DATE PALM PROPAGATION :Offshoot propagation

Offshoot propagation, also called asexual or vegetative propagation, offers the following advantages:

(i) Offshoot plants are true to type to the parent palm. The offshoots develop from axillary buds on the trunk of the mother plant and consequently the fruit produced will be of the same quality as the mother palm and ensures uniformity of produce.

(ii) The offshoot plant will bear fruits 2 – 3 years earlier than seedlings. The life span of the date palm is divided into two distinct developmental phases: vegetative, in which buds forming in the leaf axils develop into offshoots; and generative, in which buds form inflorescences and offshoots cease. From the time that the axillary bud of a leaf has differentiated into an offshoot until the time it grows outwards, takes up to three years (18 to 36 months), with another three to four years before it reaches the desired size for its separation and planting (Hilgeman, 1954).

Offshoots are mainly produced in a limited number (20 to 30 at most) during the early life of the palm (10 to 15 years from the date of its planting) depending on the variety and on prior fertilisation treatment, irrigation and earthing up around the trunks, (Nixon and Carpenter, 1978). Although 20 to 30 offshoots are produced by a palm, only three or four offshoots are suitable for planting out in one year and must still go into the nursery for 1 to 2 years before field planting. Zahidi, Berim and Hayani varieties are known to produce large numbers of offshoots, while Mektoum and Barhee varieties produce relatively low numbers of offshoots.

Offshoots are recognised by their curved form while seedlings have a straight form. Another way to differentiate between the two is that seedlings have roots all around their base with no connecting point to the palm, while an offshoot does not have any roots on the side where it was connected to the mother plant. Furthermore, an offshoot always has a mark on one side which is a result of detachment from its parent palm.

To obtain a high survival rate of transplanted offshoots, the following steps are recommended:

Offshoot selection

The offshoot selected for removal must be disease and pest free and at least three to five years old with a base diameter between 20 and 35 cm (Table 32), weighing over 10 kg but not more than 25 kg because of handling difficulties. Signs of mature offshoots are the availability of theirown roots, first fructification and the production of a second generation of offshoots (Nixon and Carpenter, 1978).

Small offshoots weighing 5 kg and less, if needed, could also be used, but their survival potential will be much lower than that of larger offshoots. They should initially be looked after, for at least two years, in a nursery, or mist bed in a greenhouse or a shade net structure (Reuveni et al., 1972). Fungi are usually a serious problem in a mist bed, and the offshoots must be treated twice a month with a large spectrum fungicide.

TABLE 32
Relationship between diameter and weight of the offshoot

The best time for the removal of offshoots and transplanting into the nursery for rooting (never directly into the field) is after the soil begins to warm up in the late spring and early summer (September/October in Southern hemisphere and March/April in the Northern hemisphere). February/March and September/October are then the most suitable period for field planting, respectively.

Offshoot rooting

Two types of offshoots occur on a date palm tree: the lower and older ones, and the upper and younger ones. It is believed that low offshoots are more active physiologically than high ones; they probably grow faster (the number of leaves produced increases with age). In fact, the high offshoots have less carbohydrates than low offshoots, resulting in low roots production and consequently low survival rate. It is also suspected that high offshoots develop when no fruit is on the palm.

Early offshoot removal is desirable because:

(1) removal allows easy access to the palm,

(2) removal improves the development and fruit production of the parent tree, and

(3) planting young offshoots is advantageous as they will in turn produce a greater number of offshoots than older ones.

Numerous factors to consider when rooting offshoots include: the size of an offshoot (often expressed in weight), type (upper or lower), origin of the offshoot, the method of removal and preparation for planting, as well as treatment of an offshoot after planting (Nixon and Carpenter, 1978).

To promote rooting, the base of the offshoot should be in contact with moist soil for at least twelve months before removal. Production of high offshoots is primarily of a varietal character but also in some cases related to a damp climate. For these high offshoots, boxes or plastic bags/Hessian material could be fastened around the base of the offshoot. Another technique is to leave them on the mother palm until they mature. They are then removed and rooted in a nursery (Figure 35a and 35b).

Offshoot pruning

When the aim is the production of offshoots, no green leaves should be removed from an offshoot until it is cut from the mother palm, since the growth of an offshoot is in proportion to its leaf area. When larger offshoots are selected for the following year’s cutting, all their leaves must be retained until the offshoots are removed. When leaves interfere with cultivation, they may be tied together.

When a date palm is crowded with offshoots, only 5 to 6 larger offshoots could be left, considering the tree’s equilibrium, and the other smaller ones could either be totally removed if not needed in the future, or have their leaves cut back close to the bud to retard their growth.

Offshoots removal

After 3 to 5 years of attachment to the parent palm, depending on the variety, offshoots will form their own roots and start producing a second generation of offshoots. Only at this stage are they ready to be removed (Nixon, 1966; Nixon and Carpenter, 1978).

Care and skill, acquired only by experience, is important in order to cut and remove an offshoot properly from its mother palm. The operation, usually carried out by two skilled labourers, starts by irrigation several days before cutting. Soil is then dug away from the offshoot(s) using a sharp, straight-blade shovel (a ball of earth, 5 to 8 cm thick, must be left attached to the roots of the offshoot, with the connection exposed on each side). Roots should at no time be cut closer than necessary, since most of the cut roots die and new roots just emerging are susceptible to injuries (Nixon and Carpenter, 1978).

