Air Layering / Marcotting

Texas A & M University   A great propagation web-site which includes all the different types of grafting.  There are “animated gifs” ie clicking on a diagram sets it alive so the transfer of budwood to scion is shown in action. If you want to have all this available at any time, you can save the main file and all the illustrations in a folder on your hard drive and view it at any time by opening the main file with your web browser.                                                                                                                                                                         

Essentials   Essentially the treatment (also called marcotting)  is to girdle a shoot and surround the wound with moist material until roots emerge, when the rooted shoot is severed from the parent and established as a new individual. This is an ancient method well described by many writers as a reliable though somewhat tedious way of propagating very many woody plants, including avocado cashew and mango.

Age of Shoots   Judging by the many results of trials with a wide range of species it would seem that young shoots, with a fast growing potential in a position on the parent tree where there is adequate light, respond best to marcotting. Firm shoots sufficiently lignified to withstand the necessary manipulation, yet young enough to respond to treatment, should be selected. Generally it will be found that one year old shoots with a good supply of leaves and leaf-buds are ideal for rapid root initiation and development, which largely depend on the downward flow of essential substances from developing leaves and shoots growing above the marcot. Thus it is that the phasic development of the actual shoot and not that of the parent tree as a whole is vital. The shoots should always be young, and can be obtained on old trees by heading them back. In mango, shoots of less than two years of age have been shown to give a higher percentage of rooting than older ones. However, with cashew it was found that one—year-old wood was better than current season’s growth.

Girdling and wounding    It has sometimes been suggested that slicing, notching or half-cutting through are alternatives to complete girdling or cincturing, but these partial interruptions of the downward movement of materials are less effective than complete girdling. Where success has followed mere wounding it may have been brought about by extensive tissue damage, and consequently girdling effect, or by excessive use of a highly concentrated growth substance.

As to the best season for the operation, work in various countries has shown that there is no major advantage in choosing a particular month. The condition of the shoot is the important factor; it should be young and active but not too soft, and one or two year old shoots have generally proved the most suitable. A practical consideration of some importance is to time the operation so that the rooted shoot is due for removal for transplanting early in the rainy season so as to facilitate establishment.

The position of the cincture, whether above or below a bud or midway along an internode, is relatively unimportant but it should be placed where it can be conveniently wrapped and be supported either by being tied to a neighbouring branch or to a stick or cane. Another practical consideration is the eventual size of the rooted layer. The rooting of very large branches leads to establishment difficulties. Size reduction by pruning, at the moment of transference, may aid survival but may result in a misshapen plant based on lateral development. Suitable shoot sizes about the cinctured position have varied from 9 to 24 inches (23 to 61 cm) in length and from pencil thickness up to half an inch (1.25 cm) in diameter. A cincture a little above the base of the shoot will be ideally placed with regard to root formation and support.

The cincture itself is made by removing a complete ring of bark. Widths advocated have varied from one-eighth to three inches (0.3 to 7.6 cm). The essential aim is to stop the downward movement of carbohydrates and growth substances so that they accumulate above the cincture and are available for the initiation and development of roots.

This interruption of flow should be as complete as possible without interfering with the upward passage of water and other materials to the shoot above. An eighth—inch (0.3 cm) girdle will heal and form a bridge too quickly, whilst a three-inch (7.6 cm) gap is excessive. An inch (2.5 cm) —wide girdling will generally serve and this can be made by two encircling cuts and the removal of the intermediate rind. The operation can be done with a sharp knife or with a double bladed tool. These cuts should not enter the wood (xylem) of the shoot for this not only checks the upward sap flow but, by weakening the shoot structure, increases the risk of loss by breakage. The vital necessity of preventing the downward flow of materials cannot be overstressed. Some workers have treated the bared wood— surface with disinfectants or other chemicals in an attempt to prevent the formation of a new rind, but with no clear benefit. A somewhat more efficacious treatment is to rub a pad of steel wool, or other abrasive, around the cincture to remove all traces of meristematic tissue. Another method is to pull twine or strong plastic thread very firmly into the wound to prevent bridging by callus:

Synthetic growth substances    It has become customary to apply synthetic growth substances to the upper part of the cincture and to about the first inch (2.5 cm) of the bark immediately above it. This is not essential for readily rooting subjects but has generally proved helpful in speeding root formation and in increasing the number and length of roots. Many substances have been used with success, the most popular being indole acetic acid, indole butyric acid and naphthalene acetic acid, or mixtures thereof. They may be applied diluted in alcohol, in inert dusts or in grease bases s as lanolin.

From a survey of the literature it would seem that the most effective single substance for assisting root formation is indole butyric acid (IBA). To prepare an alcoholic solution of IBA (or other crystalline growth substance) dissolve 1 g in 50 ml of ethyl alcohol, methyl alcohol or methylated spirit and then dilute this with an equal volume of water to make 100 ml of solution containing 10,000 p.p.m. IBA. To dilute the 10,000 p.p.m. to 5,000 p.p.m. add a further 100 ml of 50/50 alcohol/water mixture. Stored in well—stoppered opaque bottles in a refrigerator, the solutions retain their activity indefinitely, and should be strong enough for shy—rooting subjects.

Dusts containing synthetic growth substances ready for use may be purchased or can be made as follows. To make a dust containing 10,000 p.p.m. of growth substance dissolve 1 g of the substance in 40 ml of methylated spirit or 95 per cent alcohol, and stir this into 100 g of pharmaceutical talc to form a smooth paste. This should be done in a darkened room away from strong light (117). Stir the paste while it is drying until it becomes a fine dry powder. This prepared dust remains active for six months or more if stored in a closed opaque container in a refrigerator.

