees : a worldwide encyclopaedic guide, Thames and Hudson, London

• Bradbear, N. 2009, Bees and their role in forest livelihoods : a guide to the services provided by bees and the sustainable harvesting, processing and marketing of their products, Food and Agriculture Organization of the United Nations (FAO), Rome

• Chaplin, L. T. & Brandies M. M. 1998, The Florida Gardener's Book of Lists, Taylor Publishing Company, Dallas, Texas

• Crane, E., Walker, P. & Day, R. 1984, Directory of important world honey sources, International Bee Research Association, London

• Francis, J. K. 2004, Wildland shrubs of the United States and its territories: Thamnic descriptions, U.S. Department of Agriculture, Forest Service, International Institute of Tropical Forestry, Río Piedras, San Juan, Puerto Rico & Rocky Mountain Research Station, Fort Collins, Colorodo

• Francis, J. K. and Liogier, H. A. 1991, Naturalized exotic tree species in Puerto Rico, General technical report SO-82, USDA Forest Service, Southern Forest Experiment Station, New Orleans

• Gilman, E. F. 1997, Trees for urban and suburban landscapes, Delmar Publishers, Albany, New York

• Hall, N. 1972, The use of trees and shrubs in the dry country of Australia, Australian Government Publishing Service, Canberra, Australian Capital Territory

• Harrison, M. 2009, Flowering shrubs and small trees for the South, 1st ed., Pineapple Press, Sarasota, Florida

• Hocking, D. 1993, Trees for drylands, International Science Publisher, New York

• Krishen, P. 2006, Trees of Delhi : a field guide, Dorling Kindersley Publishers, Delhi

• Liegel, L. H. 1987. A technical guide for forest nursery management in the Caribbean and Latin America, U.S. Dept. of Agriculture, Forest Service, Southern Forest Experiment Station, New Orleans

• Lorenzi, H. 2002, Brazilian trees : a guide to the identification and cultivation of Brazilian native trees. Vol. 1, 4. ed, Instituto Plantarum de Estudos da Flora, Nova Odessa, Sao Paulo

• Luna, R. K 1996, Plantation trees, International Book Distributors, Dehradun, Uttarakhand

• Nair, P. K. R. 1993, An introduction to agroforestry, International Centre for Research in Agroforestry (ICRAF), Kluwer Academic Publishers, Dordrecht

• National Research Council (Board on Science and Technology for International Development) 1980, Firewood crops : shrub and tree species for energy production (Volume 1), The National Academies Press, Washington D. C.

• Perry, B. 2010, Landscape plants for California gardens: an illustrated reference of plants for California landscapes, Land Design Publishing, Claremont, California

• Perry, F. & Hay, R. 1982, A field guide to tropical and subtropical plants, Van Nostrand Reinhold Company, New York

• Sheikh M. I. 1993, Trees of Pakistan, USAID Forestry Planning and Development Project, Pictorial Printers, Islamabad

• Vozzo, J. A 2002, Tropical tree seed manual, U.S. Department of Agriculture (USDA), Forest Service, Washington D.C.

• Webb, D. B. 1984, A Guide to species selection for tropical and sub-tropical plantations, 2nd ed., Unit of Tropical Silviculture, Commonwealth Forestry Institute, University of Oxford, Oxfordshire

Articles, Journals, Reports and Working Papers

• Morton, J.F. 1964, Honeybee Plants of South Florida, Proceedings of the Florida State Horticultural Society, Vol 77:415-436.

• National Research Council (Board on Science and Technology for International Development) 1990, Saline agriculture : salt-tolerant plants for developing countries, The National Academies Press, Washington D. C.

• Queensland Department of Agriculture and Fisheries (Australia) 1990, Weed and pest animal fact sheets, Brisbane