4. Branch, stem and crown development
Irrespective of the production system used the techniques used on the tree nursery are consistent in stem and crown development. Outlined in this section are examples and explanations of good and not so good nursery practice coupled with items which can be specified to ensure young trees are 'fit for purpose'.
A significant number of young trees are either budded or grafted during the production process.
These techniques are used for the production of cultivars of a particular species with the cultivar scion wood budded onto the appropriate understock.
Acer campestre ‘Louisa Red Shine’ budded onto Acer campestre understock.
Pyrus calleryana ‘Chanticleer’ budded onto Pyrus communis.
Sorbus aucuparia ‘Sheerwater Seedling’ budded onto Sorbus aucuparia understock.
Such budding is normally carried out at the base of the stock plant above soil level and can be clearly recognised by a slight bend towards the base of the main stem above the root collar which becomes less pronounced as the young tree develops.
There will be some species where the scion wood is grafted onto a more mature main stem which may be two or more metres tall.
Chitalpa tashkentensis ‘Summer Bells’ with Chilopsis being grafted on to a Catalpa main stem.
Also Fraxinus ornus Meczek.
Some trees may have een both top and bottom worked which means there are two unions on the same tree, one at the top and one at the bottom.
An example of this is, Prunus fructicosa Globosa, where Prunus avium is budded onto a colt understock and fructicos Globosa grafted onto the top to form the crown.
The compatibility of scion wood and understock coupled with the quality of contact between cambium and other meristematic tissues are essential in the successful formation of a permanent bud or graft union.
Poor or incompatible bud or graft unions can lead to failure in the landscape, sometimes years after planting has occurred. Such poor or incompatible graft unions can be identified on the nursery where there is a visible differential in growth rates between stock and scion wood resulting in an apparent swelling of either.
A large amount of epicorminc growth emerging from the stock wood can also be an indicator of poor bud or graft union.
|A||Understock planted out in nursery field|
|B||Bud of scion wood inserted in stock wood|
|C||Bud begins to develop|
|D||Understock removed leaving scion bud to develop on main stem|
|E||Diagram of cut. At no time should the bud union be below soil level.|
|F||Young developing budded tree.|
|G||Swelling at the bud union which may indicate imcompatibility. As the young tree develops growth masks with cut area which reamins visible as a slight bend at the base of the main stem|
|H||Disproportionate growth in the area of the bud union is an indicator of imcompatibility between stock and scion wood|
Growth between stock and scion wood should be equitable with no disproportionate growth apparent between the two.
The supplying nursery should be able to indicate the understock used to produce any particular cultivar.
At no time during the production process should the bud union have been below soil level.
No epicormic growth should be apparent emerging from the stock wood.
While all tree species have clearly identifiable, individual growth characteristics all broadleaved deciduous and evergreen trees can be placed in one of two categories.
Excurrent trees have a naturally occurring straight leader which remains prominent throughout the life of the tree. (for example Liquidamber styraciflua)
Decurrent trees lose leader dominance as they develop. (for example Acer psuedoplatanus).
All best practice nursery production will aim to produce and retain a strong central leader irrespective of whether the natural habit is excurrent or decurrent. Decurrent trees should retain a strong central leader until they have reached two-thirds of their mature height. The leader can be said to be dominant when none of the lateral branches are more than quater the diameter of the main stem at the branch union. The aim of the nursery is to produce a strong central leading stem while retaining enough photosynthetic capacity to facilitate healthy growth and development.
Failure to achieve the above can result in a weak central stem which is ill defined and ill equipped to support the construction of a well balanced permanent branching system as the tree develops both on the nursery and in the landscape.
To achieve the above some nursery formative pruning will have taken place with lower branches either subordinated or removed. The resultant pruning wounds will be visible on the main stem. These wounds will heal naturally providing the branches removed do not exceed the branch/main stem ratio referred to above and that the branch collar has been retained during the pruning process. Wound healing will be clearly visible with signs of occlusion taking place even though the wound may not be completely healed.
Poor or large pruning wounds may not occlude successfully leaving to ingress points for disease, structural weak points or distorted, misshapen main stems which may not recover as the young tree develops.
Formative pruning on the nursery is designed to produce a strong balanced leading stem capable of supporting the trees future permanent branching system. This permanent structure will not begin to be developed until a clear stem of between 2.5 and 3.0 metres is achieved. All branches below this point are temporary and will be removed at some time during the trees development. This does not apply to multi stemmed trees.
However it is important that during the nursery process poorly attached branches, branches with included bark and inward growing branches are either removed or subordinated. Failure to address such problems can lead to ill formed crown development, compromised leader formation and structural weakness.
|A||Diagram of well formed nursery tree with defined central leader and well balanced branching system|
|B||Laterals subordinated to main stem and no more than 25% of diameter of the main stem at branch union|
|C||Nursery formative pruning with all laterals subordinated to the main leader|
|D||Diagram of nursery formative pruning cuts|
|E||Potentially co-dominant laterals subordinated to the central leader in crown development|
During the formative pruning process it may have been necessary to prune out the natural leader and train a new dominant leader with all laterals subordinated to that new leader.
|A||Large prunung wound with branch collar removed. Pruning cut indicates branch over 25% diameter of main stem has been removed. Unlikely to heal satisfactorily|
|B||Acceptable branch removal with branch collar intact and cut less the 25% diameter of main stem|
|C||Fully occluded wound|
All young trees should have a clearly defined strong leader. All lateral branches should be subordinated to the leader and should never be more than 25% of the diameter of the main stem at the branch union.
All formative pruning wounds should exhibit healthy and continuous bark occlusion with all pruning cuts made leaving the branch collar clearly visible and intact on the main stem.
All branches which are poorly attached, are inward growing or cross and rub other branches should be removed or subordinated to the central stem.
Young trees develop new wood at points of mechanical stress. A certain degree of mechanical stress is beneficial in the production of structurally sound young trees. The spacing of trees on the nursery coupled with the type of support and length of time that support has been left in place impact on the trees mechanical integrity at transplanting into the landscape.
Trees which have been grown too close on the nursery tend to produce elongated central stems. This leads to a proportional disparity between tree height and tree girth. This disparity leads to tall young trees which do not have the mechanical integrity to fully support the developing crown. Such trees when planted in the landscape have a tendency to lean and incur additional stress while trying to correct the imbalance. Such trees are also prone to fracture.
Support systems restrict the trees ability to bend naturally which influences the disposition of wood. Such restricted movement inhibits the development of stem taper. Each tree species develops at a different rate and some species have a more developed stem taper earlier. However all trees , during development and uninhibited by external support, exhibit a taper from stem tip to stem base which is regular and defined with the greatest circumference visible at the root flare. Trees which have not developed stem taper are mechanically ill equipped to thrive in the landscape. The result is additional stress and the diversion of often scare resources into correcting this defect.
Nurseries often use canes and ties in the crown to train leader and encourage balanced crown development. The impact of such support, if left in place for too long, is the same as with stem development with mechanical integrity compromised. All such support should be unnecessary and removed, along with the tie holding that support in place, before planting into the landscape is considered. Ties left in the crown can result in permanent damage occurring often several seasons following transplanting.
All young trees should exhibit a clearly defined stem taper appropriate to the species and evident from crown tip through to root flare.
All young trees should have a proportionate and balanced height/stem girth ratio appropriate to the species.
At the point of despatch all trees should being wholly self supporting and free from any extraneous canes or ties in the crown.
copyright © 2015 Keith Sacre. All rights reserved