Scientific Name
Eucommia ulmoides
This is a curious tree native to China that has few close relatives; it is the only genus in its family and the only species in its genus. It is (or was) likely native to higher elevations in central and eastern China; some believe it is now extinct in the wild. Either way, it has been widely cultivated in China for many years because of its importance, especially its bark, in traditional Chinese medicine. It is one of TCM’s 50 fundamental herbs.
The name is descriptive, as it is the only temperate-zone tree that produces natural rubber and tolerates temperatures of minus 20°F with ease. This rubber content can be seen by picking a leaf and tearing it gently in two; sunlight will reveal thin strands of latex holding the halves together. During World War II, the tree was investigated as a possible source of natural rubber when access to Southeast Asia was cut off, but the rubber content was too low by weight to be practical.
The hardy rubber tree is moderate in size and can reach 45 feet in height with a crown spread about 2/3 that width. In Colorado, the largest specimens have reached about 34 feet. The tree can be either male or female, and flowers of both sexes are inconspicuous. The seeds resemble those of elm, being round-winged, one-seeded samaras. The leaves, 3 to 6 inches long, also resemble those of elm species, which has given rise to the botanical name ulmoides (elm-like).
The foliage shows little if any fall color, limiting ornamental appeal. However, it is rarely affected by pests or diseases, tolerates a variety of soils, and is somewhat drought tolerant. These traits give it a place in urban forestry as a means of species diversification.