A specially designed chisel is recommended to cut offshoots. It is a rectangular cutting blade made of tempered steel, which is welded to a solid iron handle. One side of the blade is fl at and the other bevelled so as to form a sharp cutting edge. The following chisel dimensions could be suggested: Blade: 11 cm wide, 22 cm long and 2,5 cm thick; Handle: 120 cm long and 3 cm thick (Figure 36).

Lower leaves must be cut off and the remaining ones tied together in order to facilitate handling. Once the loose fibre and old leaf bases are cut away and the connection between the offshoot and the mother-palm is located, the first cut is made to the side of the base of the offshoot close to the main trunk. The fl at side of the chisel is put towards the weak point of the offshoot and the bevelled side towards the mother palm. Injury must be avoided at all times, the offshoot’s tender heart should never be damaged and the cutting operation must be only from one side to obtain a smooth cut surface.

After completion of the removal of the offshoot, the old leaf stubs and lower leaves are cut off close to the fi bre and the basal part left bare of leaves. Ten or twelve leaves around the bud are retained and tied close together 6 to 8 cm above the bud with heavy twine or wire. The terminal parts of these leaves extending beyond the tie (20 cm above the tip – centre of the offshoot) are also cut off (Figure 37). It is advised that the cut surfaces of both the offshoot and the mother palm be covered with a copper sulphate product in order to avoid infection by Diplodia and other parasites.

Survival of cut-offshoots depends to a large extent on the variety. Medjool’s offshoot is far more difficult to establish than Deglet Nour or Zahidi.

In places such as Fezzan (Libya), some areas of Iraq and Saudi Arabia, and Hadramaout (Yemen), offshoots are not at all removed and continue to grow outwards from the original mother palm, producing large clumps consisting of hundreds of shoots, none of which produces a trunk and of course with no significant yield (Dowson, 1982).

Planting offshoots

It is advisable that an offshoot never be planted into the field directly after removal from the mother plant. A rooting period of one to two years in a nursery is essential in order to ensure an optimum survival rate and to avoid uneven development of the plantation.

In most soils, the early and rapid growth of the offshoot is better when the holes are prepared one to two months before planting. The size of the hole should be one m³ and the holes should be filled with a mixture of topsoil and 10 to 15 kg of manure of high quality (with very little unmatured matter) and NPK fertilisers. The filled holes should be irrigated several times to promote the decomposition of the manure and also to allow the mixed soil to settle in the hole. Well-rotted manure can be used in holes prepared and irrigated shortly before planting, but extreme care must be taken to put the manure (and fertilisers) deep enough to form a layer of soil of at least 15 to 20 cm thick between the manure and the base of the offshoot.

The leaf base of the offshoot should be clearly above the soil level. It is important to plant the offshoot to the depth of its greatest diameter in order to avoid the rotting of the base (if it is too low) and to prevent the water reaching the loose fibre near the bud which causes its desiccation (if it is too high). The plant water basin, of 1.5 to 1.8 m in diameter and 20 to 30 cm deep, should be prepared around the offshoot (Figure 41).

The soil near the newly planted offshoots should be kept moist at all times by light and frequent irrigation. The irrigation frequency is dependent on the type of soil. Very sandy soils require daily irrigation during the first summer. Heavy soils require irrigation only once a week; while in most soils irrigation is required every second or third day. During the first six weeks (or till the appearance of new growth) the date grower should always inspect his/her planted offshoots to make sure that the surface soil does not dry and shrink away from the offshoot. A mulch of hay or straw around the offshoot will enhance moisture contention, weed control and finally improve humus in the basin (Figure 38).

Young offshoots and tissue culture-derived plants should be protected from harsh climatic conditions (sun and wind during the first summer and cold the following winter) and against some animals (rabbits, etc.). The use of shade net/hessian wrapping or a tent of date leaves is recommended (Figure 39). The top is to be left open so that new growth may push through.

Under Namibian conditions (Southern hemisphere), there are two appropriate periods for planting: February/March and September/October. The first period is preferable since it allows a longer time for the offshoot to establish itself before the arrival of the next year’s hot summer temperatures, although it passes through the cold months of winter (June, July and August) while the plant is still in its initial establishment phase. The second period (September/October) avoids the cold temperatures and later receives warm temperatures that allow an active growth followed by the hot summer (December, January).

To summarise, offshoot propagation is true to type but it is not very practical from a mass propagation point of view, and consequently does not satisfy the large needs of plant material. The following reasons illustrate this handicap:

– Offshoot production is limited to a certain period in the palm’s life span (a short vegetative phase of about 10 to 15 years);

– During this short phase, only a limited number of offshoots are produced (20 to 30 offshoots, at most, depending on the variety);

– Some varieties produce more than others (some do not produce offshoots at all);

– A mature specimen with no offshoots will be lost if not propagated through another technique;

– Depending on the care given, a low planting survival rate is frequently obtained when using offshoots;

– The use of offshoots will enhance the spread of date palm diseases and pests;

– Offshoot propagation is difficult, laborious, and therefore expensive.

In comparison to the seed propagation technique, offshoots which are axillary vegetative buds, will offer the following two advantages:

– The fruits produced will be of the same quality as the mother palm and ensure uniformity of produce (true to type).

– The offshoot will bear fruit earlier than seedlings (by 2-3 years).