The date palm can be propagated in only two ways: by seed, and by the offshoots or suckers which spring up around the base or sometimes on the stem of the palm until it attains an age of ten to twenty years.
Seedlings are easily grown, but offer little promise to the commercial grower. Half of the plants will be males, and among the females there will be such a wide variation that no uniformity of pack or quality can be secured. In regions with a large proportion of seedling palms, such as Spain and parts of Egypt, there is practically no commercial date-culture. Most growers in California plant a few seedlings for windbreak or ornamental purposes. These yield a supply of males, but males can be secured better by growing offshoots from male palms of known value.
The multiplication of the date palm, therefore, is reduced in practice to the propagation of offshoots, and skill or lack thereof in this regard will determine largely the grower's success or failure at the outset.
In California at the present time the yield of offshoots is almost as valuable as that of fruit, and growers, therefore, desire to secure as many offshoots of their best varieties as possible. For this purpose ample fertilization and irrigation must be supplied. After the fourth or fifth year of a palm's life, the owner can usually take at least two offshoots a year from it for a period of ten years. The best size for offshoots at removal is when they weigh from ten to fifteen pounds (say , 5 to 6 inches, is greatest diameter). The best season for the purpose is during February, March, or April.
Four or five days before the offshoots are to be removed from the mother-palm, their inner leaves should be cut back one-half and the outer leaves two-thirds of their length. It will be well worth while to have a special chisel made for removing offshoots. It should have a cutting bit of the best tool steel, 5 inches wide by 7 inches long, one side flat, the reverse beveled for 2 inches on the sides as well as on the cutting edge. The chisel should have a handle of soft iron 3 feet long and 1 1/4 inches in diameter, such as can be hammered with a sledgehammer. The delicate operation of cutting is described by Bruce Drummond, who is the best American authority on the culture of the palm, as follows:
"To cut the offshoots from the tree the flat side of the chisel should always be facing the offshoot to be cut. Set the chisel well to the side of the base of the offshoot close to the main trunk. Drive it in with a sledge until below the point of union with the parent trunk; then by manipulating the handle the chisel is easily loosened and cuts its way out. Next reverse and cut from the opposite side of the shoot until the two cuts come together. This operation will in most cases sever the offshoot from the trunk. No attempt to pry the offshoot from the tree should be made, as the tissues are so brittle that the terminal bud may be ruined by checking or cracking. In cutting offshoots directly at the base of the palm the soil should be dug away until the base of the offshoot is located and enough exposed to show the point of union with the mother plant. Then the chisel can be set without danger of cutting the roots of the parent tree so much as to injure or retard its growth. The connection of the offshoot on such varieties as Deglet Nur is very small, and there is no necessity of cutting deeply into the trunk to sever the offshoot from the tree."
Once separated from its parent, the moist offshoot requires a period of seasoning before it is dry enough to be planted without danger of fermentation. Offshoots from the base of a palm are usually softer and sappier than those growing some distance above ground. The evaporation should amount to 12 or 15 per cent of the total weight, which will require at least ten to fifteen days to effect. Offshoots are usually left where cut, on the ground beneath the palm, to season.
Plate XI. Left, a tropical substitute for the cantaloupe, the papaya; right, a papaya in bearing.
The Arabs plant offshoots at once in their permanent locations in the orchard, but the best results will be obtained by first rooting the young plants in a shed or frame where the two necessary conditions of high temperature and high humidity can be maintained. In California this is often done cooperatively.
A common type of shed for an individual grower is 12 by 20 feet in size with side walls 6 and 7 feet high respectively, presenting a roof-slope to the sun. The sides are usually of boards covered with tarred paper and the roof of 8- or 10-ounce canvas. In such a shed on an ordinary California summer day, the temperature will be about 115° and the humidity should be about 75.
The soil inside the shed should be a light sandy loam, well drained. Ten inches of the top soil should be removed and replaced with fresh stable manure, well packed, on which 2 inches of soil should be replaced. After a thorough flooding, the bed should be allowed to steam for a week, and then be flooded again, whereupon it is ready for the offshoots. These should be planted about 8 inches deep; in any case the bud must be above danger of flooding. During the summer the bed must be flooded at least twice a week, to keep the humidity at as high a point as possible. The offshoots must be kept in it until they are thoroughly rooted and have half a dozen new leaves. This may require one year or may need several years.
The causes that may lead to failure with offshoots are summarized by Drummond as: " (1) improper selection of the location for the nursery bed; (2) failure to construct the frame so nearly air-tight as to insure the necessary humidity and high temperature; (3) improper methods of cutting and pruning, and the neglect of seasoning before planting in the nursery-bed; and (4) the neglect of irrigation when necessary and failure to apply water properly. The points above mentioned are all essential to success, and to neglect one and observe the others may lead to as great a failure as to neglect them all." On the other hand, by using the proper care growers frequently succeed in making 90 to 95 per cent of their offshoots take root.
After they are removed to the open field, the young palms should be protected by wrapping during the following winter from the possibility of freezing, as they are tender at first. Newspaper is as good as anything for the purpose; canvas, burlap, and palm-leaves are also used.
For security, the orchardist should allow one or two male date palms for each acre of fruit-bearing trees. Care should be taken to secure males that flower early in the season and yield abundant fertile pollen; sterility is common.
The female palm ordinarily blossoms between February and June (in California usually during March and April). Flowers appearing later than May 1 are not worth pollinating, so far as commercial production is concerned. Artificial pollination has been practiced since the dawn of history, and offers no difficulties.
The flowers of the two sexes can be distinguished readily (Fig. 28). The branchlets of the male inflorescence are only about 6 inches long, and are densely clustered at the end of the axis, while those of the female are several times as long and less densely clustered. The male blossoms are waxy white in color, the female more yellowish; while also the latter are much the less closely crowded together on the branchlets.
Fig. 28. On the left, a sprig of staminate or pollen-bearing flowers of the date palm; on the right, pistillate flowers which will, if properly pollinated, develop into fruits.
The presence of pollen in the male flower is in most cases easily to be detected by shaking a cluster of the blossoms.
As soon as the spathe containing the pollen-bearing flowers opens, it should be cut and put into a large paper bag to dry, the bag being stored, open, in a dry room. Thoroughly dry pollen will retain its vitality for many years, and a small quantity should be kept in a bottle from year to year, as a precaution. In case of need it can be used with a wad of cotton.
The pistillate flowers should be pollinated as soon as the spathes crack open, the plantation being inspected every day or two with this in view. The operation is preferably carried out about midday. The split female spathe is held open, and a sprig from the male flower gently shaken over it and then tied, open flowers downward, at the top of the female cluster. A single pollination with one sprig is enough for each cluster unless rain follows within twenty-four hours, in which case the operation should be repeated. The grower should keep the situation well in hand.
The grower must not let his young palms bear too many dates, particularly if he wants them to produce offshoots at the same time. Part of the female spadices (flower-stalks) should, therefore, be cut off. In most cases a palm may be allowed to bear its first two bunches of fruit in its fourth year, and three or four bunches in each of the next two years. If even a full-grown palm is allowed to bear to its limit in any year, it is likely to bear less the following season.
In case the grower should find himself absolutely without date pollen at a time when his pistillate trees are flowering, he may have recourse to the pollen of some other Phoenix, or even of a different genus of palms, Chamaerops, Washingtonia, or whatever it may be. This will often enable him to save part, if not all, of the crop.