Palo verde tree called Desert Museum puts on show all summer long
A tree whose name translates from Spanish to “green stick” has performed a remarkable feat. Native to ranges in Southwestern deserts and once thought here as appropriate only for Phoenix or perhaps Palm Springs, the palo verde tree has become a favorite choice of Los Angeles landscape architects.
After considering the most commonly available palo verde species, Cal Poly Pomona professor emeritus and author Robert Perry selected four as suitable for the Southland: Mexican, blue, foothill and Sonoran. All offer drought tolerance, stunning spring blooms, sex appeal for bees, leaves like filigree, branches in a luminous green and dimensions suitable for patios.
Yet it was a palo verde hybrid called Desert Museum that Perry singled out as special. It is named for Arizona-Sonora Desert Museum in Tucson, where three decades ago the director of natural history, Mark Dimmitt, identified a thornless seedling that would not just flower in the spring, but would keep blooming throughout the summer. As cuttings were propagated, Dimmitt and museum staff began seeing the new hybrid around Tucson, then Phoenix, then Las Vegas.
By 2005, Southern California’s leading horticulturist, Bart O’Brien of Rancho Santa Ana Botanic Garden, was describing it as a plant that combined the best features of its parents. By 2011 it was the key courtyard tree at the new Valley Performing Arts Center in Northridge.
Faintly taken aback that his discovery is now making it big in Los Angeles, Dimmitt kindly talked about palo verdes in general and Desert Museum in particular for this edited Q&A.
Question: Where do you find palo verde trees most commonly in the desert?
Answer: The first-generation natural hybrid, between Mexican and foothill palo verdes, is found mostly in disturbed areas around Tucson and other desert cities. This is because Mexican palo verde is introduced to Arizona and is rarely found in natural desert.
When you started working with crosses of palo verdes in the 1970s, what did you see in terms of height, thorns, flowers? How many crosses did you go through to arrive at Desert Museum?
Desert Museum was a discovery, not the result of a breeding program. In 1979, I collected a few dozen seeds from a first-generation hybrid and grew them. All except one seedling were uninteresting. The one seedling grew into a superior tree. Genetic and phenotypic analysis revealed that the hybrid had been pollinated by a blue palo verde. I named it Desert Museum with the expectation that Arizona-Sonora Desert Museum would patent the tree and make some money from it. Oh, how they wish they had now!
Are bees the natural pollinators? Or do the trees attract other animals, such as bats?
Only bees, numerous species, of which a few are specialists on palo verdes.
Are the trees self-fertile? Do you get seed from a single plant? Will it produce another palo verde?
I've never seen one growing in isolation, so I don't know if they are self-fertile. Desert Museum does produce viable seeds. I have grown hundreds and all were inferior to the parent.
What months are the trees naturally dormant if not watered?
As desert plants, palo verdes will be dormant in any season if they have no soil moisture, but they will die after a year or so with no rain.
Are all palo verdes suitable to Southern California?
They should be. Coastal areas may not be warm enough for good growth. Mexican and Desert Museum palo verdes are damaged below about 15 degrees. Foothill and blue palo verdes will tolerate more frost, and palo brea, which occurs from Sonora, Mexico, to South America, is damaged at about 20 degrees.
I see Desert Museum as one of the most popular landscaping plants in L.A., increasingly used around glass-and-steel buildings. Can they tolerate lots of reflected light and heat as well as the smog from urban settings?
I've never seen a mature tree burn. The bark of young ones might be burned by extreme reflected heat, but they do well against west walls in Phoenix. They have not been tested for environmental tolerance, so I don't know about smog, shade, etc. I suspect that they need long hours of full sun because of the small leaves.
Does irrigation increase the size of the leaf? I've noticed pronounced variability between two trees about 20 feet from each another in my garden. One is closer to an area that gets overflow irrigation. Can they be evergreen?
Probably, but irrigation would mostly increase the number of leaves. They are probably evergreen as long as they have water and temperatures are not below freezing. New leaves grow only during the summer; they slowly fall off over the winter, and the canopy is normally quite sparse by spring.
