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Article written and kindly provided by Harry Harrington

Layering is a method of creating new bonsai or 'potentsai' from trees or shrubs that, in their present state or stature, are undesirable or unsuitable for bonsai cultivation.
Bonsai can be created from well formed branches of fully grown trees, poor quality bonsai can have desirable sections rooted and separated from the rest of the tree, good quality material with poor nebari can have new suitable nebari formed as air-layering produces roots emanating radially from around the new trunk.
Layering is also a straightforward method of propagation that although relatively unused in the West, has been tried and tested in China and Japan for centuries. For some species such as Acer Palmatum and Azaleas, layering is the most reliable way of creating new stock vegetively.
The principal of layering in all its forms is to injure the wood of the parent tree, so that the flow of nutrients from the parent trees' roots to the layering's leaves is kept intact, whilst the flow back from the layering's leaves to the parent trees' roots is interrupted.
The injured part of the bark slowly heals, forming a callus from which adventitious buds are able to form new roots into the growing media. The layering continues to be supported by the parent tree, however, the food energy its leaves produce go into building its own new roots. When the layering has sufficient roots of its own, it can be separated from the parent and is then able to support itself.
Layering should always be carried out in Spring when the first flush of leaves has hardened on the parent tree and the parent tree is putting on a great deal of root growth of its own. This timing allows enough time for a layer of many tree species to become established on their own new roots before the onset of the following Winter.
Ground Layering
This form of layering mimics the process by which some species propagate themselves naturally. Low branches on some plants come into contact with the ground as they lengthen and become weighed down by their foliage; from these points, adventitious buds produce roots into the ground and the root system eventually becomes established enough to support the branch as a plant in its own right.
Suitable species for ground-layering include Acer, Azaleas, Berberis, Buxus, Chaenomeles, Chamaecyparis, Cotoneasters, Euonymus, Forsythia, Hedera and Wisteria. It is always worth investigating around the base of all of these species when found growing in the garden or field to see if there are any naturally occurring ground-layers that can already be removed.
To create ground-layers artificially, try to find fairly young growth that will touch the ground; make an upward slit in the underside of the bark where roots are required. Dust with rooting hormone and wrap the wound with long-stranded sphagnum moss. The section of the branch to be rooted now needs to be shallowly buried in the soil and pegged in place with a piece of U-shaped wire.
This process should be carried out in Spring and should be left for at least three months ensuring that the area is kept damp. If the layer has failed to root after 3 months, re-cover it and leave until late-summer. If it has still failed to root by this time it is still worth leaving it in position until the following Spring. When successfully rooted, the new plant can be removed and planted up.
Don't be too eager to separate the layering, it is better to leave it intact until there is enough rootsystem to support the layering, rather than remove it too early and watch it slowly die. If there are not sufficient roots on the layering by September, it is better to leave detaching it until the following Spring as an new immature layering may not survive on its own through the Winter cold, however hardy the parent plant is.
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)
Example Air-layer: Acer palmatum
<<>>A strip of bark is removed just below an old leaf node. After the bark is removed, all of the green cambium layer below it is carefully scraped away; any cambium left could make the layer fail by allowing the tree to bridge the ring-bark.


<<>>Six weeks later and the new roots have appeared in the bag. At this point, care must be taken that the sphagnum moss is kept damp.




<<>>The tree is then potted up and pruned back heavily to reduce the stress on the new roots.

It is very unlikely that the new tree will put out any new top growth for the remainder of the year; instead it will produce a big flush of root growth.
The new root system will still be relatively weak for the first year and during the following Winter, extra protection should be given to the tree.

Larger airlayers can be taken from some tree species. Here is a 5" thick English Elm airlayer taken this year. And an Acer palmatum.

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