Flora Fact

Texas’ Rarest Tree

The tropical limoncillo almost disappeared from South Texas.

by Sumaiya Malik

The Rio Grande Valley might be at the lowermost tip of Texas on the map, but it takes the top spot in being home to plants found nowhere else in the state, including the rare Runyon’s esenbeckia, or limoncillo tree. Once thought to be extinct in the wild in Texas, the citrus tree was rediscovered in 1984 growing not far from where it was first identified in 1929.

It’s likely the rarest tree in Texas and perhaps the United States.

“Most people don’t know these trees exist,” says Mike Heep, who helped rediscover the species and grows young saplings of the tree at his nursery in Harlingen. “Only a real plant aficionado will know.”  

Requiring a tropical or subtropical climate, the fruit-bearing Esenbeckia runyonii grows 20 to 30 feet tall; in Mexico, where the trees are more widespread, they are known to grow to almost double that height.

Runyon’s esenbeckia is named after Robert Runyon, an American photographer and botanist who served as the mayor of Brownsville from 1941 to 1943. Runyon helped identify the tree as a new species in 1929 when he sent specimens he had received and collected from a newly discovered grove to the Smithsonian Institution in Washington, D.C. He had noticed that the three smooth and shiny leaves that sprouted from each branch made this esenbeckia different from a similar tree in Mexico. The biologists found that this was a new species in the citrus family not known to science and gave it the scientific name of Esenbeckia runyonii. It was the first known occurrence in the United States of this genus of primarily rainforest trees.

A few other small populations of Runyon’s wild trees were discovered, but all were lost when the land was converted to agriculture. The only known remaining tree was the one Runyon had planted in his Brownsville yard, earning it a spot on the Texas A&M Forest Service’s roster of Famous Trees of Texas.

In 1984, Heep stumbled upon additional trees growing along a resaca in Cameron County, land that became part of the Lower Rio Grande Valley National Wildlife Refuge.

“The tallest one we found there measured 26 1/2 feet tall,” Heep says.

Trees have since been planted at the Gladys Porter Zoo, Harlingen’s Hugh Ramsey Park and other places across the Rio Grande Valley.

The limoncillo’s fruit is a thick-skinned, five-lobed rounded capsule. The segmented fruit with an edible rind is green when it’s growing and turns tan-brown when ripe. The tree propagates through seeds that have a short lifespan.

The tree’s leaves are glossy and lemon-scented, and its small white blossoms bloom in dense clusters and emit a pleasant citrusy fragrance.

“This is a very pretty tree that flowers in spring every year,” Heep says.