- Author: Ben Faber
Transpiration is essentially a function of the amount of leaves present. With no leaves, there is no transpiration and no water use. The extreme case is tree removal. If canopies are pruned there is reduced water use. The more canopy reduction, the more transpiration reduction. Most citrus produces terminal flowers, so there is also a reduction in yield, but there is also typically an increase in fruit size as competitive fruit growing points are removed. There is a balance between yield reduction and tree water use, but typically a 25% canopy reduction results in a 25% decrease in tree water use (Romero, 2006).
The severity of the drought will determine how drastic the canopy should be trimmed. The trees can be skeletonized so that only the main structural branches are left. The tree is whitewashed to prevent sunburn and the water is turned off. As the tree gradually leafs out, the water is gradually reapplied in small amounts. It's important to check soil moisture to make sure the tree do not get too much or too little water. The trees if pruned in the winter will often flower a year later in the spring, but normal production will often take three years for the trees to recover their previous yields.
Skeletonizing should first be practiced on orchards that are the poorest producing. In those areas that get too much wind and have lots of wind scarring or elevated water use, those areas that are most prone to frost damage, those areas that have been always problematic, such as fruit theft. In areas that are healthy and a new variety has been contemplated, this is the time to topwork and replace that old variety. In areas that have been poor producing from disease, this is the time to get rid of those trees.
Canopy sprays of kaolinite clay have shown some promise in reducing transpiration with negligible yield reduction (Skewes, 2013; Wright, 2000). If these are used, they should be done under the advisement of the packing house to make sure the clay can be removed in the packing house.
With a reduced canopy, there are often other benefits besides water reduction. There is better spray coverage for pest control. There is also reduced fertilizer use. New growth is normally coming from nutrients that are now being mined by a large root system and fertilizer applications can be significantly reduced or eliminated altogether for a year until fruit set recommences.
Kerns, D. and G. Wright. 2000. Protective and Yield Enhancement qualities of yield of kaolin on lemon. In: Eds. G. Wright and D. Kilby, AZ1178: "2000 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture and Life Sciences, University of Arizona. http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1178_3.pdf
Skewes, M. 2013 Citrus Drought Survival and Recovery Trial. HAL Project Number CT08014 (16/12/2013). SARDI. http://pir.sa.gov.au/__data/assets/pdf_file/0004/238414/SARDI-Citrus-Drought-Survival-Recovery-Trial.pdf
Navel trees skelotinized and topworked, ready for rain and more profits in the future.
- Author: Ben Faber
Impacts of the recent drought conditions on Central Coast avocado production, and potential impacts of continued drought conditions
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction. Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Defoliation leads to sunburned trees and fruit which can be severe economic losses.
Strategies to address drought conditions
Ensure that the irrigation system is at its greatest potential and is maintained. Avocados are grown on hillsides and pressure regulation is extremely important and is frequently neglected.
Significantly prune trees to reduce leaf area. Avocado can be a very large tree, and if half the canopy is removed, there can be as much as 1/3 reduction in water use. When trees are about 15 feet tall, removing half the canopy can reduce water use by one half.
In extreme drought conditions, the canopy can be reduced to just the skeleton branches which are white washed to prevent sunburn. Water use drops to zero, and then gradually as the tree leafs out, water can be slowly reapplied, but at significantly less amounts than with the full canopy. Stumping typically results in three years' worth of crop.
In orchards that have low producing areas, because of recurrent frost, high winds, shallow soils, disease, etc. the grower could decide to completely remove those trees, thereby saving water.
White kaolin applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since if it is applied to fruit, it is very difficult to remove.
Impacts of the recent drought conditions on Central Coast citrus production, and potential impacts of continued drought conditions
Citrus is much less sensitive to salts and drought than avocado, partially because of its greater rooting depth. However, it is much more sensitive than deciduous fruit trees, resulting in smaller fruit and lower prices when drought cannot be addressed with adequate irrigation water. Drought also makes the trees more susceptible to leaf drop, and sunburned fruit.
Strategies to address drought conditions
The strategies for citrus are very similar to those for avocado. It is much more sensitive to pruning to reduce water use than avocado. Typically removing half the canopy results in half the water use. Because of thus greater control, citrus is rarely stumped.
By reducing canopy size, production can be maintained, often without loss of fruit size.
Kaolin clay can effectively reduced water use and can be applied soon after harvest without the problem of coating fruit making its removal difficult at the packing house.