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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)


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