Texas has its share of rare trees, owing to its vast geography and climate zones, but perhaps none are rarer than the species named for the South Texas botanist and photographer Robert Runyon (1881–1968).

In the spring of 1929, Runyon received a herbarium specimen of an unknown tree that had been collected by Harvey Stiles from the banks of the Resaca Del Rancho Viejo, near Brownsville. After making collections of his own from the small grove, Runyon sent a sample to the Smithsonian Institution in Washington, D.C., where botanist Conrad Morton formally described it as a new species of Esenbeckia, a genus of mostly rainforest trees in the citrus family (Rutaceae) from the Yucatan area of Mexico. Morton named it Esenbeckia runyonii.

Broader searches in Texas ultimately located only four small populations of this species, commonly called limoncillo or Runyon’s esenbeckia, and each was subsequently lost to agricultural conversions, perhaps eliminating this species from the United States. But Runyon had used some of his personal collection to germinate seeds and plant one of the trees at his Brownsville home. That tree stands to this day and once was believed to be the last of its species in the United States.

Although the species has since been discovered in a few other places in Texas—a grove of fifteen trees are now protected as part of the Lower Rio Grande National Wildlife Refuge system—it remains exceedingly rare, perhaps the rarest tree in the United States.

The tree is on private property near Charles and 8th Streets in Brownsville.