Lanolin pastes, which are particularly convenient for use in marcotting but are now regarded as an obsolete treatment for cuttings (5), are made by stirring the growth substance into the molten lanolin and then allowing it to cool (93). To make a paste containing 5,000 p.p.m. of growth substance melt 200 g of lanolin and thoroughly stir into this molten lanolin 1 g of IBA, or other growth substance required. This prepared paste will keep indefinitely if stored in a well-stoppered opaque glass vessel within a refrigerator.

The alcoholic solution is conveniently applied by means of an artist’s camel—hair brush, the liquid being contained in a small glass bottle held close to the cinctured shoot. The bottle should be stoppered between each individual application. The liquid should wet the lower inch (2.5 cm) or so of the bark close above the cincture as well as the adjacent cut edge. An inadvertent wetting of the exposed wood (xylem) is of little consequence. The solution should be allowed to dry before applying the medium. Dusts can be generously applied by means of a half-inch (1 cm) brush.  If insufficient dust adheres, the bark above the cincture should first be wetted with water or a mixture of 50/50 methylated spirit and water.Lanolin pastes are applied by means of a small pliable serrated spatula, a brush of stiff bristle or by the finger protected by a rubber finger-stall.                                                                                     

Rooting media    The choice of the rooting medium to place around the cincture has been widely studied and the present view is that a wide range of media are satisfactory (see page 15 A good medium should be open in structure to permit air and moisture movement and be able to retain its structure throughout the rooting process. It should therefore not ferment or otherwise breakdown, hence live sphagnum moss has been widely accepted as a basis, but coir fibre and coir dust, wood shaving and sawdust have all proved excellent as basic ingredients.

Work with cashew in Tanzania (89) showed that vermiculite alone was superior to fibre, moss, coir, sawdust or a 50/50 mixture of vermiculite and sand. It is an advantage if the selected material is light in weight and holds together whilst being placed in position during the wrapping procedure. Whilst an open, light—weight medium provides an excellent rooting environment, it should be remembered that the main hazard in marcot propagation occurs during transference and establishment. From general experience it has been learned that transference to soil or mixtures thereof, whether in containers or the open nursery, is more readily accomplished if the marcotting medium contains a modicum of loan or clay soil. A medium of equal parts clay soil and sieved leaf mould has proved superior to moss alone in regard to rooting and survival of mango air layers in India (120) but it should be remembered that heavy additions of soil increase the need for extra support for the marcot.

Wrapping the air layer   The medium should be well moistened. Fibrous material should be thoroughly wetted and squeezed free from excess water at the moment of application. It may be convenient to pack the material round the cinctured part before placing the wrapping over it. Often a better way is to place the medium on the wrapping and so lift it into position. Polyethylene sheet is an ideal wrapping material, being both water-tight and transparent, obviating the need to water the marcot and enabling the new roots to be seen when they come through the rooting medium. As with many other kinds of enveloping material polyethylene may be punctured and torn by birds. Where this occurs the only remedy appears to be an outer cover of really tough, resilient material such as a piece of inner tube from a car tyre. The polyethylene film must be thin enough to be wrapped and folded round the shoot to form an air—tight enclosure. Thicknesses of from 100 to 400 gauge (0.001 to 0.004 in or 0.025 to 0.1 mm) are suitable Thicker films are difficult to fold sufficiently tightly to prevent drying of the enclosed medium. Occasionally it is possible to maneuver a length of polyethylene lay—flat tube, three or four inches (8 or 10 cm) wide, over the top of a shoot bearing only small leaves and to fix this below the cincture and then to fill it with rooting medium before closing it tightly above. Mostly a Square of sheeting must be used, the size varying with the situation. Pieces measuring about 7 x 10 inches (18 x 25 cm) are normally large enough to contain a ball of medium some three inches (8 cm) long and 2½ inches (6.5 cm) in diameter, leaving two inches (5 cm) for a double-fold lengthwise and two inches (5 cm) at either end for closely wrapping round the shoot.

Having made the cincture and applied the growth substance, a handful or so of moist medium is placed in the middle of the wrapping and immediately lifted into position round the shoot, the two edges of the wrapping are matched together and folded over twice to hold the medium quite firmly. The lower open end is closed by a twisting motion very firmly round the shoot and there tied or taped with plastic strip or strong adhesive tape. The medium is firmed below the open top, using the fingers, before tightly taping. The wrapping and tying must be done in such a way that no opening is left for evaporation from the medium. The tie at the top end should always begin on the bare shoot and be spiralled down on to the wrapping to prevent excessive rain seeping down into the marcot. Sometimes, in spite of all wrapping precautions, marcots become waterlogged. This appears to be due to overheating of plastic wrapped marcots by direct sunshine followed by heavy rain which, flowing down the shoot, rapidly cools the marcot, setting up a strong suction press which draws water into the marcot through even the smallest opening. An outer loose covering such as that SUgge5 as a protection against birds, or a white paper Covering, Will reduce excessive water intake. In the absence of plastic materials or rubber sheeting a great variety of wrapping materials have been used such as hessian cloth, various fibres, barks or leaves, or even old boots, but the distinct advantage in efficiency of using transparent, foldable plastic sheet, not least the absence of the need to water a plastic-covered marcot, ensures its present and future popularity.                                                                                                                           

Transference of the rooted air layer   When the marcot has rooted sufficiently to ensure successful transplantation (the period may vary from as little as 30 to more than 100 days) the procedure of separation should begin. Ideally this is done in stages, the branch is half nicked close below the wrapping and completely severed one or two weeks later. At this moment of separation the rooted marcot is vulnerable to adverse weather conditions and it is advisable to place it in a container and to take it through the full nursery procedure as described for young seedlings or newly rooted cuttings, including adequate nutritional treatment to build a fully self-contained plant for transference to the plantation.