Layering techniques for Bonsai

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Air-layering

Air-layering is a similar to ground-layering except that it utilizes branches that are growing off the ground and so the injured bark/rooting point has to be encased with growing media wrapped in polythene (or similar). Branches of up to 2" (or much more) diameter can be successfully rooted leading to the possibility of creating new plants with great potential for bonsai.

Branches on fully-grown trees have particularly vigorous growth and can be pruned over successive seasons to form thick tapering trunks for bonsai, which can then be air-layered from the parent tree.

For deciduous trees, air-layering is carried out in April to May as new growth hardens off and changes to its Summer colour. For evergreens, air-layers should be left until a little later; from late April until July.

There are two methods of injuring the parent tree to provide a site for new roots to grow. The most frequently used technique is ring-barking. Use a sharp knife to cut two parallel slits at least 2 times the diameter of the branch around the circumference of the branch. Remove the ring of bark between these two cuts and the underlying cambium layer (which is green and 'soft').

Make the ring barking point just below the section of the parent trees' branch that roots are required, if it is possible, try to make the point of ring barking just below an old leaf node as it will contain many adventitious buds.

Do not be tempted to leave a strip of bark across the ring-barking as this can allow the parent tree to bridge the air-layering and no new roots will be produced. For the same reason, ensure the ring-barking is wide enough so the parent tree is unable to bridge the gap as it heals.

The cambium must be entirely removed; this means removing the entire green layer below the bark as well as any other soft white pithy material, leaving just the 'shiny' white wood underneath the cambium layer.

One of the main reasons that airlayers fail is when the cambium is not entirely and thoroughly removed. With many tree species, the tree will try and bridge the ring-bark; this is easier for the tree than producing a new root system. Purposely leaving a 'bridge' of cambium as is occasionally recommended and tried, is a sure-fire way to ensure that the tree does not have to produce new roots.

The alternative method to ring-barking is the tourniquet. This is suitable for species that are unable to cope with the removal of a complete ring of bark. A piece of wire is wrapped very tightly around the branch below the proposed rooting site, as the branch grows the tourniquet bites into the bark and then the cambium layer slowly interrupts the flow of nutrients from the leaves down to the roots. The tourniquet method however is slower to work and more vigorous species can bridge the tourniquet as they grow resulting in a failure to root.

With both methods, dust the section to be rooted with rooting hormone and tightly wrap wet long-stranded sphagnum moss around the whole area. The sphagnum moss is then held in position with clear polythene or a clear plastic bag. Tie the bag securely and make a small hole in the top to facilitate watering.

Whilst waiting for the air-layering to root, ensure the moss is kept wet. After anything between 3 weeks and 3 months, dependent on species, white roots will be seen growing inside the bag. Allow the bag to completely fill with roots; ensuring the moss is kept damp at all times. When the roots have matured and turned brown, the layering can be removed from the parent tree.

Remove the plastic bag or polythene but leave the moss in place as the roots are very easily damaged at this point. Remove as much of the branch below the new rootball as possible and plant the air-layer in a pot of bonsai compost or pure sphagnum moss. Ensure that the layering is tied into place with string, wire or raffia to stop it rocking about in the wind and damaging the new root system. Keep the newly potted layering in the shade and mist regularly until it is established.

Winter Protection for Airlayers

There seems to be general panic where the Winter and airlayers are concerned. The cold and frosts will not damage the airlayer itself. The airlayer itself is just a wound that will have callused over. You don't pamper a wound from a recently removed branch during the Winter so why an airlayer?
Any new roots that are already growing from the airlayer will be more susceptible to extremes of cold in the same way as roots in a small bonsai pot are. However, they are insulated in sphagnum and plastic (you can add a layer or two of fleece or bubble wrap if you wish). If the new roots are damaged or dieback during the Winter, they will be replaced in Spring when the parent tree starts growing again.

New trees (produced by airlayers) should be separated at least 6 weeks before the first frosts, this allows the new roots enough time to grow and strengthen before Winter. If there are insufficient roots on an airlayer to separate it from the parent plant in the Autumn, leave it until the following Spring.

Genera/Species suitable for the tourniquet method include;

Abies, Acer, Cedrus, Cercis, Chamaecyparis, Cornus, Fagus, Juniperus, Larix, Lonicera, Malus, Picea, Pieris, Pinus, Podocarpus, Prunus (don't use copper wire), Pyrus, Quercus (with difficulty), Azaleas and Rhododendron, Stewartia, Taxodium, Taxus, Thuja, Ulmus, Virburnum, Wiegela, Wisteria and Zelkova.

Genera/Species suitable for the ring-bark method include;

Acers (Red leaved varieties can be very slow to root) Berberis, Buxus, Camellia, Carpinus, Cornus kousa, Corylus heterophylla, Cotoneaster, Cryptomeria, Gingko, Hamamelis japonica, Hedera, Jasminium, Juniperus, Ligustrum, Lonicera, Morus, Magnolia stellata, Myrtus, Parthenocissus, Prunus, Punica, Pyracantha, Rhododendrons and Azaleas, Serissa, Syringa, Tamarix juniperina, Thea sinensis and Ulmus.

(These lists are by no means exhaustive; most woody trees and shrubs that backbud / readily produce adventitious buds on old wood can be air-layered with a good chance of success)

 

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