CHAPTER IV: CLIMATIC REQUIREMENTS OF DATE PALM

By A. Zaid and P.F. de Wet
Date Production Support
Programme

The climatic factors influencing date growth and production are discussed in
this chapter. Temperature, rain, humidity, light and wind are the most important
climatic factors which determine the suitability of a specific site for growing
date palm.

1. Temperature requirements

The highest maximum temperatures found in the date growing areas of the world
are the result of low humidity, great insolation and long days in summer.

There are several ways to record temperature, and the analysis of its effect
will be conducted accordingly. First, there is the average daily temperature
with a daily maximum and a daily minimum (Table 19), then the values of average
annual temperatures are also used (Table 20). The average of a larger number of
daily records translated into monthly values will, however, give a more accurate
result (Table 21).

TABLE 19
Average maximum and minimum daily temperatures of
various date growing areas (°C)

Station/Country

Length of record (years)

Maximum May to Oct. incl.

Minimum (Jan.)

Turbat/Pakistan

41

6.7

Basra/Iraq

19

37.4

6.4

Muscat/Saudi Arabia

38.5

15.9

Cairo/Egypt

33.7

7.6

Gabes/Tunisia

30

29.3

6

Touggourt/Algeria

15

35.9

3.4

Erfoud/Morocco

12

36.4

1.3

Elche/Spain

28.2

6.9

Indio/California; USA

25

37.6

3.7

Bahrein/Bahrein

12

34.3

13.3

Oued Haifa/Sudan

30

40.2

1.1

Keetmanshoop/Namibia

37

35.1 (Jan)

6.2 (July)

Source: Saeed, 1972; Al Bakr, 1972.

TABLE 20
Average annual temperatures for various date growing
areas

Place/Country

Length of record (years)

Temperatures (°C)

Basra/Iraq

4

24.2

Baghdad/Iraq

9

22.5

Tozeur/Tunisia

49

21.3

Touggourt/Algeria

25

21.4

Biskra/Algeria

25

21.8

Niamey/Niger

-

31.5

Kidal/Mali

-

27.7

Keetmanshoop/Namibia

37

20.7

Source: Dowson, 1982.

To compare the climates of different places with regard to their suitability
for date cultivation, it is appropriate to use the average of the two extremes.
However, in order to differentiate between places which may grow dates and those
that are not suitable, it is recommended that use be made of the annual average
of the daily averages.

Date palm is cultivated in arid and semi-arid regions which are characterised
by long and hot summers, no (or at most low) rainfall, and very low relative
humidity level during the ripening period. Exceptional high temperatures (±
56°C) are well endured by a date palm for several days under irrigation. During
winters, temperatures below 0°C are also endured. The zero vegetation point of a
date palm is 7°C, above this level growth is active and reaches its optimum at
about 32°C; the growth will continue at a stable rate until the temperature
reaches 38°C/40°C when it will start decreasing.

Even though date palm is a thermopile species, it withstands large
temperature fluctuations. Below 7°C growth stops and this stage is called a
resting period. When the temperature decreases for a certain period to below
0°C, it causes metabolic disorders which lead to partial or total damage of
leaves. At -6°C pinnae margins turn yellow and dry out (Figures 29a and 29b).
Inflorescences are also heavily damaged by frost (Figure 30). When frost periods
are suspected, inflorescences should be protected with craft paper bags
immediately after pollination (Figure 31). From -9 to -15°C, leaves of medium
and outside canopy will be damaged and dry out. If these low temperatures are
maintained for a long period (12 hours to 5 days) all leaves will show frost
damage and the palm will look as if it was burnt (cases of Morocco date
plantations in 1952, and 1965; Iran, 1964; USA, 1913, 1937, 1949 and 1959
(Mason, 1925a; Nixon, 1937). The more leaves damaged, the greater the
possibilities of poor fruit quality that year. Poor flowering could also be
expected the

TABLE 21
Average daily and annual maximum air temperatures at
various date plantations in northern and southern hemispheres

