Flooding and Almond Tree Survival

When periods of high amounts of rain occur, orchards can become saturated and flooded. During these periods, tree loss may occur, but this is dependent on the duration of soil saturation, the timing of year, and rootstock.Soil saturation reduces the ability for oxygen to infiltrate into the rootzone. Roots require oxygen to respire, and long periods of saturation can lead to root loss by asphyxiation. Fine feeder roots are often the first affected, with larger secondary roots affected during periods of extended saturation or flooding. Tree survival is dependent on how many roots survive and can regrow before periods of high transpiration occur.The sensitivity of almond roots to saturation is dependent on the timing and duration of the flood event. During dormancy, trees are reasonably tolerant due to lower soil temperatures and low rates of respiration. Flood events that are shorter than 7 days will not have any affect on tree performance. Extending beyond this, however, some root loss will occur, and poor spring tree growth may occur. With extended periods – beyond 10 days – tree loss may occur.After the trees have leafed out, the period of tolerance is much shorter. Standing water within an orchard for 5 days will kill mature trees. This is due to the higher activity of the root system due to warmer soil temperatures, and overall tree activity. In these cases, efforts to drain as much water as possible should occur to reduce the impact of the flooding event.If water is moving through an almond orchard, the effect of soil saturation/flooding is reduced. Since moving water carries oxygen, some movement of oxygen into the soil can occur. This will extend the period before negative impacts are seen by several days. For example, an orchard near the Merced River was flooded in April, 2016 for several weeks. Trees were alive during the period of 7 days of moving water, but areas of standing water declined and died within the same period.Some rootstocks are more resistant to soil saturation and flood events. Plum- or plum-hybrid rootstocks tend to tolerate soil saturation for a greater period than peach or peach-almond hybrid rootstocks. The cause of this is unknown, but it is most likely due to the shallower, more laterally formed roots that are common with these rootstocks. This allows more oxygen to move into a larger area/volume of roots.Post flooding, there are also problems. High water tables or poorly drained areas can continue to kill trees. Phytophthora, a water borne disease, can affect roots that have been saturated as these roots are weaker. Additionally, if the orchard is under water, Phytophthora can affect the aerial parts of the tree, often leading to a quick death once temperatures increase. To manage these root and trunk disease, a foliar spray of potassium phosphite should be considered after leaf-out to improve the tree’s resistance to infection. Please note that there are several ongoing import/export concerns with this potassium phosphite, so consultation with local regulations prior to use is required. Please note that any suggestion of pesticide use is not a recommendation, and local regulations must always be followed when using any pesticide. Please consort with your local authorities before using any pesticide.