Janine Hasey, UCCE Farm Advisor, Sutter/Yuba/Colusa Counties, and Greg Browne, USDA Plant Pathologist, UC Davis, and Astrid Volder, Plant Sciences, UC Davis, and Bruce Lampinen, UCCE walnut Specialist, UC Davis

In 2017, high and fluctuating water flows, unprecedented for their duration, passed through the Sacramento and Feather Rivers. Orchards were hurt by these flows due to direct flooding, indirect flooding via under-levee seepage, and loss of land through river bank erosion. Many orchards had standing water from January through mid-May. In other orchards, ditches overflowed with nowhere to pump water out. Unlike previous years where floods occurred from levee breaks, such as 1986 and 1997, trees in 2017 had a much longer exposure to waterlogged conditions. We have written previously about flooding damage generalities and past flood events. Part 1 of this article detailed our observations on flooded orchard damage as of early June and current research plans; Part 2 explores management considerations; and Part 3 will provide an update on potential resources available for flood damaged orchards.

Management Considerations

• Flooded orchards will likely respond to irrigation differently than normal, since root systems are compromised.
• In saturated soils, fine roots die, and depending on the extent of flooding, larger roots can die as well. It takes time for the root system to regain functionality and re-start new fine root production after flooding.
• If the water table level below the soil surface drops gradually, trees may be able to continue to produce functional roots at increasingly deeper levels over time. Initial irrigation could be delayed for some time, as deeper roots may maintain water uptake.
• However, if the soil remains saturated at some level below the surface, the only functional roots may be at a very shallow depth and irrigation may have to be initiated earlier than normal. In this case, shorter but more frequent irrigations could help avoid further damage to roots in the upper zone.
  • Carefully monitor both soil water levels and tree water status so that the trees can be gradually brought back to health. Soil based monitoring, using Watermark^® soil moisture sensors installed at different depths, will let you know the level where the soil is saturated as the water level drops, as well as the amount of water that is being moved up above this level of saturation by capillary rise. Apply enough water to bring the soil to field capacity to enable plant water uptake from the upper soil, but avoid prolonged periods of saturation that may damage roots. It is easy to saturate the soil, particularly when the water table is close, so irrigate judiciously, ideally using both soil moisture monitoring and plant pressure chamber data to aid in determining irrigation duration and frequency.
• Do not initiate irrigation until pressure chamber measurements show trees are 2-3 bars below the fully watered baseline (more dry) and only irrigate enough to bring the trees back up to about 0.5 to 1 bar below the baseline by applying short sets of irrigation.
• It is not a good idea to prune or fertilize trees after flooding. Pruning will reduce both carbohydrate reserves and leaf area, while fertilizer applications may mostly go to waste and delay or damage root production. The best approach would be to wait, and let the tree adjust to the altered environmental conditions while managing irrigation as described above to avoid further damage to the root system.