Station/Country

Length of record (years)

Average daily maximum temperature (°C)

Average annual maximum temperature

Northern Hemisphere

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sept

Oct

Nov

Dec

Touggourt/Algeria

15

17

19.5

22.7

28

32.3

37.2

41.3

40.1

36

29

21.8

17.4

28.6

Biskra/Algeria

27

16.1

18.3

21.7

26.1

30.6

36.1

41.1

40.6

34.4

27.8

21.1

16.7

27.2

Tozeur/Tunisia

9

15.2

18.6

22.5

27.4

31.7

37.6

41.7

40.3

34.3

28.4

21.4

15.7

27.9

Basra/Iraq

19

15.6

18.4

23.2

28.9

34.6

38.1

40.2

40.5

38.5

32.6

25

17.8

29.4

Indio/Ca, USA

25

20.9

23.8

26.4

30.1

33.7

38.9

41.4

40.8

38.1

32.6

26.9

21.5

31.3

Southern Hemisphere

Jul

Aug

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Alice Springs/Australia

46

19.4

22.2

26.5

30.5

33.4

35.3

35.9

34.9

32.4

27.9

22.8

19.8

28.4

Finke P.O./Australia

24

20.2

22.3

26.6

31.1

34.4

36.4

37.8

36.6

34.0

29.2

23.5

20.7

29.4

Kakamas/RSA

15

21

24

27

31

33

36

36

35

32

29

24

22

29.2

Goodhouse/RSA

15

23

26

29

33

35

38

39

38

36

32

27

23

32

Keetmanshoop/Namibia

37

21.2

23.4

27.4

29.9

32.6

34.4

35.1

33.7

31.8

28.1

24.1

21.2

28.6

Hardap/Namibia

20

22.9

25.1

28.7

31.4

34.4

36.4

35.6

33.6

31.8

28.9

25.7

22.7

29.7

It is worth mentioning that owing to the fibrillium, the meristematic area of
a date palm (growth centre) is well protected against frost. After frost
occurrence and during early spring, the damaged palm restarts its normal growth.
Nixon (1937) noted that date gardens that were irrigated during frost periods
were less damaged than the ones that were not irrigated.

The Zahidi variety (Iraq) was found to be the least damaged by frost while
Khalas (also Iraq) is the most sensitive.

According to Mason (1925), the growth of a date palm does not cease if:

(i) minimum daily temperature does not fall below freezing point,
and
(ii) maximum daily temperature at the growth centre does not fall below
9 to 10°C.

The temperature requirements presented below are those of normal growth,
flowering and fruit maturation. Date palm flowering is initiated after a cold
period, when the temperature becomes high enough and reaches a level known as
the flowering zero (0). This temperature level varies according to varieties and
to local climatic conditions. Table 22 indicates the flowering time of several
date growing regions.

TABLE 22
Flowering time and related average daily temperatures
of various date plantations around the world

Plantations/Country

Flowering Period

Temperatures (°C)

Tafilalet/Morocco

March - April

21

Touggourt/Algeria

Mid March - Mid April

18

Atar/Mauritania

February

22

Basra/Iraq

March

18

Aswan/Egypt

March

20

Kakamas/RSA

July - August

22

Keetmanshoop/Namibia

July - August

21

Alice Springs/Australia

August - September

17

Source: Dowson, 1982.

Flowering temperatures represent the average daily temperatures from
initiation till the end of the flowering period, while the fruiting period of
date palm starts at fruit set and ends at fruit maturation; its length (of about
120 to 200 days) varies from one date variety to another and depends on
environmental conditions (Table 23).

TABLE 23
Average fruiting periods of various date growing
areas

Plantation/Country

Fruiting period (days)

Months

Touggourt, Bechar, Laghouat and Alkantara/Algeria

180

May - October

Kankossa and Atar/Mauritania

135 to 150

March - July

Nema/Mauritania

135

Mid February - June

Bilma/Niger

150

Mid March - Mid August

Kidal/Niger

135

March - Mid July

Kayes/Mali

150

January - End May

Basra/Iraq

165

March - August

Turba/West Pakistan

200

February - Mid August

Cairo/Egypt

239

End February - End October

Alice Springs/Australia

185

August/Sept. - March/April

Keetmanshoop/Namibia

195

September - March

Kakamas/RSA

200

September - April

Source: Djerbi, 1995; McColl, 1992.

