How much water will my olive trees need as they grow?

There are many factors which can affect the water requirements of any tree. The soil type is a variable e.g. a tree planted in sand is obviously going to need more regular watering than a tree in clay because of the fast draining nature of sand. The orchard’s local climate – if trees are planted in an area which receives 350 days of sunshine per year they will need more water than those planted in a cloudier climate which may only receive 200 sunny days per year.

Annual rainfall – the tree’s supplementary irrigation needs. In places which usually receive a Mediterranean type winter rainfall such as Vic., S.A. and W.A. the supplementary winter irrigation given in North New South Wales and Queensland is generally not needed.

Although the olive tree is very resistant to drought, when continued water shortage occurs it survives at the expense of the crop.  There are scores of papers and books written on the subject of olive orchard irrigation, however, one of the simplest to understand is by Goldhamer, Dunai and Ferguson, University of California, 1994. Excerpts have been reproduced with permission from ‘Acta Horticulturae’ below.

The trial was carried out on a mature Manzanillo olive orchard in a very dry region of California. The trees were planted at a spacing of 4.57 x 9.14 m (231 trees/ha). Eight different rates of irrigation were applied to a number of plots within the orchard over three years (1990-1992). There was a total of six plots under each rate of applied irrigation (6 plots x 8 different irrigation rates = 48 plots total). The applied annual irrigation ranged from just 232 mm (9″) to 1016 mm (40.6″). Each plot was assessed for a number of variables, the most relevant of which was the fruit yield in kg/ha.

Trees which were only ‘supplementary’ irrigated with 232 mm (9″) (on top of the 100mm (4″) natural rainfall) yielded an average of 45.5kg/tree (10,500 kg/ha), whereas, trees fully irrigated with up to 1016 mm (40.5″) yielded an average of 95.7 kg/tree (22,100 kg/ha). Average yields per tree with their corresponding irrigation levels were as follows: 232mm-45.5kg, 338mm – 53.7kg, 424mm – 56.7kg, 599mm – 66.7kg, 729mm – 75.8kg, 838mm – 85.3kg, 945mm – 94.8kg, 1016mm – 95.7kg. All of these figures were averaged over the two years 1991 and 1992 to take into account the effects of alternate bearing. NB. Alternate bearing is reduced when irrigation is applied to an orchard. It should also be noted that irrigation increased the actual dollar value of the fruit due to its healthier weight and appearance.

Although this Californian paper gives an average of up to 95.7kg/tree in a mature fully irrigated Manzanillo orchard and the Australian Mildura trial averaged 93.1 kg/tree under similar conditions, a conservative figure for Australia, we estimate for a mature tree to yield in the same conditions at 70 kg/tree.

The final line of the paper reads, “This suggests that meeting the water use requirements of the trees over the season is preferable to sustained deficit, even if water costs are relatively high.” Sustained deficit irrigation refers to the trees inability to get enough water to remove all water stress during the year. If the olive grove receives its optimum water requirements throughout the year, then much greater yields can be expected than if it goes through periods of water shortage.

To keep in good health, olive trees need at least two full waterings to field capacity (full depth of roots eg. 75 cm (2’6″) in 10-year-old trees), each winter. If water can be applied more regularly during winter or at other times of the year then this will be most beneficial and will result in increased crops. Five lots of applied irrigation to field capacity spread throughout the year will ensure the ongoing health of the orchard, however, commercial crops will need greater amounts of water as outlined below. Trials have shown that mature trees which receive over 800mm (32″) of combined rainfall and irrigation water give the best commercial crops. If you have regular rain during summer then most of your applied irrigation will be needed in winter and vice versa for winter rainfall areas. It must be remembered, however, that the olive’s only real enemy is too much water.  So keep a good eye on the moisture levels around the trees.  You can monitor the water available to the tree by installing a Tensiometer system.  Use your water wisely.

Olive trees have two main growth stages – an intensive stage in the Spring and early Summer and a less vigorous stage in early Autumn.  Late winter watering will help the tree to flush out in fresh growth which is an important part of flower setting for the following two seasons. A study by Ruggieri found 52.6% of sterile flowers in olive trees growing under dry soil conditions compared with only 7.7% in trees growing under irrigation. The need for adequate water supplies to ensure the formation of large numbers of perfect flowers begins during the previous Summer when it is important to avoid excessive leaf dropping due to drought as this reduces the tree’s photosynthesis ability.  Stress from water shortage during pre-emergent flower development in the winter can also seriously affect the production of perfect flowers and therefore reduce the overall crop.

The actual volume of water required by a mature olive tree in a year will vary due to the factors in paragraph 2. However, we have decided to provide an estimated per watering requirement for a mature olive tree grown in the recommended 5 X 8m orchard layout (spacing recommended for Mechanical Harvesting). This amount is an estimation of a tree’s total requirement for each watering with no rainfall.  In other words, the water requirements below are for olive trees growing at Oodnadatta or Birdsville during a ten-year drought!

Firstly, it is being taken for granted that the ground beneath the tree canopy is mulched and free from weeds, grass and cover crops. If you want to supply enough water to irrigate grass, weeds or other crops directly under the trees then you will need 30% more water than the following suggests. To establish a newly planted tree (0 – 1-year-old) we recommend about 10 litres (2galls) per week in a single application during the summer and less in winter. However, for our smaller commercial orchard size trees (300mm size), use about 3 – 4 litres per weekly watering in the first couple of months and then slowly increase to 10 litres per weekly watering as the tree grows.

A five-year-old tree should have a root system covering at least 3mX3m (10ftx10ft) and the roots will go about 600mm (2ft) deep. If the soil is of medium texture (ie not too sandy and not too heavy) then approximately 75mm (3in) of water will penetrate dry soil to the full depth (field capacity) of the tree’s root system.  This represents about 675 litres (150galls) of water per tree site if the soil is bone dry, which it would not normally be.  One hectare (250 trees at 8X5m spacing) would, therefore, require a maximum of 169,000 litres (37,500 galls) per watering – in Oodnadatta!

A ten-year-old tree will have a root system covering approximately 5m X 5m (16ftx16ft) and going down about 750mm (2’6″). Maximum water per tree site would be 1,875 litres (416 galls) per watering in bone dry soil. Therefore the maximum water required per hectare per watering (with no rain), would be 468,750 litres (104,000 galls). The root systems of mature trees will vary in their spread and depth depending on the soil types and irrigation methods used. As such, the following international research is a valuable guide for irrigation planning.  Irrigating olive trees with the correct sprinkler will but down water wastage. The Olive Sprinkler has been specifically developed for olive trees and has been tested in the field for many years.

The International research concludes that one hectare of a mature olive orchard will need between 6 – 10 megalitres of water (rainfall and irrigation combined) per year. (NB. 100mm (4″) of steady rainfall gives one megalitre of water per hectare).

After stating all of the above, it is still impossible to give a perfect ‘watering table’. As noted in the introduction, there are so many variables such as soil type, rainfall quantity and regularity, evaporation and transpiration rates, just to name a few. There are many valuable methods to scientifically estimate the irrigation needs of an olive orchard. Some of these are well outlined in the Californian “ Olive Production Manual“. In the long run, it comes down to the growers common sense (which is increased by experience) to understand whether the orchard needs water or not.

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