By Rob Paratley 

Sweetgum is a native southern tree, widespread in riparian areas, swamp margins, and moist ravines.  In Kentucky, the tree is found throughout the state, but is scarce in the Bluegrass and on alkaline soils generally.  It tolerates a moderate amount of flooding or saturated soil.  Most of us know the tree as sweetgum, but in parts of the deep South it is called alligator wood, gum tree, or incense tree.  In the European timber markets it is known for some reason (marketing?) as satin walnut.

Sweetgum does not tolerate northern winters.  The horticultural literature considers it a Hardiness Zone 5 tree, and therefore can be successfully planted in most of Missouri and the southern half of Illinois in the Midwest, to southern New England and the Atlantic coast.  It can be grown successfully in the northern part of Florida (Zone 9).  But Florida is not the end of the story for sweetgum.  Disjunct populations considered the same species are found in higher elevations of Mexico, Guatemala, and Honduras.  In fact, the first written accounts of the tree were from Spanish observers in the 1500s.  A journal from a conquistador accompanying Cortez recorded observing the Aztec emperor himself enjoying a resinous stick from the tree.  The Spaniard recognized its similarity to the well-known Asian species (and close kin) that exuded the same resin (see below).  Spanish explorers of the American South also noted the resemblance and recorded the tree as they trudged through southern swamps.  The first formal appearance of the tree in botanical literature was in the first Spanish-language Mexican herbal in the late 16th Century.

Sweetgum is known as Liquidambar styraciflua to botanists.   The genus name refers to the viscous resin, literally liquid amber- liquidus and ambar.  The species name translates as flowing with gum (styrax or storax).  The genus Liquidambar has four species worldwide. Two close relatives of our North American sweetgum are Formosan sweetgum (Liquidambar formosana) of southern China and Taiwan, and Oriental sweetgum (Liquidambar orientalis) of Turkey and the Caucasus.  The former species is an uncommon ornamental in the U.S.  The latter is the storax tree, the “Balm of Gilead”, a medicinal perfume and salve important in the Middle East and mentioned in the Bible. 

Traditionally, the genus Liquidambar has been placed in the witch-hazel family.  Modern research using DNA “fingerprints” has separated the genus from the witch-hazels and placed it in its own tiny family, called the Altingiaceae (after a tropical close relation, Altingia).  Because sweetgum doesn’t much resemble witch-hazel anyway (sweetgum is wind-pollinated; witch-hazel pollinated by flies; sweetgum is resinous, witch-hazel isn’t), the change seems to make sense.

Sweetgum is a very popular street and park ornamental tree.  It does well in most urban conditions, is resistant to most insect pests and is relatively disease-free.  In city conditions, the tree typically reaches 30 - 60 (to 80) feet in height, but on best riparian sites in the South it can top 100 feet.  The national champion is a North Carolina sweetgum 136 feet tall with a four-plus foot diameter.  It has much to recommend it as an ornamental, with rapid growth, an appealing conical growth form, and shiny leaves displaying excellent fall color- yellow, purple, orange, red, often on the same tree.  A downside of sweetgum is the messiness of the fruit, a dense spiky cluster of 2-parted capsules.  These fruit clusters are produced in great numbers, falling to the ground around the tree.  Walking over them in bare feet is not recommended.  In addition to its popularity as an eastern shade tree, sweetgum is also used in Europe and in California, where it somehow manages to endure the long, dry California summer by making use of coastal fog.  (It is a little amusing to observe these eastern trees trying to figure out what to do with their leaves as the mild, moist, California winter approaches.) 

The most interesting cultural feature of sweetgum is the property it shares with the storax tree—its resin.  Cut the bark, and an aromatic resin flows.  Sometimes called American storax, the resin is yellow and variably viscous.  Apparently the yield (flow) is greater in hotter climates, so the highest yielding sweetgum populations are in Mexico and Central America.  In traditional North American Indian medicine, the resin and inner bark were used to treat diarrhea, and, topically, as a salve for wounds and skin irritations.  Its antiseptic and antibacterial properties are useful in both contexts.  The Cherokee and other tribes used the resin to calm nervousness.  For this it was taken as an infusion (tea).  The Cherokee and Choctaw also combined the resin with strawberrybush to make a beverage of unknown appeal, and the resin, when hardened and sticky, was chewed as a gum.   

The Euroamerican settlers adopted many of the Indian uses for sweetgum, either through observation of native practices or discovered independently.  In Appalachia, a concoction of the resin mixed with whiskey was chewed to clean teeth, heal gums and mouth lesions, or to relieve toothache.  Applying the balsam to the skin might help with chigger and other insect bites.  It may also have been used as an insect-repellent, although other botanical sources were more effective.  It fulfilled these roles for soldiers in the Confederate army during the Civil War.  The resin was tapped in the South especially before the Civil War, but subsequently it couldn’t compete economically with Asian storax and other resin-producing trees.

Sweetgum wood is medium in density and strength, easily worked, but has poor elasticity.  The sapwood is creamy, the heartwood is pink to reddish brown.  They are often marketed separately.  It is used for lower-end furniture, trim and veneer, plywood, joinery, and low-end uses like boxes and crates.  It is marketed as red gum (no relation to blackgum).  Sweetgum is an important crop tree in many managed Southern bottomlands, usually mixed with other bottomland hardwoods sold for timber or for pulp.

Photography

1. Sweet gum leaves (Wikimedia commons, CC)
2. Fall color sweet gum leaves (Pixabay, CC)
3. Sweet gum fruit (Wikimedia commonc, CC)

About the Author

Rob Paratley is an Agriculture Research Specialist in the University of Kentucky Department of Forestry and Natural Resources. Rob has taught several courses at the University of Kentucky including Taxonomy of Vascular Plants, Economic Botany, and is the Curator of the UK Herbarium. Email Rob at rparatl@uky.edu.