Summer Irrigation of Established Oak Trees

July 14, 2005

Many recommendations on irrigation of established native California oak trees caution against summer water application, especially near the base of the tree. Fungal diseases such as oak root fungus and crown rot are often associated with the death of oak trees under a condition of high soil moisture. Even though oaks are known to be drought tolerant,

summer irrigation may be beneficial to oak trees, especially during periods of prolonged drought, if water is applied in the outer two thirds of the root zone. Watering every two to three weeks during the summer will be helpful to the tree. There was a study conducted to test the validity of these recommendations and to present the findings on the effects of summer irrigation on established oak trees.

In trial 1, a group of 165 native valley oak trees, growing in Oak Grove Regional Park near Stockton, California, were evaluated in 1988 using an oak tree hazard evaluation system that included an objective measure of tree vigor. The trees had an estimated average age of 125 years, based on a sampling of increment borings. The soil type is clay and relatively shallow allowing the trees to establish shallow but extensive root systems that are prone to water stress during periods of drought. The trees were evenly disbursed throughout the test area and the region is currently experiencing a drought conditions. The natural areas were not irrigated. However, the root systems of some of the trees in the natural area extended into the landscaped areas and therefore received summer irrigations. To study the effects of summer irrigation on these oaks, the trees were re-evaluated in 1992. Thirty trees located in the unirrigated natural areas were near irrigation, being within 2x the dripline of the landscaped areas and therefore received some irrigation water. The other 135 trees did not receive summer irrigation water.

The results: By 1992, 73 of the non-irrigated trees were in serious decline or dead. Significantly fewer, only 27, of the trees near irrigation were in similar condition. Root sampling in the areas for diseases did not show the presence of damaging pathogen and no significant levels of insect pests above ground were found. A second trial was conducted on a group of 24 native valley oaks to study the effects of irrigation on improving vigor on drought-stressed trees. All trees were in a non-irrigated area of the same park and had generally poor vigor. Twelve trees received drip irrigation approximately 6 feet from the trunk at the average rate of 0.10 inches per hour for 30 hours in mid-July. Soil moisture sensors were placed in the soil at a depth of 14 inches (depth to the hardpan in the park). Irrigation continued until adequate moisture was detected at that depth. The other 12 trees did not receive supplemental irrigation. The results: Results of the drip irrigation trial showed signs of improved tree growth by mid-August, four weeks after the application of supplemental irrigation. Eight of 12 irrigated trees showed signs of new shoot growth. The 12 trees not receiving supplemental irrigation showed no signs of new shoot growth. After five weeks 11 of 12 irrigated trees had new growth, while only 2 of the non-irrigated trees showed similar growth. One year after irrigation, 83 % of the irrigated trees continued to show improved vigor while significantly less of the non-irrigated control trees showed an improved vigor. Conclusion: Under periods of prolonged drought, supplemental irrigation of established oak trees may prevent their decline and death. If irrigation water is applied well away from the tree root crown area but within the tree rooting zone, irrigating every two to three weeks in the summer months will be helpful.

For more information please contact your local tree care specialist. 688-5580

A Western Arborist Article


Santa Ynez Valley Tree Care
P.O. Box 1147, Santa Ynez, California 93460

(805) 688-5580

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