Most palo verde trees put on their bold yellow flower show in April and May, but some of them are bursting into brilliant bloom even as summer begins and daily high temperatures soar past 100 degrees.
Among the late bloomers are the Mexican palo verde and the Desert Museum palo verde, a hybrid discovered at the Arizona-Sonora Desert Museum west of Tucson.
The bloom could continue well into summer.
PEAK BLOOMS VARY
A museum database indicates that the Desert Museum variety of the tree can flower at least into July, said Mark Dimmitt, retired director of natural history at the museum and the scientist who discovered the variety in 1979.
“Analysis revealed that the hybrid had been pollinated by a blue palo verde,” a variety that typically reaches its peak bloom in late April, Dimmitt said. Other types commonly peak a little later, with Foothills palo verde trees blooming heavily in May and Mexican palo verdes peaking after that.
“Desert Museum palo verde flowering spans the whole range” of the other types, with sporadic blooming continuing into summer, Dimmitt said.
HYBRIDS EXPAND BLOOM
“The mass blooming of palo verdes will end sometime this month, but light blooms can occur all summer with irrigation or monsoon rains,” said Mark Fleming, curator of botany at the Desert Museum.
Palo verde hybrids contribute to that prolonged bloom.
“Palo verdes hybridize naturally, on their own,” Fleming said. “Over the last 30 years, a lot of Desert Museum palo verdes have been planted. Also, there are other hybrid palo verdes being offered in nurseries that are not Desert Museum” variety.
Some of those trees now hybridize with native species and with each other — “and so bloom periods can appear inconsistent with our memories of the past, especially in urban situations and especially with hybrid trees,” Fleming said.
WEATHER PLAYS A ROLE
Pinpointing the start and extent of blooms for the various types of palo verde trees can be a challenge.
“The answer is not simple because flowering is triggered by a combination of fall rain and winter-spring temperatures,” Dimmitt said.
The bloom periods “could be two weeks earlier or later (than normal), depending on the preceding year’s weather,” he said.
Take home message: Enjoy the late bloomers now and keep an eye out for flowering overachievers as the summer sizzles on.