One of the most reliable methods for propagating material for bonsai is by layering. It is almost foolproof. Layers can be taken from many plants that are difficult from cuttings. You can even layer from a mature bonsai that you wish to redesign. This gives you two or more plants that are “instant bonsai”. Another advantage is that a dramatic flare of many fine, radially spread roots is produced. This allows you to choose which roots will be allowed to develop into the ideal nebari, by eliminating any that are unnecessary or badly placed.
This copies a natural process where low branches, that are in contact with the soil, sometimes root and supplement the food supplied by the parent plant. Some species use this tactic to reproduce in nature. It is a good way of producing plants suitable for developing in a windswept style.
Ground layering is used for shrubs and trees that produce low branches that can be pegged to the soil. Here they often root with a minimum of fuss. There are one or two things that can be done to encourage the process and speed it along. Choose growth that is fairly young wood and in spring make an upward slit in the bark on the underside of the piece to be buried. Dust the cut with rooting hormone and hold open with a twist of sphagnum moss. Peg with a piece of wire bent into a long U shape and bury shallowly, in garden soil, lightened with the addition of some sharp sand. Leave for three months and then carefully scrape the soil away, above where the stem was slit. If the layer has rooted fairly strongly, it may now be severed. Cut through the stem a couple of inches toward the parent plant from the roots. See under “Treatment after severing” for advice on potting up. If few roots have appeared, or none at all, cover again and wait ’till later in the year. Some plants may be stubborn and refuse to root. Others from the same plant can surprise you by rooting in as little as a fortnight.
Use ground layering for; Acer carpinifolium, Akebia quinata, Berberis thunbergii, Buxus microphylla (Box), Caragana sinica, Celastrus orbiculatus, Chaenomeles, Chamaecyparis pisifera varieties, Cotoneasters, Cytisus scoparius, Euonymus radicans, Forsythia suspensa, Hedera (Ivy’s), Wisteria.
This is a simple technique that is used to provide many plants from one parent. The plant should have multiple stems for this to be successful. Coppicing (beheading) it a year beforehand is a good way to provide these shoots. Then raise the soil level in a mound around the base of the plant and leave for about a year. Prunus such as P. amygdalus “Flore-Plena” respond well to this technique.
Air layering was developed hundreds of years ago, probably by the Chinese, to propagate plants. It is a method that has a distinct advantage for bonsai, as you may consider the shrubs and trees in your garden at leisure, looking for a piece that might become an “instant” bonsai. You could prune or wire an area with a view to it becoming a future air layer. A suitably shaped piece should have many of the characteristics desired for your bonsai – trunk thickness, bark development, branch placement, possibly even twig ramification. Another big advantage is that layering provides material that is mature flowering or fruiting wood, so long as the branch that it comes from flowers and fruits. Seed or cuttings from the same tree would take years to reach flowering maturity.
The aim of the air layering process is to fool the tree into producing roots at a point where it normally would not. For this to happen, the tree must feel a need to supplement the nourishment it is receiving through the normal processes within the bark layer. I.e. You must somehow interrupt or slow down the flow of sap. The usual time to attempt this is from spring to summer. Several methods have been devised and some suit one species better than another.
Tourniquet Air Layering
Materials required:- aluminium, copper or plastic coated wire, thin polythene bag (sandwich type), sphagnum moss (the type sold in garden centres for hanging baskets), and pliers.
A wire loop is placed around the trunk and crossed over itself so that it encircles the point where new roots are required. It should be tightened, by twisting with pliers, to the point where it is biting in. This will slow down the sap flow and the constriction will increase as the tree builds in girth. On some subjects it is helpful to dust the area around the wire with rooting hormone. Wrap some moist sphagnum moss, (soaked and squeezed out to a damp sponge stage) around this area. Secure in place loosely with another piece of wire. This helps prevent the next process from becoming very fiddly and requiring three pairs of hands or great dexterity. Split the bag open fully into a long rectangle and wrap it around the whole area until it has encircled it two or three times. Then the top and bottom ends are secured with insulation tape or twists of wire (if too tight it may scar the trunk). An alternative to polythene is tinfoil, which is great for the less dextrous!
Check that the moss hasn’t dried out with a visual inspection through the polythene every fortnight, or weekly in hot or windy weather. If damp, there will usually be some condensation on the inside of the bag and if gently squeezed it still feels “squashy”. If it is drying out, it feels more resistant to a gentle squeeze. When the moss requires watering, the easiest way is to use a syringe. The needle is used to puncture and squirt water into the bag.
