5. Root system development and management
The three production systems used predominantly in the UK all affect young tree root development in some way. A series of root modifications occur irrespective of the system. Each of these has the potential to impact positively or negatively on transplant success. This sections highlights good and not so good practice for each of the three systems and provides items which can be specified specifically.
The nursery production method chosen primarily effects the development of the young trees root system. It is important that there is an understanding of best practice for each of the systems and what is actually specifiable and why. It is only by understanding nursery best practice and the problems which occur post transplanting into the landscape that successful procurement specifications can be written.
Irrespective of the final production system all trees are at some stage lifted from the nursery bed or field. The process of lifting from the nursery field inevitably leads to the root system being damaged or modified.
The earliest modification occurs when the seedling is lifted from the seedbed. The naturally occurring tap root is severed during the process. At the point where the lifting cut is made an adventitious lateral root system is formed which is deeper than the lateral root system which would have formed had the young tree remained undisturbed. This adventitious root system is often more vigorous than that developing just below the root flare. The root area between the adventitious and naturally occurring lateral root system is known as the root shank. it has been recorded that even if the bud union and the root flare are clearly visible the adventitious root flare may be 300mm below the soil surface when trees are lifted from the nursery for transplanting into the landscape.
With the adventitious root flare too deep the liklihood of deep planting into the landscape is increased. Reasearch has demonstrated reduced crown growth on some species when planted as little as 80mm too deep. See The Barcham Line
|A||Nursery seedling in the seed bed|
|B||Seedling undercut and replanted. Proliferation of the new root development at the cut point|
|C||Root shank development between natural laterals root system and adventitious root system formed at the cut joint. This tends to be more vigorous|
The seedling produced can be either grown on to mature specimens or be used as understock for the production of clonal cultivars.
A similar process occurs when trees are produced from cuttings with prolific root development occurring at the base of the cutting where callous has formed. The root system can also be deeper than would be desirable.
The depth of the primary lateral root system is important as, when it is potentially coupled with deep planting, eventual failure in the landscape is likely. Unfortunately the length of the root shank and the depth of the adventitious root system where this occurs can only be examined easily on bare root trees.
Bare Root Trees
It is very difficult to produce a definitive description and universal specification for the root system trees as they vary enormously with each tree having its own individual characteristics.
|A||Idealised bare root system|
|B & C||One directional root systm caused by nursery production|
All root systems exhibit certain characteristics which are peculiar to the species under consideration. Even within a species there is variation. Providing generalised guidance is difficult but it is fair to suggest that all bare root systems ideally display a good lateral root spread, with at least four obvious and well developed lateral roots evenly spread around the stem circumference. (This cannot be precise in definition)
The method used by nurseries to line young trees out in the field often produces a wholly one directional root system which has been colloquially described as ‘the hockey stick root system.’ Once this type of system has thickened the one dimensional characteristic is retained throughout the life of the tree. This can be easily identified and avoided.
Trees with one directional root systems and or a lack of evenly spaced lateral roots are more likely to be unstable as they develop in the landscape.
Bare root trees provide the opportunity to examine root structure and while it is difficult to provide difinitive guidelines the depth of the adventitious root flare, as described above, needs to be considered. Overly vigourous adventitious root development at the lower end of the root shank can contribute to failure in the landscape especially coupled with deep planting.
It is possible to indicate the expected diameter of the root spread of bare root trees as illustrated in the following table: The amount of fine root is important for the absorption of water and nutrients after transplanting into the landscape.
Root Spread for bare root tree stock.
|Young tree height in metres||Diameter of root spread in millimetres|
|2.5 to 3.0||450|
|3.0 to 3.5||550|
While some damage is inevitable when lifting bare root trees from the nursery field it is important that the root system retains a significant amount of fibrous root to facilitate early development in the landscape.
When selecting bare root trees, the vigour as well as the structure of the root systm needs to be considered. A simple iodine test can be performed to confirm he presence of stored carbohydrate. The darker the stain the higher the carbohydrate content present. Stored carbohydrate is essential to stimulate new root growth following transplanting into the landscape.
All root systems should be typical for the species and should display a well-balanced radial root system comprising of at least four obvious and substantial lateral roots.
Root systems should not be predominantly one directional.
All bare root systems should meet the height size parameters as outlined in the table above.
All bare root systems should have a significant amount of fibrous root commensurate with the species.
Bare root trees on the nursery should have several lateral roots near the soil surface.
A simple iodine test should be carried out to assess the amount of stored carbohydrate present.
