A bear that does not fatten up before the winter suffers during hibernation. This analogy is closer to perennial crop dynamics than many people realize. Almonds, walnuts, pistachios, and even grapes are not fully dormant during the winter. Metabolic processes are occurring, including respiration, which requires a steady source of stored starches as fuel. If trees use up all of their carbohydrates to build the crop and are not supported to replenish the storage pool of carbohydrates, they will suffer during the winter. They will burn through whatever carbohydrates they have left and show weak vigor in the spring. This will lead to poor bud break, few flowers and a limited nut set.
Starches, like sugars, are a form of carbohydrate. They need to be produced by the trees during the summer and fall months to survive the winter and still have enough left over to fuel the energy-demanding stages of bud break, bloom and nut set. Figure 1 shows the dynamic of carbohydrates for almonds during the year. The summer is the period of the lowest carbohydrate supply. It is during the late summer and fall that the trees restore that supply.
Figure 1: UC Davis Carbohydrate Observatory
The canopy is the source of carbohydrate production. Healthy leaves are more efficient at creating sugars. Healthy leaves require the same things necessary to produce the crop: proper nutrition and irrigation management. Soil nutrients are likely near their lowest levels post-harvest. That is why a soil and perhaps a foliar application are necessary after harvest: to resupply the trees with the nutrients and water they need to create carbohydrates.
The most critical nutrients to apply post-harvest to support carbohydrate production are the three main macronutrients: N, P, and K, and two crucial micronutrients, Zn and B.
Nitrogen is often very low post-harvest. Growers’ concerns about hull rot limit nitrogen availability past July. But it is crucial for a functioning canopy.
Phosphorus is necessary for the production of sugars and is also the most critical nutrient supporting root growth. Both of these activities are vital post-harvest. Additionally, many growers do not apply phosphorus after the spring, so levels of this nutrient are also likely low.
Potassium is required for starch formation and carbohydrate movement. Nut crops are notoriously huge potassium users, so this nutrient is, again, likely not at optimal levels during the fall. Additionally, potassium is the most critical nutrient related to drought resistance. Crops well-supplied with potassium produce higher yields with low water supply compared to potassium deficient crops.
Zinc and boron are the two most essential micronutrients to consider post-harvest. Zinc boosts chlorophyll production, which is needed for optimal photosynthesis to create sugars. It is also used during carbohydrate production. Boron is necessary for healthy flowers and nut set. Although spring is another timing to consider, fall is considered by many to be the optimal timing for boron applications in perennial crops.
These nutrients can be applied directly to the canopy with a foliar application, which is very efficient. But they should also be applied to the soil, especially the macronutrients. This can actually reduce the effect of “off” years in tree crops. The alternate bearing cycle is directly related to carbohydrate supply: after an “on” year, reserves are low, and yields decline the following year. By supporting the trees with proper nutrition, the effect of “off” years can be minimized.
Harvest is not the end of the growing season for perennial crops. The health of the trees post-harvest is just as important in the fall as it is in the spring. Soil and foliar nutrient applications and proper irrigation support the production of carbohydrates necessary for winter survival and spring growth. Fatten up those bears, and they will be ready for strong yields the following year.