The Mesa, sometimes called Engelman oak, is a single trunk evergreen tree native to slopes, foothills and woodlands below 4200 feet. The rarest oak, due to urban sprawl and climate changes, makes protection of this species a priority.

Mesa oaks can reach heights from 15—45 feet with a 55 foot spread. While most Mesa oaks average up to 50 feet on a short, crooked trunk there is a specimen in Pasadena, California that stands 84 feet tall with a 106 foot spread and a circumference of 144 inches, according to the National Register of Big Trees.

Mesa oaks tolerate full sun and are very low maintenance. They tend to hybridize, or crossbreed, with other oaks making leaf identification challenging in some regions. The clue is to look for the distinguishable leathery texture and blue-green color of the leaf. When contrasted with other native oaks, the silvery-blue/green foliage helps to distinguish the Mesa species among the assembly. If you would like to take an up close look at one of these trees, there are a few locations in our area that have exemplary Mesa oak specimens:

• The Los Angeles County Arboretum in Arcadia
• Huntington Library and Gardens
• Caltech campus just east of the library

Leaves, Bark, Flowers & Fruits

A true Mesa oak has long elliptical leaves up to 4 inches long and 1 1/2 inches wide; looking more blue in color when compared to other oaks. The leaf has a dull sheen with smooth margins that may only exhibit a few sharp teeth. The underside of the leaf is greener than the top and is also somewhat pubescent (fuzzy). In contrast, the Coast Live oak has dark green oval leaves with serrated margins.

The bark on a Mesa oak has a distinctive gray mottled appearance with shallow furrows and scaly ridges.

Mesa oaks flower in the spring, with male flowers appearing in long (2-4 inches) narrow, drooping catkins that are yellow-green in color; female flower are inconspicuous reddish-green spikes that are found in the leaf axils. Mesa oak flowers are wind-pollinated as are all other species of oaks. The acorns, which mature in the fall, are stout, almost egg-shaped, 1 inch long with a warty, thick cap.

This acorn is stubbier than the more elongated acorn of other oaks and is a very prolific producer, dropping thousands of acorns in a single season. With an abundance of acorns comes a reliable food source for small mammals, birds and insects.

It is not unusual to see the acorn woodpecker (Melanerpes formicivorus) hard at work creating tidy holes in a variety of woodland trees, (and sometimes other wooden structures like telephone poles), for the purpose of storing acorns. These cache trees are called “granary” sites, and while generally every carefully hoarded acorn is consumed, some acorns escape that fate by sprouting and taking root in the decaying wood of a dead tree. 

Acorn Ecology

Fragmentation of habitat and isolation of some trees species affect availability of pollen and will reduce acorn production. Animals and insects feeding on acorns and browsing from deer also contribute to reduced populations of Mesa oak. In addition, male flowers do not open and release pollen unless humidity drops below 45% for several hours. Climate continues to influence species stability while pressures from land over-use are causing these oaks to hybridize with species of scrub oaks, producing a smaller, shrubsized version that exhibits lots of leaf variation. Every year oak trees provide storm water control and removal of nitrogen, sulfur, ozone and particulate matter from our environment. This is a beneficial service worth protecting.

Protect & Enjoy

Protecting our native trees requires our active partnership and contribution to the common goal of resource management. Oak, sycamore, and bay trees hold a unique place in our local ecosystem and provide habitat for 81 species of resident and migratory birds.

Urbanization continues to present a challenge within the sustainable capability of the ecosystem. Open spaces, clean air and homes for wildlife are worth protecting, but will demand support for the native trees existence in the local landscape.

Trees are living resources and assure healthy living habitats for interconnected flora and fauna, and the people that live within these habitats. To ensure that our trees will survive and thrive, please protect them by following these suggestions:

• Do not injure the trunk with objects.
• Keep all machines and weed whips away from tree trunks.
• Practice conservative pruning.
• Do not over water.