The above two criteria (flowering temperature and fruiting period) allow the
calculation of the heat units, which correspond to the sum of the average daily
temperatures from flowering time till fruit maturation, that are necessary for a
given variety under a given environment. Different methods of calculating this
heat unit value were used by the following authors:

- In 1879, Cosson estimated the sum of heat units necessary for Algerian
date varieties as 6,000 °C. Similar results were obtained by Fisher (1883)
while working with Egyptian varieties (6,136°C). Those authors' techniques
were based on using the earliest date in each place when the temperature first
reached 18°C in the year, and the last date of the year when the temperature
dropped below that value.

- De Candolle (1883) considered 10°C as the growth's zero value. By
subtracting it from the average daily temperature and then adding all the
obtained values together, he found that 5100°C was the required heat units to
ripen fruits of some date varieties completely.

- Swingle (1904) added the daily temperature maxima for a period of 184
days. The growth zero value used was 18°C (since the flowering process does
not commence below 18°C) and consequently the value of heat units for several
date plantations are shown in Table 24.

- Munier (1973) used the same technique as Swingle but differentiated
between varieties such as Gharas, Degla Beida with 180 days and a heat unit
value of 1,800°C, and Deglet Nour with 200 days and a heat unit value of
1,890°C. His results, presented below (Table 25), concluded that the value of
1,000 °C is the minimum limit for growing a productive date
palm.

TABLE 24
Value of heat units at various date growing areas in
Algeria and Iraq

Plantation/Country

Heat Units (°C)

Laghouat/Algeria

2,327

Biskra/Algeria

3,049

Ayata/Algeria

3,295

Touggourt/Algeria

3,666

El Golea/Algeria

3,990

Baghdad/Iraq

3,898

Source: Swingle, 1904.

TABLE 25
Heat units at various date growing countries

Plantations/Country

Heat Units (°C)

Elche/Spain

792

Laghouat/Algeria

990

El-Kantara/Algeria

1,170

Touggourt/Algeria

1,854

Bechar/Algeria

1,620

Atar/Mauritania

1,860

Kankossa/Mauritania

1,836

Nema/Mauritania

1,903

Basra/Iraq

1,872

Kidal/Mali

1,841

Kayes/Mali

1,980

Agadez/Niger

1,827

Bilma/Niger

1,860

Largeau/Chad

1,860

In conclusion, the value of the heat unit is of great importance for defining
the suitability of a site in which to grow a productive date palm, to eliminate
areas that cannot grow date palm, and also to help in variety selection.
Furthermore, the temperatures shown in the above table can be regarded as
optimum for date cultivation.

2. Rain effect

Date palm culture has mostly been developed in areas with winter rainfall
which does not cause harm to the date fruits, but benefits the soils of the
plantations by lessivating the deposited surface salt and avoiding the upward
movement of salt from lower layers. Table 26 shows that the main date-growing
regions are almost rainless until November. In these countries harvest begins
mid August until the end of October. Rain during the flowering and harvest
season is likely to cause some damage to the fruits.

TABLE 26
Rainfall in the main date growing regions

Grove/Country

Length of record (years)

Rainfall (mm)

Northern Hemisphere

Jul.

Aug.

Sept.

Oct.

Nov.

Multan/Pakistan

-

60

50

8

0

2

Maskat/Oman

37

0

0

0

2

10

Bushire/Iran

52

0

0

0

2

40

Bahrein/Bahrein

21

0

0

0

0

10

Basra/Iraq

24

0

0

0

2

22

Cairo/Egypt

25

0

0

0

1

3

Tozeur/Tunisia

49

1

2

7

9

12

El Oued/Algeria

25

0

0

3

7

13

Biskra/Algeria

25

1

2

7

9

12

Indio, Ca/USA

33

0

1

1

1

1

Yuma, Ariz/USA

60

1

2

1

1

1

Southern Hemisphere

Jan.

