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A common misconception amongst newcomers to the art of bonsai is that trees (bonsai) with large, thick trunks must have had decades of training to become the size they are and that a thin-trunked seedling will one day acquire a thick mature trunk even though it is planted in a bonsai pot.
Unfortunately, once a tree is growing in the confines of a small pot, with its roots restricted and upper growth regularly pruned, the trunk and branches of the tree will only thicken very slowly.
Large bonsai with thick trunks are nearly always developed in the ground prior to being planted into a pot; some are purposely field grown, some are collected mature trees.
As a tree develops new growth during the growing season, it lays down new wood to feed and supply its new shoots and leaves. The more new shoots and foliage the tree produces, the more new wood is developed to support this new growth. This new wood grows around the outer ring of the trunk and branches in an almost direct passage from the new shoots, back through the trunk to the root system, gradually increasing the trunk's diameter. Therefore, the greater the amount of new growth a tree achieves in a season, the greater the increase in the girth of its trunk.
A tree that is allowed unrestricted growth will always thicken faster than a tree that is pruned.
The best way to promote unrestricted growth in any tree or shrub is to plant it into the ground; a large container is an alternative but not equivalent to growing in the ground. (This is chiefly due to the difference in dynamics of soil held within a container and that of a large mass of ground-soil; be wary of planting trees in overly large containers, this can in fact slow growth. See Overpotting).
Field-growing techniques can be used within any area of ground, if an area of land is unavailable to you (as is often the case) trees can be grown on (and enjoyed) in the garden amongst ordinary garden schemes, as 'temporary' 5-10 year hedges or as 'temporary' garden specimens. It is also possible with a little work to build raised beds specifically for the purpose of field growing; raised beds can be walled with brick or wooden planks and filled with good quality soil.
Any tree/shrub species can be used for field growing as long as it is hardy in your local climate. Native species naturally thrive in your local climate and will therefore respond to give the best results; other species will develop well but can take longer to establish in the ground before growing with real vigour.
Any age or size of tree is suitable for field growing as long as it is well developed enough to compete with any grasses or weeds that might compete for light or moisture. Generally, cuttings, seedlings or saplings should be at least 2 years old before planting out unless you are able to cosset them for the first year.
Growth rates to expect
Typically, trees will spend the first year establishing in the ground with some reasonable top growth. The second year will start to show strong growth and bulking up of the trunk. By the third year you should expect to see extremely vigorous growth; some species such as Trident Maples, Elms and Hornbeam will have easily reached heights of 12ft+ by now if left unpruned.
With regard to the trunk diameter, quick growing species such as Tridents, Field Maples, Hornbeam, Elm and Scots Pine can see an increase in trunk diameter from 1/2" as saplings to 2" to 3" at the end of their third year in the ground. Peter Chan of Heron's Bonsai has described how a 1" diameter Field Maple he planted into the ground 18 years previously, had achieved a 15" trunk!
Nebari and root techniques
Always consider the formation of the trunk base/nebari when first planting your trees as the roots will grow strongly in the ground and this is a good opportunity to ensure that your future fat trunk has an equally impressive nebari.
If the tree you are planting has downward growing roots or poor surface roots, consider ground layering it by tying a wire around the base of the trunk (see New Nebari ). If there is already a good root pattern and strong lateral roots, prune the rootball so it is reasonably shallow and plant the tree on top of a tile, brick or flat piece of wood. As the tree grows, new roots will be unable to grow downwards and the resulting lateral rootgrowth will ensure the tree is easier to collect and have a much improved root spread and nebari.
This Tilia cordata or Small Leaved Lime has been planted on top of a tile in the ground for the past 4 years. Last Spring it was lifted and root pruned for the first time. As can be seen below, a year later the result is a flat rootball with strong lateral root growth, ideal for bonsai.