Common name: Sweet gum

Sweet gum is an ornamental and resin yielding tree originating in North and Central America, its natural range extending from the Midwest United States to the eastern and southern states and through Texas to highland areas in Mexico, Guatemala, Honduras and El Salvador.

Across its range, there are different varieties with different climate adaptations. This description is limited to varieties occurring naturally in Central America and subtropical parts of North America.

A tall tree, it may reach up to 50 m (165 ft) in natural forests with a trunk diameter of 1.5 m (5 ft), though on open sites is more commonly 10 to 20 m (33 to 65 ft) tall with a straight trunk supporting a densely leafy crown, pyramidal on young trees, becoming rounded with age. The bark is red-brown on young trees, on mature trees greyish, thick and deeply furrowed.

The leaves are similar in size and shape to maple leaves, being up to 15 cm (6 in ) wide with five deeply cut, sharply pointed lobes and toothed margins. They create a stunning and striking colour display at the turn of summer to autumn, changing from bright green to deep red or burgundy then detach and fall to the ground.

The flowers are small and insignificant, greenish and either male or female in clusters arising at the sides and ends of the branches, the male flowers at the top of the cluster, situated above the female flowers. They come into bloom in the dry season, coinciding with winter in its native range, and are followed by small beak-shaped seed capsules tightly packed in spiky, round clusters up to 3.8 cm (1.5 in) in diameter. The capsules are green when young, become dark brown and woody when mature with two winged seed enclosed.

Use

Sweet Gum is occasionally cultivated as a specimen or landscape tree for its shapely form and showy maple-shaped leaves, their spectacular autumn colour a bonus.

The trunk on wounding yields an oleo-resin (an essential oil resin mixture) known as 'Storax Balsam'. It is a commercially important resin long used in traditional medicine and in more recent times as a fragrance and flavouring. It has a balsamic character with a soft cinnamon-like aroma and is distilled into an essential oil used as a fragrance component in branded perfumes and colognes such as 'Soir de Paris', 'Bel Ami' and 'Tweed', as well as to scent soaps.

The essential oil is also used in small amounts as a flavouring for alcoholic and non-alcoholic beverages, frozen dairy desserts, hard candy confectionery, baked goods and puddings.

To tap the resin, a canal is cut in the bark about 15 cm (6 in) long, 4 cm deep and at a 45-degree angle, which allows the resin to drain into a container attached to the trunk. The tapped resin is collected about once a month, heated and skimmed to remove impurities and then bottled for transporting to an essential oil distilling facility. Unrefined storax is a viscous semi-fluid brown substance that on refining develops a yellow or amber yellow hue.

The wood has commercial value and well-formed logs are sawn into lumber for fine furniture and cabinets, interior joinery, flooring and panelling. Selected logs are sliced for decorative veneer. A medium-weight wood, it averages 560 kgs per cubic meter (35 lbs per cubic ft) and has lustrous reddish-brown heartwood, which is demarcated from the nearly white sapwood. Natural resistance to rot, decay and wood-boring insects are low to moderate and it has a tendency to warp badly if not properly dried. It is therefore not recommended for outdoor construction.

Health use

Storax Balsam has expectorant and antiseptic properties and is used in Chinese and other traditional medicines to loosen mucus congestion in the respiratory tract as well as to treat wounds and skin problems such as scabies.

General interest

It is estimated that Sweet Gum accounts for up to a third of the hardwood forest stock of the Southern United States.

Climate

Grows naturally in moderately humid subtropical and tropical highland climates, generally in areas with annual lows of 6 to 18°C, annual highs of 20 to 31°C, annual rainfall of 800 to 2500 mm and a dry season of 6 months or less.

Growing

New plants are usually started from seed. Mature capsules are collected, dried under shade for about a week and the seed extracted and sown without pre-treatment. Seed kept under cool, dry storage conditions remain viable for several years.

Performs best on deep, rich, free-draining clay and loam soils of a moderately acid to slightly alkaline nature, generally with a pH of 5.0 to 8.0, and on sites with full to partial sun exposure. It has poor tolerance to slow-draining or permanently wet soils.

Problem features

Sweet Gum is recorded as a weed of agriculture in Chile and as naturalised in parts of Europe but there does not appear to be any record of it anywhere as a serious weed or threat to the natural environment.

The roots are moderately invasive, with 5 m (16 ft) the recommended minimum planting distance from underground pipes, paved areas and foundations.

Books

• Arctander, S. 1960, Perfume and flavor materials of natural origin, Elizabeth, New Jersey

• Bonner, F. T & Karrfalt, R. P. 2008, The woody plant seed manual, U.S. Department of Agriculture, Forest Service, Washington D.C.

• Burns, R.M. & Honkala, B.H. 1990, Silvics of North America (Volume 2) : Hardwoods, Agricultural Handbook 654, U.S. Department of Agriculture (USDA), Washington D.C.

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

• Coppen, J. J. W. 1995, Gums, resins and latexes of plant origin, Food and Agriculture Organization of the United Nations (FAO), Rome

• Food and Agriculture Organization of the United Nations (FAO) 1986, Databook on endangered tree and shrub species and provenances, FAO Forestry Paper 77, Forest Resources Division, Rome

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

• Groom, N. 1997, The new perfume handbook, 2nd ed., Blackie Academic & Professional, London

• Howes, F. N. 1949, Vegetable gums and resins, Chronica Botanica Company, Waltham, Massachusetts

• Khan, I. A. & Abourashed, E. A. 2010, Leung's encyclopedia of common natural ingredients : used in food, drugs and cosmetics, 3rd edition, Wiley Publishing, Hoboken, New Jersey

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

• Porter, T. 2012, Wood : identification & use, Compact edition, Guild of Master Craftsman Publications, Lewes, East Sussex

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

• Scheffer, T. C & Morrell, J. J. 1998, Natural durability of wood : a worldwide checklist of species, Forest Research Laboratory, Oregon State University, Corvallis, Oregon

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