December 14, 2009
What’s Blooming: Sweet Gum’s Bittersweet Horticultural Legacy
On the south side of the creek that meanders through the Arboretum’s Meadowbrook section three tall, upright trees can be seen right now covered in brightly colored fall foliage. Commonly called “Sweet gum,” Liquidambar styraciflua, is native to Central America, Mexico, and the Southeastern United States where it is a significant percentage of newly regenerated hardwood forests. Liquidambar grows rapidly in regenerating forests because it has the ability to fix nitrogen in the soil. Liquidambar’s name derives from the liquid, amber-colored, resin the tree produces to protect itself from insect attack. This resin has been used by Native Americans in Mexico and North America as a chewing gum and as a medicine for healing wounds. The Aztecs were even more creative with the gum, using it as ceremonial incense and adding it to smoking tobacco to create a potent sleep-aide. Liquidambar wood is beautiful; polished cross sections reveal red heartwood surrounded by a contrasting band of bright white sapwood. Unfortunately the moisture content of the wood and its poor durability make it almost impossible to work with; items made from the wood warp and wear out rapidly. Furniture makers can still exploit the Liquidmabar wood’s natural beauty by applying it as a veneer to furniture constructed of more durable but less beautiful woods like pine. During the late 1970’s and throughout the 1980’s, Liquidambar styraciflua was an extremely popular landscaping and street tree. Three varieties introduced in the late 50’s and early 60’s by the Saratoga Horticultural Foundation in Palo Alto came to dominate the California urban landscape because of their pleasing forms and reliable fall coloration. ‘Palo Alto’ has orange and red fall leaves, ‘Festival’ has a confetti-like combination of red, yellow and pink, and ‘Burgundy’ is a variety that reliably turns a dark red here in late fall. These are the varieties planted along the stream in the Meadowbrook section.
For a while Liquidambar seemed like the perfect tree for California; a deciduous tree that seemed to thrive here and gave migrants from the east coast a taste of the colorful fall display of leaves they were used to back home. It was also fast grower that didn’t get out of bounds (cultivated Liquidambar rarely get over 60-70 feet tall) and a tree that seemed to thrive anywhere because of its nitrogen fixing abilities. But when the Saratoga Horticultural Foundation introduced the trees they paid little attention to a characteristic of the tree that would later create what could be termed a horticultural disaster. As Liquidambar matures its nature changes; the trees develop a propensity to form damaging surface roots and their production of spiny seed-balls increases. This makes mature Liquidambars quite a nuisance and liability; a fact that many property owners and cities would become painfully aware of after removing tens of thousands the trees and repairing the expensive damage caused by them to nearby structures. A study published in Western Arborist that surveyed tree damage in several Southern California cities found that the ubiquitously planted Liquidambar accounted for the greatest number of trees causing damage. In the city of Alameda it was found that 69% of the Liquidambars planted in that city were damaging or starting to damage structures. Currently Liquidambar styraciflua is not the easy landscape choice it used to be, but it still has advantages in some situations; it is now recommended that the trees be planted in locations that are at least 15-20 feet from any structures, and in areas where their copious production of spiny seed balls is not a problem. They should never be planted as street trees.