Planting and Protecting California Oaks

The Council for Watershed Health organized a day-long conference on sustaining native oak woodlands in Los Angeles on Nov. 17. The meeting was held at Descanso Gardens, which is home to more than 1,300 oak trees. More than 120 participants, many directly involved in oak tree maintenance on private and public lands, took in four hours of presentations followed by four hours of field observation.

Tom Scott from UC Riverside gave an overview of Southern California earth science and ecology, noting heavy clay soils or fast-draining alluvial soils select the appropriate oak canopies for a local geography. Scott referred to the “plastic” quality of oaks, noting their tenacity with limited resources and debunking the notion that oaks are slow-growing trees. With ample water and limited competition, oaks can be fast-growing. 

Scott also noted Since 2004 the GSOB has contributed to the decimation of 80,000 acres of mature oak in San Diego's Cleveland National Forest. Researchers continue to search for biological and chemical controls for the invasive beetle.  Scott urged the audience to buy firewood only from reliable sources, noting the continued quarantine on firewood transport out of San Diego County. Firewood is the way this pest moves. 

Bart O’Brien, of Rancho Santa Ana Botanic Garden, gave a slide presentation and later a field ID session on the varieties of oaks. They can and do hybridize within species, when growing near enough for wind pollination.

Larry Costello, of the UC Cooperative Extension, illustrated the power of oaks to access water deep in the soil, move it up through the tree’s vascular system and transpire or sweat it through the leaves, thus releasing the water back into the atmosphere. From there it will be pulled back to the soil by gravity.

Rosi Dagit, Senior Conservation Biologist, presented an overview of the Los Angeles County Oak Woodlands Conservation Management Plan, a volunteer effort that surveyed the historic Los Angeles oak populations as they stand, not individual trees.

The Oak Management Plan, adopted by the L.A. County Board of Supervisors in August, augments the Oak Tree Protection Ordinance of 1982. Dagit told the assembly during her address, [the Plan offers more] “… big picture things done for the management of bigger stands of oaks,” including tax incentives for restoring oak woodlands in LA County. Details are available online.

In a presentation on the history of Descanso Gardens, Brian Sullivan, director of horticulture and operations, referenced the evolving mission of Descanso. Originally developed as a camellia farm for the cut flower industry, owner Manchester Boddy bought two entire camellia nurseries in 1942, Star and San Gabriel, before the owners were sent to Manzanar. Boddy planted 50,000 camellias over 19 acres under the mature oaks.

When the gardens were transferred to a public botanic garden in 1953, maintenance of the camellias continued to include hand watering each plant. In the 1980s oscillating sprinklers were installed for efficient overhead watering.

In the 1990s the Descanso Guild, nonprofit overseers who manage this L.A. County Parks Department property, began to review the foundation of the garden: the California oaks. The conditions so favorable to camellias, including shade, leaf-enriched soil, and overhead summer watering, were detrimental to the oak overstory. In nature, the oaks would be re-seeding and replacing themselves. The density of the camellia plantings edged out new oaks and curbed the camellias. Planted so closely, these C. japonicas were unusually tall and gangly, with sparse blooms.

A long-range study was conducted last year to review and revise the garden’s maintenance, with an eye to the future health of the camellias and oaks. The resulting plan is being enacted, concurrent with growth and dormancy cycles. The camellias are being pruned, which will force prolific flowers. And the camellias will be gently “teased apart” from the oaks and re-established in a separate 10-acre “estate garden,” complete with micro-spray irrigation. The oaks will slowly return to the cyclic rains of Southern California’s Mediterranean climate with cool, wet winters and hot, dry summers.