They grow quite quickly. Does it make sense to plant them when they're small?
My original seedling and cuttings grew six feet per year for the first three years. They slow down greatly in later life. I suspect that a 5- or 15-gallon tree would catch up to or even surpass a large boxed tree in a few years.
How long do they live?
Not enough data to know that. Blue and Mexican palo verdes are usually rather short-lived. It is written that they become senescent in about 30 years. But I know a blue palo verde in my neighborhood that was a big tree when I moved here in 1970, and it still looks healthy. My original seedling of Desert Museum broke apart last year at the age of 31. It may have been damage from palo verde root borers. It is resprouting from the roots, though.
If a palo verde purchased from a nusery is pot-bound, with roots swirled inside the container, can you disentangle the roots, essentially create a bare-root tree and still plant it successfully?
I have only one experience with this. We planted a 36-inch boxed tree at the Desert Museum. It was not well established, and the soil fell away from the root ball, effectively bare-rooting it. This was in summer. We planted and watered it, and it grew without skipping a beat.
Should gardeners amend the soil around the root zone to improve drainage? How deep do the roots go?
This has a complex answer, but the short reply is no. Roots of plants tend not to grow beyond the amended soil; they won't grow into soil of a very different texture, effectively confining the plant to a big pot in the ground. Amendments will not improve drainage if there is a hardpan layer beneath the planting site.
Desert plants simply do not need amended soil as long as the local soil is fairly friable. Research on a number of desert trees revealed that almost all of their roots are within three feet of the surface. That's true even of mesquite, which is famous for putting roots down hundreds of feet; most roots are still where the rain reaches. Desert Museum does not grow vigorously in very hard or rocky soils. In such locations, a hole should be dug 2 to 3 feet deep and as wide as you can afford to go -- up to 10 feet or more. Backfill with a medium that will not compact. Lots of humus is not recommended for desert plants.
What would you suggest as companion planting?
Desert Museum casts very light shade, so in sunny climates you can grow many sun-loving plants beneath them, including cactuses. The small leaves and flowers are not nearly as messy as the long ones of Mexican palo verde, but they will still get caught in plants that are spiny or debris-catching. Choices should be warm-season growers with similar watering needs.
I see them commonly mulched with crushed glass or fancy gravel. It looks hot. I prefer wood mulch. Should one cool the root zone?
It's a desert tree, so the roots don't mind hot soil. But a good mulch will greatly reduce irrigation needs. A thick gravel is almost as effective as an organic mulch. If the glass is clear, it might heat the soil and cause more evaporation.
Mine were planted in December, and instead of flowering all at once in spring, they have been issuing successions of flowers through the summer.
Young trees tend to flower all summer. Older trees have the longest flowering season of any palo verde. They begin when the earliest species blooms (blue palo verde) and continue through the late season of Mexican palo verde. Here in Tucson, Desert Museum flowers heavily from mid-April through June and continues to flower sporadically all summer into fall.
Are there any diseases that gardeners should be aware of?
The hybrid seems to be as resistant to pests. All palo verdes are pretty tough. Some have been attacked by the eriophyid mite that causes witches broom; it eventually kills the tree and is not worth treating. A good precaution for any landscape is not to plant monocultures, large areas with a single plant, which can facilitate the spread of problems.
-- Emily Green
Corrected: An earlier version of this post said the palo verde hybrid Desert Museum was planted at Los Angeles Police Department headquarters. We cannot confirm whether those trees are Desert Museum or some other type of palo verde.
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Photos, from top. The yellow flower of Desert Museum brightens Green's backyard. Credit: Emily Green. The palo verde Desert Museum seen in winter in the courtyard of the Valley Performing Arts Center at Cal State Northridge. Credit: Emily Green. Palo verde trees bloom outside Los Angeles Police Department headquarters. Credit: Jamie Rector / For The Times. A bee scrambles over a Desert Museum flower at Tree of Life Nursery in San Juan Capistrano. Credit: Christine Cotter / Los Angeles Times.