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.
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. 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.
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
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 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 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.
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.
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.
This allows for a bonus and smiles rather than frowns in the future!
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.
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.
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.
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. 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!
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.
five year old tree will 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!
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.
research concludes that one hectare of a mature olive orchard
will need between 6 and 10 megalitres of water (rainfall and irrigation
combined) per year. (NB. 100mm (4") of steady
rainfall gives one megalitre of water per hectare).
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.