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Irrigation for Olive Orchards

"How much water will my olive trees need as they grow?" Although this is one of the most common questions asked regarding olive trees it is also the most difficult to answer.

There are so many factors which can effect the water requirements of any tree. Soil type is an obvious variable - 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 - this is the most obvious variable when considering the tree's supplementary irrigation needs. Due to the inconsistent rainfall patterns in California and most of the US southwest Santa Cruz Olive Tree Nursery recommends some type of irrigation system in all cases.

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 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 15 x 30 ft (93 trees/acre). Eight different rates of irrigation were applied to a number of plots within the orchard over three years (1990-1992). There were 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 9 inches to 40.6 inches . Each plot was assessed for a number of variables, the most relevant of which was the fruit yield in pounds/acre.

Trees which were only 'supplementary' irrigated with 9 inches (on top of the 4 inches natural rainfall) yielded an average of 113.8 lb/tree (10,580 lb/acre), whereas, trees fully irrigated with up to 40.5 inches yielded an average of 210.5 lb/tree (19,580 lb/acre). Average yields per tree with their corresponding irrigation levels were as follows: 9 inches-114 lb, 13.3 lb-118 lb, 16.7 in -125 lb, 23.6 - 147 lb, 28.7 in - 167 lb, 33 in - 188 lb, 37.2 - 209 lb, 40.5 in - 211 lb. 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 211 lb/tree in a mature fully irrigated Manzanillo orchard and an Australian Mildura trial averaged 205 lb/tree under similar conditions, Santa Cruz Olive Tree Nursery believes that a more conservative value would be an estimated mature tree yield in the same conditions at 154 lb/tree. This allows for a bonus and smiles rather than frowns in the future!

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. 2.5 ft 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 32 inches 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.

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 16 x 26 ft orchard layout. 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 in Death Valley 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 2.5 gallons per week in a single application during the summer and less in winter. However, for our smaller commercial orchard size trees 1 ft, use about 0.8 gallons per weekly watering in the first couple of months and then slowly increase to 2.5 gallons per weekly watering as the tree grows.

A five year old tree will have a root system covering at least 10 x 10 ft and the roots will go about 2 ft deep. If the soil is of medium texture (i.e., not too sandy and not too heavy) then approximately 3 inches of water will penetrate dry soil to the full depth (field capacity) of the tree's root system. This represents about150 gallons of water per tree if the soil is bone dry, which it would never normally be. One acre (100 trees at 26 x 16 ft spacing) would therefore require a maximum of 37,500 gallons per watering - in Death Valley!

A ten year old tree will have a root system covering approximately 16 x 16 ft and going down about 2.5 ft. Maximum water per tree would be 416 gallons per watering in bone dry soil. Therefore the maximum water required per acre per watering (with no rain), would be15,000 gallons.

International research concludes that one acre of a mature olive orchard will need between 2.4 and 4 megalitres of water (rainfall and irrigation combined) per year. (NB. 4" of steady rainfall gives 0.4 megalitre of water per acre).

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.