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7 drought-tolerant trees for Southern California gardens and landscaping

June 24, 2023

John Winkler emailed me in search of seven desert trees.

These trees are suitable for landscaping throughout Southern California and are noteworthy, in addition to their low water requirement, for their small to medium size; they typically reach only 30 feet in height although most are fast growing. Most (2-6 on the list below) are also leguminous, meaning they do not require fertilization since they make their own nitrogenous fertilizer with the assistance of symbiotic bacteria that dwell in nodules on their roots. Another characteristic of leguminous trees is their multi-leaflet leaves, typically small and feathery, as well as seed pods. These pods are sometimes edible, sometimes toxic, and contain hard seeds which, in order to germinate, must be placed in moist peat moss in the refrigerator for eight weeks, or scored with a knife and then given a thirty-minute bath in sulfuric acid.

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Leguminous trees play a vital role in desert ecology. The soil beneath them is more fertile than surrounding soil due to the nitrogen that is manufactured in their roots and the enrichment provided by decomposing fallen leaves. Such soil is an invitation for seeds of other plants to germinate there. Many leguminous trees are also considered nurse plants since they protect tender young seedlings – of saguaro and organ pipe cactuses for example – from scorching summer sun and winter cold. Such seedlings are also protected from herbivores by the thorns on the low-lying branches of these trees.

Winkler’s list and the nurseries that grow them follow below:

1. Desert willow (Chilopsis linearis): The deciduous desert willow is so named because of its leaves, which are as slender as those found on true willows (Salix spp.). Like true willows, desert willow – a California native – grows naturally along water courses even if, in its case, they are arroyos that go dry in summer months. Desert willow is extremely drought tolerant and handles blistering direct sun as well as cold down to zero degrees. In its habitat, it grows in proximity to native palo verde (Parkinsonia species) and mesquite species. Desert willow’s trumpet flowers, which are mildly fragrant, attract hummingbirds and the tree serves as a nesting site for a variety of songbirds. Flower color is white, pink, lavender, purple, or burgundy, depending on variety. Desert willow reaches a height of 30 feet with a spread of 25 feet. There is an excellent parkway planting of desert willows on the east side of Hayvenhurst Avenue south of Magnolia Boulevard in Encino.

2. Chilean mesquite (Prosopis chilensis): This is a glorious tree with attractive black bark as it ages and a weeping demeanor. It grows quickly to 30 feet tall with an equal spread and may live for more than 100 years. Soft, pendulous, ferny foliage belies its durability as a desert tree. Its trunk is famously gnarled and can grow to a diameter of three feet. Chilean mesquite is variably thornless so if its two-inch thorns are a concern, you will want to ascertain that you are acquiring a thornless specimen. There is a four-month winter dormancy period when branches will lose their leaves and, in the manner of many desert plants, foliage may drop off in periods of prolonged heat or drought as a water-conserving measure. Chilean mesquite produces suckers that should be removed unless a multi-trunk tree is desired. Pollen from its yellow catkins is allergenic, so if you are sensitive to pollen that wafts through the air, you will need to carefully consider whether you want it nearby. Its curvaceous yellow pods indicate that it’s a legume affiliation. After its first year in the ground, it should never be watered since water will encourage its shallow roots to be more aggressive than usual. In any case, it should not be planted closer than 20 feet to any structure. Still, it is a widely recommended selection due to its fast growth into a manageable, medium-sized tree that provides shade, in addition to its ornamental virtues and unmatched drought tolerance.

3. Texas mountain laurel (Sophora secudiflora). It is noteworthy for fragrant flowers, mauve in color, that smell like grape juice. Foliage on some varieties is silver, adding to its decorative appeal. It grows naturally into a large shrub and may be used as an informal hedge. However, it is commonly trained into a small tree that reaches 20 feet in height. Be aware that all parts of this species are toxic. Texas mountain laurel is sometimes referred to as mescal bean. However, although mescal, a drink derived from around 30 agave species (tequila comes from only one species, the blue agave) has the customary intoxicating effect associated with imbibing alcohol, consumption of mescal beans will lead to hallucinations.

4. Palo Brea or Sonoran Palo Verde (Parkinsonia praecox). This species is scarce in the nursery trade and grows larger than more common palo verde species, but otherwise shares its features of a golden yellow cloud of blooms along with sculptural trunks covered with lime green bark. These days, Parkinsonia x Desert Museum is the most widely planted palo verde due to its long bloom period, extending on and off from spring to fall, and its thornlessness. It grows to a height of 25 feet. Palo verde means “green stick” in Spanish and references its green bark. Most plants perform photosynthesis – by which they make their sustaining carbohydrate – with the assistance of the green chlorophyll pigment in their leaves. The palo verde has chlorophyll in its bark as well and most of its photosynthesis happens there. The palo verde is not only deciduous in the winter but may lose its leaves in the summer as a way to minimize water loss. The same leaf pores that open to admit carbon dioxide – an essential ingredient of the photosynthesized carbohydrate – are a liability during hot weather since, while open, water escapes through them. Thus, a desert plant that can photosynthesize through its bark is at an advantage when its leaves disappear. Ocotillo (Fouquieria splendens) has this capability as well.

5. Texas Ebony (Ebenopsis ebano): Another drought-tolerant arboreal legume, Texas ebony produces clusters of fragrant and fuzzy cream-colored flowers from May to October. In addition to its legendary drought tolerance, Texas ebony withstands windy conditions and can also grow in compacted soil. It grows to 30 feet with an equal spread.

6. Desert ironwood (Olneya tesota): The Sonoran Desert’s most senior of citizens, desert ironwood specimens have been found that are 800 years old. This tree is especially famous as a nurse plant with more than 200 species discovered growing in its shade. In addition, dozens of reptile and mammal species consume its seeds or leaves and lay their young under its sheltering branches. Although its striking blue-green leaves do not remain on the tree for more than a year, it is still considered evergreen since new leaves immediately grow where old ones have fallen. Its twisting branches make it a memorable accent in any garden setting, with a mature height and spread of 30 feet.

7. Thevetia thevetiana (Giant Thevetia): An oleander relative and equally poisonous, this is a small tree that grows to around 15 feet. Silky yellow blooms proliferate from summer to fall among shiny, narrow, emerald-green leaves.

Plants 1-4 (including Desert Museum palo verde, but not palo brea) are grown at Monrovia Nursery (monrovia.com), 5 & 6 at Mountain States Wholesale Nursery (mswn.com), and 7 at San Marcos Growers (smgrowers.com). These are production nurseries that supply retail outlets throughout Southern California. When you reach their websites, click around until you find a list of retail nurseries that carry their plants; you can then special order the trees that you desire from these nurseries.

California native of the week: Vine maple (Acer circinatum) is a stunning deciduous shrub or small tree growing up to 20 feet that “rivals Japanese maple in beauty” according to Carol Bornstein in “California Native Plants for the Garden.” It is eminently suitable for container growing or shady exposures. Leaves emerge tinted red and then turn a pale green. In fall, foliage will turn yellow, orange, and scarlet before dropping off. It has a tendency to sprawl in the shade so may have to be staked to grow upright. When planted in the open along the Northern California coast, it will grow into a symmetrical shrub, although in Southern California full sun may scorch it. It is widely available in Northern California nurseries but I have not found it locally. If anyone knows of a local source for this plant, please advise.

If you have experience with any desert tree that you wish to pass along, please email me at [email protected]. Questions, comments, or photos regarding any plant species or gardening subject are always welcome.

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7 drought-tolerant trees for Southern California gardens and landscaping

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