After between one month and possibly as long as two years, for a slow rooting pine subject, you will notice white roots beginning to appear against the polythene. Wait until there are many of them and then cautiously check in the moss at the top to see if the roots are issuing evenly all around the trunk. If they are, it is time to sever the new plant by carefully cutting through the trunk below the wire ring. If the roots are not even, dust with hormone again and wait a few more weeks. For potting up, see the general notes on treatment after severing, below.
Use tourniquet air layering for; Most Abies, Acer palmatum & many var’s, Cedrus deodara, Cercis, Chamaecyparis, Citrus, Cornus, Crataegus, Fagus crenata, Fraxinus sieboldiana, Juniperus, Larix, Lonicera morrowii, Malus, Picea, Pieris, Pinus, Podocarpus, Prunus but don’t use copper wire, Pseudotsuga japonica, Pyrus simonii, Quercus, Rhododendron, Sciadopitys verticillata, Stewartia pseudocamellia, Taxodium, Taxus, Thuja, Ulmus, Viburnum, Vitis vinifera, Weigela, Wistaria, Zelkova serrata.
Ring Bark Air Layering
This is a slightly more dramatic method than the tourniquet. When I began in bonsai, I thought that it looked like a good way to kill the tree. It involves removing the bark for a length of about 1″ to 2″ all the way around the trunk or branch that is going to form the new plant (ring barking). The edges should be a clean cut and right down to the harder wood beneath. Any remaining soft sappy tissue, the cambium layer, must be scraped down to wood. Otherwise it provides a nutrient path and hinders rooting or calluses over again completely. The top of the cut area is then dusted with rooting hormone. Use a small paintbrush for this and try to get all of the bark/cambium edge covered. Be careful not to breathe in the dust. The same procedure of wrapping with damp sphagnum moss and covering with polythene, as used for a tourniquet layer, is followed. Suitable species survive the trauma and root so well, that they often develop a better radial root system than a seedling or cutting.
Use ring bark air layering for; Most Acers, Berberis, Buxus (Box), Camellia, Carpinus (Hornbeams), Cornus kousa, Corylus heterophylla, Cotoneasters, Cryptomeria, Ginkgo, Hamamelis japonica, Hedera helix & H.h. var’s, Jasminum, Junipers, Ligustrum, Lonicera, Morus, Myrtus, Parthenocissus, Prunus, Pseudocydonia sinensis, Punica (Pomegranate), Pyracantha (Firethorns), Rhododendrons, Serissa, Syringa (Lilac), Tamarix juniperina, Thea sinensis, Ulmus (Elms).
There are other refinements of the ring barking method, which suit species that are reluctant to root quickly. These retain a bark bridge by which the tree may continue to receive a reduced supply of sap. Have a look at the diagram to see the options.
Bridge Air Layering
This method involves leaving small bridges of bark across the area ring barked. These allow a decreased flow of sap to maintain the portion of growth above, until roots are developed. Several variations of this method have been developed. See the diagram for details.
Use bridge air layering for Pines and other slow to root species.
Treatment After Severing
Remove as much of the trunk below the new roots as you can, using branch or knob cutter to nibble away the wood. Take care not to damage the fragile root ball. It is not advisable to try to remove the moss from the roots or to try to sort the root spread at this stage. The new white roots are too fragile. Pot up the new tree working a light-weight soil mixture gently around the root ball, ensuring that there are no spaces. Mixtures of moss peat, coir, leafmould or composted bark with as much as 50% perlite or biosorb are best.
Any layer is in danger from knocks or wind rocking, in the first few months of its life, as the roots are so brittle at first. Protection in a greenhouse, cold frame or sheltered area is best. The plant should be tied, wired or secured with cane stays to the container.
Treat as a cutting for a month or two, before gradually introducing the new tree to wind, direct sun and rain. After a year, it is time to inspect the roots and select those that are well placed, removing any that are badly positioned. The remaining stub of trunk beneath the roots may be reduced and dusted again with rooting hormone. Repot in a normal soil mixture for the species.
Another approach for air layering from bonsai trees is to use a split flower-pot with a mix of peat and vermiculite 50/50. The pot is joined together with insulation tape and secured in position with wire or plastic cable clips. The planting mixture is watered every time that the tree requires it. Rapid rooting normally results. The rooting mixture is easier to remove from the tender new roots than moss. This allows for some cautious re-positioning of roots when potting up.
Once you’ve experienced the advantages of air layering, it becomes a part of your armoury of skills that will allow you to develop better bonsai faster.