Feb.

Mar.

Apr.

May

Alice Springs/Australia

17

43

41

33

16

13

Finke/Australia

26

22

40

17

14

11

Keetmanshoop/Namibia

37

25.2

43.4

41.3

16.3

4.4

Mariental/Namibia

50

35.9

54.5

47.2

15.7

3.1

Source: Nixon and Carpenter, 1978; Dowson, 1982; and McColl, 1992.

There is controversy about the effect of rain on pollination and fruit
set. Rain that occurs just after pollination is considered as a washing
agent that takes away most of the applied pollen. Another negative effect of
rain on fruit set also results from low temperatures that accompany or follow
rain. A third factor is the reduction of the flower's receptivity when in
contact with water.

Date growers must assume that the rain will affect pollination/fruit set, and
any pollination operation immediately preceded or followed by rain (4 to 6
hours) must be repeated.

Rain is also responsible for increasing the relative air humidity creating
favourable conditions for cryptogamic diseases that result in the rotting of
inflorescences. A light spring shower with ahigh relative humidity followed by
warm temperature just before pollination causes the Khamedj disease
(Mauginiella schaettae). This high relative humidity is also associated
with the pollen's explosion (Further details on this subject are to be found in
the Chapter "Pollination and Bunch Management").

The major damage caused by rain occurs when either the rain is early, or the
dates are late in ripening. In fact, rain does not seriously damage the dates
when they are still at the early Khalal stage, but rather has a beneficial
effect by washing away all dust and sand particles from the fruits. Rain can
however cause severe checking and cracking in the Kimri and late Khalal stages.
In some areas of California (USA), where rain damage is frequent, date growers
commonly use craft paper to protect fruit bunches (Figure 32). Rutab and Tamar
stages are the most sensitive since rain and associated humidity cause severe
damage including rotting and fall-off of the fruit. Rain or cool weather
occurring near harvesting is also inclined to delay ripening.

It is worth mentioning that the amount of any particular rain is of less
importance than the conditions under which it occurs (Nixon and Carpenter,
1978). A light shower accompanied by prolonged periods of cloudy weather and
high relative humidity may cause more damage than heavy rain followed by clear
weather and dry winds.

Varieties of dates differ in their susceptibility to rain and humidity.
Dayri, Khastawi, Thoory, Khadraoui, and Sair, under Coachella Valley conditions,
were found to be the least damaged of sixteen varieties (Nixon, 1950). Zahidi,
Khalas and Barhee varieties had average damage, while Deglet Nour, Yatima,
Hayani and Ghars were the most sensitive. Both very early (Al Mehtari/Iran) and
late varieties (Khissab and Hilali/Iraq) escape the rain and consequently their
fruits are not affected.

The chances of damaging rain (in August, September and October for Northern
Hemisphere and January, February and March for the Southern Hemisphere) in any
given season is more important from a practical point of view. Hence, a season
could be considered: Good to average, if less than 50 mm of rain fell in each of
the three months; average to poor, if more than 50 mm of rain fell in one of the
three months, poor, if more than 50 mm of rain fell in two of the three months;
and very poor, if more than 50 mm of rain fell in each of the three months.

The extension of date palm culture to the southern part of the Sahara Desert
(Sahel) came as an exception to the above with heavy rain in July, August and
September, and a dry period during October, November, and December (Table
27).

TABLE 27
Rain distribution and fructification periods in the
Sahel date growing countries

Source: Djerbi, 1995.

