Common name: Mexican willow

Mexican willow is a fast-growing landscape tree originating in Central and South America, its native range extending from Mexico to Chile. Nowadays, it is also found as an introduced species in Australia, Asia and the Caribbean.

It is a narrow tree up to 20 m (65 ft) tall, with a columnar shape created by long, slender, almost vertical branches held close to the trunk, which is up to 20 cm (8 in) in diameter. The bark is grey, smooth when young, becoming rough and furrowed with age. A less common variety with wide-spreading, drooping branches occurs in the southern part of its range.

The leaves are slender, 5 to 13 cm (2 to 5 in) long, tapered at both ends, finely toothed on the margins and dull green top and bottom. They are alternately arranged along the branches and remain on the plant in all seasons.

The flowers are small and inconspicuous, held in slender clusters, and either female or male, borne on separate trees. The male flowers are yellow, the female flowers green. They bloom from winter to spring, though for reasons unknown may be absent in some areas. Where present, the flowers on female trees are followed by small seedpods up to 5 mm (0.2 in) long, containing many tiny tufted seed designed for wind dispersal.

Use

Mexican willow is occasionally cultivated as a landscape tree for its tall columnar shape, an unusual feature in subtropical and tropical trees. Planted closely spaced, it serves as a tall windbreak or privacy screen.

It has a water-loving habit and vigorous rooting system that makes it suitable for planting along watercourses to minimise soil erosion. However, such plantings have led to it becoming a serious weed in some areas.

The long, slim, flexible cane-like branches are cut for use in woven craft, in what is commonly referred to as 'Willow craft', an art form practised for centuries and used in basketry and for making cane furniture.

The wood is lightweight, averaging 400 kgs per cubic meter (25 lbs per cubic ft), with low natural resistance to rot, decay and attack from termites and marine borers. This makes it generally unsuitable for use in construction or craft, other than for making non-durable lightweight articles such as wooden boxes and pencils. When properly dried, the wood and branches make an excellent firewood.

The bark is a rich source of Salicylic acid, a plant defence phytohormone that can be extracted and given to plants as a tonic to boost their immune system, making them more resilient to pests and disease. It is prepared as an infusion by soaking the bark and stems in water, then straining off the liquid and applying it to plants in the form of a foliar spray.

Salicylic acid is used mainly in cosmetics, including makeup, face masks, cleansing products, acne treatment products, deodorants and suntan lotion, for its preservative, antimicrobial, antiseptic, anti-itch and skin-sloughing (exfoliating) properties.

The leafy stems and bark from mature willow trees have long been used as a natural dye to produce a range of colours, including pinky-tans, yellows and greens, usually with an alum (aluminium) mordant to brighten the colours. Varying shades of grey are produced by adding iron to the dye pot. Both hot and cold dyeing methods create colours that work well with animal and vegetable fibres.

Health use

Salicylic acid is a forerunner of aspirin used in medicine to relieve pain and fever. Unlike aspirin, it does not thin the blood or irritate the stomach lining.

Climate

Grows naturally in humid subtropical and tropical mid- to high-elevation climates, generally areas with annual lows of 12 to 21°C, annual highs of 24 to 32°C, annual rainfall of 900 to 6000 mm and a dry season of 4 months or less, extending to 7 months with irrigation or groundwater. 

Growing

New plants are usually started from cuttings, which readily root wherever planted. Performs best on free- to slow-draining clay and loam soils of a moderately acid to neutral nature, generally with a pH of 4.5 to 7.0 and on sites with full to partial sun exposure.

It has good tolerance to seasonal flooding and thrives in permanently wet soils alongside marshes, lakes, ponds and rivers.

Problem features

Helped by moving water, pieces of broken stem or root can travel far from the parent plant and have the ability to set root easily. This has contributed to its spread outside cultivation, and it usually naturalises along waterways or generally any wet area.

It is listed in the Global Invasive Species Database (GISD) as a weed, is a declared noxious weed in Puerto Rico and is considered a serious weed in many other countries, including Australia. 

The vigorous roots can block sewerage and other types of underground pipes. 

Pollen released by male flowers is known to cause hay fever and allergies in some individuals.

Books

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

• 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

• Berni, C. A & Bolza, E. & Christensen, F. J. 1979, South American timbers - the characteristics, properties and uses of 190 species, Commonwealth Scientific and Industrial Research Organization (CSIRO), Division of Building Research, Highett, Victoria, Australia

• Cassidy, F. G. & Le Page, R. B. 1980, Dictionary of Jamaican English, 2nd ed., Cambridge University Press, Cambridgeshire

• Chevallier, A. 2000, Encyclopedia of herbal medicine, 2nd American ed., Dorling Kindersley, New York

• Dean, J. 2010, Wild Color : the complete guide to making and using natural dyes (Revised and updated edition), Watson-Guptill Publishing, New York

• Gargiullo, M. B & Magnuson, B. L. & Kimball, Larry D. 2008, A field guide to plants of Costa Rica, Oxford University Press, Oxford

• Groves, R. H & Australia. Dept. of the Environment and Heritage & Australia. Bureau of Rural Sciences 2003, Weed categories for natural and agricultural ecosystem management, Bureau of Rural Sciences, Canberra, A.C.T

• 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

• 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.

• Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

• Randall, R. P. 2007, The introduced flora of Australia and its weed status, Cooperative Research Centre for Australian Weed Management, Glen Osmond, South Australia

• Record, S. J. & Hess, R. W., 1972, Timbers of the New World, Yale University Press, New Haven, Connecticut & Arno Press, New York

• Standley, P. C. 1920, Trees and shrubs of Mexico, Government Printing Office (GPO), Washington D.C.

• Vázquez, Y. C. 1999, Potentially valuable Mexican trees for ecological restoration and reforestation, Institute of Ecology, Database SNIB-REMIB-CONABIO, Project J084, Mexico

• Winter, R. 2009, A consumer's dictionary of cosmetic ingredients : complete information about the harmful and desirable ingredients found in cosmetics and cosmeceuticals, 7th ed, Three Rivers Press, New York