While the date palm grows luxuriantly in a wide range of warm climates, it is, for commercial cultivation, adapted only to regions marked by high temperature combined with low humidity. Properly speaking, it belongs to the arid subtropical zone. A heavy freeze will kill back the leaves, but the plant may nevertheless be as healthy as ever in a year or two. Thus, date palms have withstood a temperature of only 5° above zero and have borne satisfactory crops in subsequent years. Ellsworth Huntington speaks of seeing the date palm in Persia where twenty inches of snow lay on the ground; many generations of natural selection in such an environment would doubtless produce a hardy race, but such a region would scarcely be thought adapted to commercial date-growing.
At the other climatic extreme, the date palm apparently finds no limit, being at its best where the summer temperature stays about 100° for days and nights together. The combination of warm days with cool nights is unsatisfactory; unless there is a prolonged season during which high temperatures prevail night and day, the best varieties of dates will not ripen successfully.
Humidity is an important factor with many varieties. Dates coming from the Sahara usually demand a dry climate; yet the Coachella Valley in California has sometimes proved too dry, and the fruit has shriveled on the tree unless irrigation was given while it was ripening. Persian Gulf and Egyptian varieties will endure more humidity, since they come from the seacoast or near it. Dew at night or rain coming late in the season when the dates are softening is almost ruinous to the crop, for which reason dates cannot be produced satisfactorily in some parts of Arizona. In regions of India where the summer rains begin in July, it has been possible to bring dates to maturity before the rains arrive.
In general, the best varieties require: (1) a long summer, hot at night as well as in the daytime; (2) a mild winter, with no more than an occasional frost; (3) absence of rain in spring when the fruit is setting; and (4) absence of rain or dew in the fall when the fruit is ripening. In regions lacking any of these characteristics, date-growing will be profitable commercially only if special care is taken to secure suitable varieties and to develop, by experiment, proper methods of handling them.
Date palms grow well in the stiff clays of the Tigris-Euphrates delta, in the adobe soils of Egypt, in the sand of Algeria, and in the sandy loam of Oman and of California. No one type of soil can be asserted to be necessary. Thorough drainage and aeration of the soil are desirable, but even in these regards the palm will stand considerable abuse, and is found to grow fairly well in places where the ground-water level is comparatively near the surface. Naturally, however, the palm responds to good treatment as do other plants. On the whole, it is probably best suited on a well-drained sandy loam.
The palm's tolerance of alkali has been noted from very early times, and has led Arab writers to believe that it throve best in alkaline soil. This is unlikely. Dates can indeed be grown successfully in ground the surface of which is white with alkaline efflorescence, provided the lower soil reached by the roots is less salty; but it is probable that the limit of tolerance is somewhere about 3 per cent of alkalinity, and the grower who looks for the best results should not plant on soil whose total alkaline-content exceeds one-half of 1 per cent. Naturally, old date palms will stand more alkali than young ones. It should be noted that the so-called black alkali, consisting of carbonates of sodium and potassium, is more harmful than the more or less neutral chlorids, sulfates, and nitrates of sodium, potassium, and magnesium which go by the name of white alkali.
If the irrigating water is free from alkalinity, it will, of course, help to counteract any alkali present in the soil; whereas the grower who needs to irrigate with brackish water must plant his palms in fairly alkali-free soil. Desert landowners sometimes calculate that soil which is too salty for anything else is good enough for a date plantation. This is short-sighted reasoning. Date-growing is, when rightly conducted, so profitable that it is worth giving the best conditions available, and the wise grower will plant his palms in his best soil. The ground should be tested to a depth of six or eight feet to determine its alkali-content, particularly if there is salt evident on the surface. Unless at least one stratum of alkali-free soil is found not far from the surface, the ground should not be used for date palms.
It is the custom in the United States to plant date palms 50 to the acre. The grower with plenty of land may find that 40 to the acre (33 feet apart each way) is more convenient. Arabs plant them much closer but do not cultivate their plantations frequently. The question of spacing is affected both by the nature of the soil and by the variety planted; according to Bruce Drummond, such kinds as Saidi and Thuri give the best results if spaced 35 or 38 feet apart.
Drummond gives the following advice about planting:
"The rooted offshoot when ready for transplanting should be pruned from three to five days before removing from the frame. The new growth should be cut back to one-half the original height, leaving from three to five leaf stubs to support the expanded crown of leaves. The holes in the field should be 3 ft. in diameter and 3 ft. deep, with from 12 to 16 in. of stable manure placed in the bottom of each, with 6 in. of soil on top, then irrigate thoroughly. The rooted palm when removed from the nursery should carry a ball of earth large enough to protect the small fibrous roots from exposure to the sun or dry winds. The average depth for planting should be 16 in., but this may be varied somewhat with the size of the shoot. In any case, the depth should be as great as can be without danger of covering the bud.
"It is not advisable to transplant rooted offshoots later than June. April and May are considered the best months of the entire year for the transplanting of either young or old date palms.
"In southern California, where the dry winds occur from March to June, the transplanted palms should be irrigated thoroughly every week; in sandy soil two irrigations a week should be given until new strong growth is established."
Arabs usually follow the basin method of irrigation, and it has been satisfactory in many other parts of the world. The most skillful American growers who irrigate in basins make them 15 feet square and a foot deep, filling them with a loose mulch of straw or stable manure.
Most American growers, however, prefer to irrigate in furrows, and use no mulch. The function of the mulch in reducing evaporation is covered by giving a thorough cultivation with a surface cultivator or spring-toothed harrow as soon as the ground has dried out enough to be workable. This involves cultivation of the ground every week or two.
Adequate fertilization of the soil is absolutely necessary in order to make date palms produce fruit as heavily as commercial growers desire and at the same time yield well in offshoots. Nitrogen-gathering cover-crops are much in favor, sesbania or alfalfa being preferred in California. The long roots of the latter are useful to break up any hardpan or layer of hard silt which may be present. Many growers plant garden-truck between the rows of palms, especially while the latter are young and making no financial return.
The soil in which date palms are usually grown is of a kind that benefits by the incorporation of rough material, and stable manure is, therefore, the fertilizer of first choice. Wheat-straw or similar loose stuff is frequently added with advantage. An annual application of fertilizer is required in most localities, and if the soil is sandy the grower must be more liberal. For palms producing offshoots, half a cubic yard a year is advised; for older palms a full yard is desirable : both in addition to such cover-crop as the grower may select.
In regard to irrigation, it is to be borne in mind that the soil must be kept moist during the entire year, and that the roots of the palm go deep. The character of the soil must be carefully and experimentally studied before the grower can be certain that he has arrived at the correct method for irrigation. The amount of water that the palm can stand in well-drained land is strikingly illustrated in the great plantings around Basrah, where fresh water is backed into the gardens by tidal flow, so that there are two automatic irrigations each day throughout the year.
In the Coachella Valley, with furrow irrigation, a twenty-four-hour flow each twelve days from April to November has generally been satisfactory, although in many soils weekly irrigation is required. During the winter the rainfall usually suffices. Each application of fertilizer must be followed promptly by several irrigations.
Pruning is not so important with date palms as with many fruit-trees. Dead leaves should be removed from young palms, and if the top growth is heavy the two lower rows of leaves may be removed when the palm is four years old. Regular pruning should begin about the sixth year, after which one row of leaves is usually removed at each midwinter. Drummond advises that "the leaves should not be pruned higher than the fruit stems of the former crop, which will leave about four rows of leaves below the new fruit stems, or approximately 30 to 36 expanded leaves."