Rainfall in Southern California has been higher than usual in recent years, leading to a range of challenges for citrus and avocado growers. In the winter of 2023, Carpinteria experienced an unusual four inches of rain in July. Some young trees with underdeveloped root systems collapsed due to asphyxiation, which occurs when excess water pushes out oxygen from the soil, depriving roots of air.
Avocados are generally more vulnerable to this problem than citrus trees, and some rootstocks are particularly susceptible. “Asphyxiation is a physiological problem that may affect certain branches, whole limbs or the entire tree. Leaves wilt and may fall, the fruit withers and drops and the branches die back to a greater or lesser extent. The condition develops so rapidly that it may be regarded as a form of collapse,” according to experts at UC Agriculture and Natural Resources.
The risk is especially high in fine-textured or shallow soils with poor drainage. During periods of frequent rainfall—such as those seen in 1997-98 and again recently—even well-drained slopes have suffered because persistent wet conditions displace oxygen needed by roots. When stressed by drought after such events, damaged roots cannot provide enough water to leaves, causing trees to collapse further.
Excessive moisture also allows ethylene gas—a plant growth regulator—to accumulate around roots, accelerating leaf and fruit drop. Iron chlorosis often follows due to reduced root function.
Despite these risks, two consecutive wet winters have had some positive effects: lower rates of leaf burn from salt accumulation and less damage from Botryosphaeria fungi compared to dry years when growers rely heavily on salty irrigation water.
With heavy rains continuing into early 2026—San Marcos Pass recorded 32 inches since October 1—growers face new decisions about orchard management practices like mulching. Mulch can help prevent erosion, control weeds, improve infiltration, increase organic matter, reduce soil temperature, and minimize evaporation loss. However, during very wet seasons it can keep soils too moist by reducing evaporation further.
“So, what do you do with a thick layer of mulch when there is too much rain? You pull it away several feet from the base of the tree so that sun can dry out the soil,” advises UC experts.
For orchards on flat ground without raised mounds or berms—or where heavy soils dominate—the risk remains higher for asphyxiation problems after major storms. Besides adjusting mulch practices during rainy spells, allowing weeds or cover crops to grow can help draw moisture out of saturated soils through their own transpiration processes.
“When confronted with new conditions it’s time to make changes to your regular practices,” experts recommend.