Table 27 illustrates the case of Mali, Mauritania and Chad where there is a
perturbance of the date production cycle resulting in two periods of
fructification:

i) One first short cycle, of about 130 to 150 days, characterised by
flowering during the dry season and fruit maturation at early rainy season
with an abundant yield but of a low quality.

ii) A second long cycle, of about 180 to 200 days, where flowering occurs
during the rainy period and the fruit maturation at the dry season; the yield
is less abundant but of a high quality. Table 28 illustrates the above two
cycles in both Mali and Niger.

TABLE 28
Fructification cycles and rain distribution in two
Sahel growing countries

Source: Djerbi, 1995.

3. Air relative humidity

Depending on air humidity at the locality of a date palm plantation, various
advantages and/or disadvantages are found. In fact, the date palm eco-system is
mostly of an arid nature where air relative humidity has a large influence.

In the presence of high air humidity, some leaf diseases, such as Graphiola
leaf spot (Graphiola phoenicis Moug. Poit.), are becoming more prevalent,
while others, such as the Date mite (Bou-Faroua), become rare or absent. On the
other hand, when air humidity is low, fungal diseases are absent, while pest and
mite attacks are dominant.

Air humidity also affects the date quality during the maturation process. At
high humidity, fruits become soft and sticky, while at low humidity they become
very dry (case of Northern Sudan and in-land plantations). This phenomenon is
strengthened when low humidity is coupled with hot and dry winds (called Chili
in Tunisia, Chergui in Morocco, N-W in Iraq, etc.).

Dry and hot winds cause a rapid maturation process leading to fruit
desiccation and the appearance of a yellow or white ring at the fruit base.

When air humidity is high during maturation (date plantations in Bahrein,
Minab/Iran), the skin of the date fruit shows several cuts or breaks with an
edge-blackening (Blacknose), the soft fruits fall to the ground and consequently
lose their commercial value.

The number of these breaks and the manner in which they occur (longitudinal,
transverse, or irregular) varies between varieties. Deglet Nour's checking
occurs mainly near the tip of the fruit.

The above effect occurs only when high humidity is experienced immediately
before the Khalal stage. After the fruit acquires the Khalal colour, checking no
longer occurs. After the flesh softens in the Rutab stage, the skin does not
break readily upon contact with moisture, but the fruit absorbs moisture and
tends to become sticky and more difficult to handle. After the Tamar stage is
reached, air humidity causes little damage to the fruit unless it is
neglected.

Most commercial date plantations in USA and Israel are not intercropped with
any other plants/fruit tree species (citrus, lucerne, etc.) because of the
increasing effect of these on the humidity level of the date plantation.

The following table illustrates the high relative humidity of four Middle
East date plantations and at one place in Namibia.

TABLE 29
Examples of relative humidity of various date
plantations (at 08:00 am)

Place/Country

Length of record (years)

Averages of Relative Humidity (%)

July

August

September

October

Baghdad/Iraq

80

37

40

42

51

Basra/Iraq

18

51

51

55

60

Bahrein/Bahrein

15

68

74

74

77

D.I. Khan/Iran

10

72

75

75

76

Jan.

Feb.

March

April

Keetmanshoop/Namibia

37

43

52

57

58

Source: Dowson, 1982.

4. Wind

Compared to other plant species, the date palm shows no damage under windy
conditions. In fact, date palm can withstand strong, hot and dusty summer wind
and consequently protects the other cultures by breaking the force of the wind
and sheltering more susceptible vegetation (Dowson, 1982).

Wind is, however, a carrier of dust and sand that adheres to the date fruits
in their soft stage (Rutab and Tamar). When the fruits are at their early
development stage (green/Habakouk), nearly black-indurated patches are sometimes
seen on the fruit because of the wind beating the tender fruits against the hard
fronds. Recently planted small offshoots can, however, easily be uprooted by
strong winds.

