Summer Irrigation of Established OAK Trees

Trial 1

A group of 165 native valley oak trees (Quercus lobata), 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 (8). The trees had an estimated average age of 125 years, based on a sampling of increment borings. The soil type in the park is a clay loam with a dense, root limiting calcareous layer at 35 cm (14 in). As a result of this relatively shallow soil, the trees have developed extensive but shallow root systems. Therefore, these trees may be quite prone to water stress during periods of drought (4).

The average density of trees was 50 per hectare (20/ac) and they were distributed evenly throughout the test area. The region is currently experiencing a drought, with winter rainfall averaging only 58 percent of normal for the last six years.

The park was first developed in 1978, with sections designated as either landscaped or as natural areas. The landscaped areas received regular summer irrigation from sprinkler systems. The irrigation was applied between the months of March and October, four times per week at 45 minutes of irrigation. Water application rates were 5 cm (2 in) per week, approximating evapotranspiration (ETo).

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 irrigation.

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 dripline of the landscaped areas, and therefore received some irrigation water. The other 135 trees did not receive summer irrigation water.

Trial 2

A group of 24 native valley oaks were selected 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 2 m (6 ft) from the trunk at the average rate of 0.25 cm (0.10 in) per hour for 30 hours in mid-July. Soil moisture sensors (Watermark, Irrometer Co.) were placed in the soil at a depth of 35 cm (14 in), the depth to hardpan in the park. Irrigation continued until adequate moisture measured as < 10 centibars, was detected at that depth. The other 12 trees did not receive supplemental irrigation.

Trial 1

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 a similar condition, X² = 20.58, P <. 01 (Tablel). Root sampling in the area for diseases did not show the presence of damaging pathogens, and no significant levels of insect pests above ground were found.

Trial 2

Results of the drip irrigation trial showed signs of improved tree growth by mid-August, four weeks after the application of supplemental irrigation (Table 2). Eight of 12 (67%) irrigated trees showed signs of new shoot growth ranging from five - 31 cm (2-12 in) in length. The 12 trees not receiving supplemental irrigation showed no signs of new shoot growth. After five weeks, 11 of 12 (92%) irrigated trees had new growth, while only 2 (17%) of the non-irrigated trees showed similar growth. One year after irrigation, 83% of the irrigated trees continued to show improved vigor overthe 1988 ratings. Significantly less of the nonirrigated control trees showed an improved vigor. Only 8% of these trees in 1992 had an improved vigor rating since the 1988 evaluation and 92% showed no change or had declined, X² = 10.47, P <.01.