To help your tree maximize its fruit production, water it throughout the summer’s dry season. Since soils and microclimates vary widely, there is no set amount of water or frequency of watering per week that works for all locations. There are guidelines that can be followed, however, that will help in the development of an appropriate irrigation program.
Olives are shallow rooted trees. While they do have some deep roots that reach down for stability, the majority of the actively feeding roots are closer to the soil’s surface, making deep watering unnecessary. Watering a few times per week with shorter duration is typically preferable than one long drink per week. A shovel or soil probe can be used to determine how many hours it takes to reach the depth of the deepest active roots and irrigation durations can be timed to reach that depth. Most orchards have more than one soil type and often the irrigation program is a compromise that has to satisfy many varying soil compositions.
When establishing a young orchard, information for watering according to crop development can be disregarded. In the first few years, the goal is to increase the canopy as much as possible, by judicious watering through the warm season. The vegetative growth in the initial years will help increase fruit production in the following years. A producing orchard’s water requirements will fluctuate throughout the season, according to the physiological stage of the fruit.
It is important to make sure the trees are quite hydrated during bloom. The flowers necessitate a great deal of moisture to remain viable, as dry, desiccated pollen will not be fertile. Often during bloom there is still actively growing, green grass on the orchard floor which can look deceptively lush. This grass is using up water from the soil too, so it is crucial to provide enough water for the trees to feed their own copious blossoms. As each spring will be different, the soil’s moisture level as the buds are forming and during the blooming period should be carefully monitored.
Like many drupe fruits, olives size up in a double-sigmoid pattern. After fruit set, the fruit grows very quickly via cell division and cell enlargement. Then there is very little change in fruit size as the pit matures and hardens. The last stage of growth is all cell enlargement as the fruit matures and finally accumulates its oil. The first stage of fruit growth (cell division and enlargement) requires a good deal of water from the tree. This period is a good time to keep the trees nicely watered. Pit hardening requires less moisture from the tree and irrigation can be less frequent in this period. The last stage requires more water again as the fruit sizes up and accumulates its oil.
A few weeks before harvest, irrigation should be reduced. Overwatered fruit is hard to mill, and being plumped up with water (and not oil), will cost more if the charge is by the ton! On the other hand, extracting oil from dry fruit is equally difficult; if the fruit is shriveling, it is too dry. Most mills prefer a moisture level between 52% and 54%. In addition to the fruit’s hydration level, the tips of the branches will indicate the stress level of the trees: turgid, erect leaf tips indicate a tree with sufficient water reserves but wilted, flagging tips are symptoms of drought stress. Butte County Farm Advisor Joe Connell's extensive research into olive orchard irrigation management is another excellent resource.
For specific advice regarding your olive trees' irrigation requirements visit our Consultations page for more information.