In most date growing areas the latter part of the pollination season is
usually characterised by severe hot and dry winds which dries out the stigmas of
the female flowers. Cold winds disturb the pollen germination. It seems,
therefore, that dry wind storms lead to a faster drying of the styles which
shortens the time for the pollen to reach the ovule (Reuveni et al., 1986). The
speed of the wind could also have an effect on the pollination efficiency; light
winds are beneficial and favour pollination while high speed winds will blow
away a great deal of the pollen, especially on palms found at the edges of the
plantation. In some cases severe wind could also break the inflorescence's fruit
stalk (rachis), blocking the movement of nutrients and finally causing the death
of the bunch. It has also been suggested that mites are carried from palm to
palm by wind.

The falling down of an old date palm may be caused by strong wind but only in
the following cases:

- If the palm is very tall with a large crown and grows in shallow
soil;

- If a large number of offshoots are removed form the trunk of a palm at
one time, leaving the palm without basal support; and

- If rats have gnawed away the roots on one side of the
palm.

It is highly recommended that wind breaks be planted one to two years before
the establishment of any commercial date palm plantation. Beefwood (Casaurina
cunninghummiana, or C. glauca) is the most appropriate wind break for
the protection of a date palm plantation (Figure 33) since they are both drought
and salt tolerant.

5. Light

According to Mason (1925b), the growth of a date palm is inhibited by light
rays at the violet and yellow end of the spectrum, but enhanced by rays at the
other end of the spectrum i.e. red light. These latter rays are most active in
promoting photosynthesis.

Clouds could reduce light intensity, but unfortunately, the sky is un-clouded
in the date growing countries during the ripening period (July to October in
Northern Hemisphere and February to May in Southern Hemisphere). Table 30 gives
the average number of cloudless days in three famous date growing areas of
Algeria (Lasserre, 1922), while Table 31 shows the sunshine and radiation values
obtained in Keetmanshoop area (Namibia) for a period length of 37 years
(1948-1985).

TABLE 30
Average number of cloudless days in Algeria

Place

Length of records
(Years)

Number of cloudless days from
July to
October

Biskra

52

Touggourt

5

38

Eloued

75

TABLE 31
Sunshine and radiation (calories per square cm) in
Keetmanshoop in Namibia

Month

Mean total of sunshine in hours

Mean solar radiation/day

Global

Diffuse

January

351.4

767.1

129.5

February

296.4

697.2

121.3

March

301.7

542.4

142.7

April

305.9

521.4

67.3

May

304.5

428.6

62.2

June

286.9

372.5

64.9

July

305.6

375.3

67.9

August

320.5

465.6

78.6

September

315.1

542.8

110.2

October

337.7

696.3

103.0

November

348.2

741.0

132.6

December

370.9

765.4

125.6

Mean

320.4

In conclusion, the five important factors which determine the suitability of
a site for date palm culture, are temperature, rain, humidity, light and wind.
The effect of these factors on the growth and productivity of date palm is to be
considered as a combination of all the factors and not each one separately.

Furthermore, because of the nature of date palm, its growth is not as easy as
some other plant species and attention should be given to the following
characteristics:

- Date palm cannot be classed as arenaceous, nor aquatic. Although
frequently found growing in sand, its roots are characterised by air spaces
similar to the ones in Banana roots, but it also grows well where the subsoil
water is close to the surface.

- It is not a true halophyte despite its healthy growth in exceedingly
salty places, as it grows better when the soil and water are sweet.

- It is not a xerophyte; although it has a thick waxy cuticle, leaflets
with the two halves of their upper surfaces to the sun, reduced leaf surface,
efficient insulation of the growing point and its trunk's vascular bundles; it
requires a copious water supply.

Figure 29. Effect of low temperatures on:

A - Five years old Barhee palm; Note the yellowing and drying out of
pinnae margins (-6°C);

B - Eight months tissue culture Medjool plants: Note the destruction
of large part of the leaflet (-3°C)

Figure 30. Frost damage on date palm female
inflorescence

Figure 31. Protection of female inflorescences with craft
paper bags at Hardap Research Station (Namibia)

Figure 32. Protection unit made of craft paper to avoid rain
damage to date fruit bunches (USA)

Figure 33. Wind breaks (Casaurina cunninghummiana)
used for the protection of a commercial date plantation (Naute, Namibia)