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Currently, the Island oak is a relatively unknown California native tree throughout the Inland Empire. It is native to the coastal islands where it is adapted to winter rain, summer drought, fog, wind and humidity. Over time, a number of these trees have been planted throughout Southern California, including the Rancho Santa Ana Botanic Garden, that have increased its visibility and landscape interest.

Island oak becomes a large and stately tree over time with a dense foliage habit and rounded canopy. Mature sizes can range 40-50 ft. tall and as wide. Horticultural experience suggests that this oak is adapted to more summer water than the Coast live oak; young trees benefit from well-drained soils and moderate amounts of moisture through winter and spring, and monthly deep watering during summer. Mature trees show good adaptability to normal rainfall conditions, and tolerate heat, aridity and low amounts of summer moisture.

Water Needs

The Island oak is recommended for planting in well-drained soils with normal winter rainfall. Low amounts of summer irrigation help younger trees to grow faster and sustain the character of mature trees. The chart shown below provides a baseline guide to the monthly irrigation schedule and volume of supplemental water needed to maintain healthy growth throughout the average year. It should be noted there are several months indicated by an asterisk (*) when winter rains can provide sufficient moisture and irrigation is not needed. The high and low range of moisture indicates it can grow with varying amounts of water depending upon exposure conditions and size.

Irrigation Schedule and Graph

Low Water Use Plants

Irrigation Schedule 2

  Jan* Feb* Mar* Apr May Jun Jul Aug Sep Oct Nov* Dec*
Runs per Month 0x to 2x 0x to 2x 0x to 2x 1x to 2x 1x to 2x 1x to 2x 1x to 2x 1x to 2x 1x to 2x 1x to 2x 0x to 2x 0x to 2x
Inches per Run 1" 1" 1" 1" 1" 1" 1" 1" 1" 1" 1" 1"
Inches per Month 0" to 2" 0" to 2" 0" to 2" 1" to 2" 1" to 2" 1" to 2" 1" to 2" 1" to 2" 1" to 2" 1" to 2" 0" to 2" 0" to 2"

Range of supplemental summer water: 7"-14"
Range of supplemental winter water: 0"-10"

  Jan* Feb* Mar* Apr May Jun Jul Aug Sep Oct Nov* Dec*
For more information on how to use this Irrigation Schedule and Graph, follow this link.

For information how to calculate your irrigation system’s schedule and precipitation rate, please follow this link.

Plant Properties

Plant Type: Tree, Native
Foliage Character: Evergreen
Habit: Upright, Spreading
Growth Rate: Slow
Height: 35 ft. - 50 ft.
Width: 20 ft. - 30 ft.
Water Needs: Low 2
Foliage Color: Dark green, Dull green
Flower Color: Insignificant
Flower Season: Spring
Soil Adaptations: Well-draining soil, Clay
Exposure Adaptations: Heat, Drought, All day sun, Partial sun
Function: Wildlife value, Specimen, Shade Tree, Screening, Residential spaces, Parks and open space, Commercial spaces, California native, Background plant, Attracts butterflies


It is important to only prune island oak during the warm, dry season (July - September) to avoid disease issues. Healthy trees generally do not need significant or hard pruning. Prune young trees to set a good branch structure to avoid the need for much pruning once the tree is older. Remove accumulated small dead branches every number of years once the tree is mature (3). Any significant pruning which will expose previously shaded areas to sun should be done at the very end of the pruning season (September), because bark will be at risk of long term damage from sun burn if exposed mid-summer (S).



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