Engelmann oaks, better than beautiful


This article was originally on a blog post platform and may be missing photos, graphics or links. See About archive blog posts.

The former librarian at the Los Angeles County Arboretum & Botanic Garden doesn’t remember exactly when the visitor wandered into her office and let drop that he was a descendant of George Engelmann. What Joan De Fato does remember is telling him that there was a grove of rare oaks on the site that had been named for his ancestor.

You don’t have to be a descendant of one of the fathers of American botany to share in what De Fato recalls as his pleasure and amazement. The arboretum’s grove of Quercus engelmannii, pictured above, is one of the last local stands of a native tree once so common to the foothills that an alternate common name is the Pasadena oak.


The first thing that strikes you upon reaching this group of roughly 200 trees is how much more animated it is by birds, butterflies and scampering lizards than the more cultivated parts of the garden.

The second is that it is drop-dead beautiful.

Better than beautiful. Engelmanns are the oak lover’s oak. At least this is the case with Bart O’Brien, plantsman at Rancho Santa Ana Botanic Garden in Claremont, who has long argued that the arboretum’s stand deserves special status.

Although other Engelmann groves are in Riverside, San Diego and Los Angeles counties, according to O’Brien, molecular studies have shown that the arboretum’s Engelmanns are clearly distinct. The stand is their largest expression, with trees that were probably once part of the population also found at Santa Anita Park and the Huntington Library, Art Collections and Botanical Gardens.

Engelmann oaks do not breed with the relatively common coast live oak. However, the ability to hybridize with the region’s vanishing population of valley oaks, and the Engelmann’s long-suspected relationship with the Mexican or Sonoran blue oak, make the arboretum’s Engelmanns part of a larger biological puzzle.

According to the arboretum’s curator of living collections, Jim Henrich, right, one challenge in putting that puzzle back together is that many of the pieces disappeared in the last climatic cataclysm, when the old ranges from Arizona became desert. Although the Los Angeles County tribe of Engelmanns may have survived the prehistoric events that dried out the American Southwest, they did not fare so well in the 20th century, when importation of water led to sprawling suburbs and tastes for the kinds of trees that didn’t grow naturally here.

Recognition that our water-strapped region will need to turn to better-adapted plants has renewed interest in our native Engelmanns. Since joining the arboretum staff years ago, Henrich has been leading acorn-collection projects and aggressively studying the Engelmann collection, which may or may not be typical of the ones that used to thrive in the region. The site they occupy on a high and dry south-facing knoll has a shallow layer of soil underlaid by hardpan, a layer of earth that’s practically impenetrable to water and roots. Many of the trees have extensive lateral roots and a flopping form. Many have fallen. ‘If they have deep soil, they don’t flop,’ Henrich said.


O’Brien notes that given their druthers, Engelmanns appreciate a heavier clay soil over coarse alluvial earth. ‘So they’re really an ideal plant for many people’s gardens,’ he said.

If you have room for an oak, these are stunning. Smooth bark in saplings gives way to deep, characterful fissured trunks and limbs in adult trees. Whereas all oaks have balletic, winding growth patterns, Engelmann branches seem just that bit more eye-catching in their swerving manner.

There is so much variation in leaf size, Henrich said, that the variations can be enormous even on the same tree. The elliptical-shaped bluish foliage might be slightly lobed or serrated, or might not. In an adaptation shared with many chaparral community plants (but not coast live oaks), Engelmanns are able to tilt their leaves so as not to burn in the sun. This quality, shared by manzanitas, means their canopies hold light in a way that allows them to shimmer.

Unlike coast live oaks, Engelmanns have funky phases. Engelmanns shed old leaves in spring just before putting out new growth. This can panic the uninitiated, O’Brien said. When stressed by drought in the summer, they also can shed their leaves, but Henrich noted that if forced to do this in the wild or allowed in cultivation, acorn production diminishes. When the arboretum’s trees were vividly stressed in the heat of the Station Fire, the 2009 blaze that was the largest in L.A. County history, he irrigated them.

Their relationship to the Southwestern Mexican blue oak may indicate a tolerance or even an appreciation of summer moisture, which Southwestern plants receive in the form of monsoons. Henrich is interested in exploring the need for summer water in the face of climate change, and to help Arboretum visitors who may later want to plant the trees near irrigated settings.

Tree of Life Nursery horticulturist Gene Ratcliffe has an Engelmann in her home garden in San Juan Capistrano. She recommends summer water for the trees becoming established. Her tree, put in four years ago from a 15-gallon container, is now 18 feet tall. She still waters it in the summer, once a month, slowly and deeply.


O’Brien, who is watching new Engelmanns put in along Indian Hill Boulevard in the foothills of Claremont, says that the growth rate is handsome and that the trees easily reach 20 feet or more in 10 years.

The need for water depends on precipitation. ‘This winter, it would obviously not be necessary unless it dries out suddenly, then you may want to water in March,’ O’Brien said.

O’Brien does not water established trees in summer after a good rain year. He added that when a tree’s roots have reached the water table, you should stop watering in the summer.

Ample rains the last couple of years have led to some good acorn harvests, which is good news for preservation. Thanks to volunteers and Henrich, the arboretum has seedlings in production and plans to plant successor oaks in careful proximity to parent plants.

The Rancho Santa Ana Botanic Garden also will have plenty of arboretum-type Engelmann seedlings for sale to homeowners and landscapers next fall, O’Brien said. Rancho will be identifying the seedlings by parent, so those planting a number of trees can choose from siblings or trees with different parents.

Tree of Life’s Ratcliffe reported good stocks of seedlings generated from the genetically distinct Riverside and San Diego communities of Engelmanns.


The arboretum’s Engelmann oak grove may be seen by visitors willing to climb the knoll of the Arcadia grounds. Given the extraordinary relationships that native oaks have with local flora and fauna, Henrich will be leading a March 19 tram tour of the Engelmann grove as part of a lecture on the life cycles of bees, wasps and ants.

-- Emily Green

Green’s column on low-water landscaping appears here every Friday. Follow our coverage via our Facebook page dedicated to gardening in the West.


Another source for native plants

Does rain mean an end to drought?

Curtain rises on a theatrical landscape design


Best ways to kill your lawn