The rootballing of trees is a process. This process involves the regular undercutting or transplanting of field grown trees. During each undercutting or lifting sequence the root system is cut. New prolific root growth is apparent at the cut points. Unfortunately evidence as to whether this practice has been carried according to best practice is hidden inside the rootball and cannot be easily inspected. On lifting from the nursery field for despatch the resultant soil ball is wrapped in hessian and supported by non galvanised wire mesh.
An indication of bad practice is undue movement of the soil ball in relation to the stem of the tree. If one can be moved independently of the other then further investigation would be advisable.
The following table indicates the size of rootball and the number of times it should have been undercut/transplanted on the nursery.
Girth of tree measured at one metre in centimetres
|Minimum diameter of rootball in millimetres||Minimum number of times transplanted/undercut on the nursery|
The lifting of trees from the nursery field to produce rootballs where this best practice has not been followed can result in as much as 95% of the trees root system being left in the ground. A rootball which has a smaller diameter in relation to tree girth than recommended in the table above is unlikely to have been prepared according to best practice and will have had a significant percentage of its root system left in the nursery field.
Poor practice in the preparation of a rootball is likely to result in poor performance in the landscape or failure.
|A||Diagram of undercutting process and the impact of the process on root development|
|C: first cut|
|B: Second cut|
|B & C||Impact of not undercutting. Note the amount of fibrous root left in the ground|
Nursery preparation of trees for rootballing can result in mounding with the root flare either totally or partially, buried. This mounded soil can be lifted as an integral part of the rootball itself. This results in the natural rootflare being too deep.
This can lead to failure in the landscape. This eventual failure may not manifest itself for a number of years after transplanting. The final depth of the rootflare can be further comprised if the tree is actually transplanted too deep. This increases the likelihood of failure.
The root flare needs to be clearly visible prior to planting.
|A||Soil mounted during the production process results in root flare buried and lifted inside the rootball.|
|B||Deep planting in the container results in the root flare being buried and fibrous root growth in compost above root flare.|
All trees (SIZE/STEM GIRTH) should have been transplanted or undercut (NUMBER) and have a diameter of (MEASUREMENT) (see above table)
The root flare should be clearly visible at the surface of the rootball.
The final root pruning cut should be at least 200-300mm from the exterior of the rootball.
The soil ball should not move independently of the central stem.
Containerised trees are produced when young trees are lifted from the nursery field and containerised. The period between containerisation and despatch is used to re build the root system following the inevitable damage incurred at lifting. The system ensures that root systems are undamaged and fully functional when the young tree arrives at the planting site.
The major problem with containerised trees occurs when the tree is left in the container for too long. When this happens the root system circles around the outside of the container and in extreme cases will girdle completely. This can happen at any stage in a young tress development and from the smallest to the largest containers.
Despite many claims to the contrary root circling and eventual girdling will occur in all container production systems if the young tree is left in the container for too long.
Cross sectional view showing development of root circling leading to root girdling (seen from above)
|A,B,C||Illustrate stages in the development of circling/girdled roots in a container|
The only satisfactory method of ensuring this has not happened is to enquire of the supplying nursery as to how long the tree has been in a container. This is often referred to as ‘shelf life’
Circled or girdled roots when planted in the landscape can lead to instability as lateral root development is restricted and in extreme case self strangulation. This often leads to failure, which can often be dramatic, several years after planting.
Another problem occurs when the tree’s root flare is buried too deep in the container at containerisation. On occasions this results in matted fibrous root development in the extraneous compost above the root flare.
The presence of this matted fibrous root above the root flare can exacerbate deep planting.
All containerised trees should be free of root circling or root girdling.
All nurseries supplying containerised trees should be able to specify the ‘shelf life’ date of individual or batches or trees.
The beginnings of the root flare of all containerised trees should be clearly visible on the surface of the container.
Containerised trees should never be confused with container grown trees. Containerised trees are trees lifted from the nursery field and grown on in containers for a limited period of time. Container grown trees have been grown in containers for the whole of the production period. The trees are moved from container to container as they develop. Any root defects created in the smaller container are transferred to the larger container. This can lead to structural weakness in the root system once transplanted into the landscape with failure occurring at the point when tree roots were transferred from on container to another.
Where trees are moved from smaller to larger containers any circling or defective roots should have been shaved off leaving only lateral roots in evidence before re-containerising.
Where trees are transferred from smaller to larger containers evidence should be provided to demonstrate that any root defects/circling roots have been shaved from the container ball prior to re-containerisation.
copyright © 2015 Keith Sacre. All rights reserved.