The living trees of Eucommia ulmoides, an endemic species in China, grow from 200 to 1700 m above sea level, within the geographic range from 102° E to 118° E and from 25° N to 35° N. Spring temperatures in these regions vary from 12.3°C to 20.1°C. A physiological study (using germination tests) of E. ulmoides has been undertaken to test the role of spring temperature as a factor controlling the distribution of Eucommia. Results show that the spring temperature is a limiting factor for Eucommia seed germination and hence for the distribution pattern of the genus. The suitable range of temperature for seed germination, established experimentally, is from 13°C to 22°C, with an optimum of 18°C. Specimens of fossil Eucommia cf. ulmoides, preserved as a branch segment and leaves, showing the distinctive latex, were found in Middle Miocene sediments of Shanwang Formation, Shandong Province, East China. If the climatic tolerances documented here for E. ulmoides are extrapolated to Shanwang, they are in fact consistent with other predictions of the paleoclimate at this site, indicative of the potential value of Eucommia as a biothermometer. These Miocene fossils, and one previously described Eocene fruit specimen, prove the former existence of Eucommia in China in addition to North America and Europe. This confirms that the genus is not a recent arrival in China and extends our understanding of the past biogeography of the genus.