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

Green’s column on low-water gardening appears here every Friday. For an easy way to follow future installments, join our Facebook gardening page.

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. Desert Museum blooms 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.

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The leguminous trees of the desert

How to water in summer

A sunflower or two (or 200,000)

Those planning a garden or learning to garden should use his book to choose the right plants and irrigation.

Perry was kind enough to pass on this most general, no-guarantees, advice for summer: Water every three weeks or four to five times from June to October. Run drip systems on two- to three-hour cycles with .9 gph emitters. This puts about three to four inches of water down on every foot of soil where emitters have been placed. Three to four inches of water will percolate 24 to 36 inches deep in sandy soils, 12 to 18 inches deep in loamy soils, and six to nine inches deep in clay soils. “Clay soils are such a challenge!” Perry said.

You can also set your sprinklers for multiple run times to also provide this amount of water if you know the precipitation rates of the nozzles, he said.

For more thirsty ornamentals, “I look at two-week intervals,” he said, adding that you’re essentially doubling their water budget.

There, in a nutshell, you have the reason why it is preferable to landscape with natives. Whatever type of plants you have, if they are young with poorly established root systems, you’ll need to water more frequently. It also merits giving your trees and shrubs a short, gentle shower with the hose once a month to rinse off urban grime.

To test how well water is sinking into the soil, Galbreath recommends a T-shaped soil probe that can produce a foot-long sample not much wider than a roll of quarters, so you’re not disturbing the earth. After checking the moisture content, you can simply plug the sample back in. Landscape instructors also encourage beginning gardeners to use a probe so they can learn their soil type. If your sample balls up in your hand when you add water, you have clay. The sweetest condition of soil will be lightly moist but also airy so roots can reach air and water.

The secret to making the most of your irrigation in summer while avoiding pitfalls of disease is mulch. This is the blanket of organic material — wood chips, leaves, straw, coffee bean hulls if you can stand the smell — that will keep the soil cool and reduce evaporation. By August, most yards can use a topping of it. You want a two- to three-inch layer in beds, less around tender vegetables where mulch can overwhelm them.

Bagged redwood chips sold in box stores work fine, but the city of Los Angeles gives free mulch made from green waste to anyone who can haul it away. Commercial suppliers, such as Cal Blend Soils, have mulch starting at $19 per cubic yard. According to Cal Blend, this should cover 150 square feet to a depth of roughly two inches.

A great source for mulch can be tree trimmers; however, if the wood and leaves are freshly ground, you should ask if the mulch came from healthy trees, not from a specimen felled because of disease or pestilence. There is also the issue of seeds. Be prepared for them to sprout into volunteer plants. A school where I once helped to mulch now features an unplanned eucalyptus among its courtyard willows.

The greenness of fresh mulch will have a deliciously woodsy smell that will intensify as the nitrogen in the leaves briefly heats and breaks down. Be careful about having a big pile of cooking leaves near existing plants because it might heat up the roots. At two to three inches deep, though, green mulch won’t heat up. Whatever nitrogen is leached as the leaves compost will be swiftly returned to the soil.

Don’t pile any mulch right up to the base of the plants, but leave three to six inches for small plants, at least a foot for trees.

Uncured stable straw can have manure and urea capable of burning the root zones of plants, so let it stand for a while. Then spread it on your vegetables and fruit trees first. Most natives and many Mediterranean plants are nitrogen fixing, meaning they take their fertilizer from the air, so they won’t need the manure. Some plants, such as lavender, even seem stunted by helpings of nitrogen. Buying new hay and straw bales, using them for garden seating, then un-baling them and spreading the straw is a fun option. Beware, straw can make the ground slippery for running kids, and there will be a certain amount of drift in wind.

The best way to build up mulch around trees is to ban leaf blowers and rakes, particularly around oaks and avocado trees. The needles of pines and cedars give a particularly healthful shot of acid to soils.

As for the other summer chores: Harvest tomatoes, thin grape leaves, clear dead wood in fire zones. Book an arborist for autumn tree trimming, then pore over seed catalogs for October planting season.

– Emily Green

Photo credits, from top: Robert Gauthier / Los Angeles Times; Mladen Antonov / AFP / Getty Images