Trunk caliper is the measurement of the diameter of the trunk recorded at approximately 150mm above the soil level. The trunk caliper should be thicker towards the base of the tree and taper up towards the top. Good trunk caliper and taper will stabilise the trunk to hold the crown and withstand climatic forces such as wind. 


Good trunk caliper is important to:

  • Stabilise the tree
  • Support the crown, and
  • Withstand climatic forces such as the wind.

Trees with good trunk caliper are able to self support themselves without needing to be staked. Maintenance practices undertaken in the nursery will determine whether a tree will develop into a strong or weak tree. Correct practices will ensure that the lower trunk caliper will become thicker, a stronger root system will develop and the tree will be able to stand upright unsupported. A weak trunk will result in reduced growth overall.


Air pruning is the process of pruning the roots when they are exposed to the air. This process results in the development of new roots and overall the development of a healthy, fibrous root system. In hard wall containers, the roots are redirected when they reach the walls developing kinked, girdled root systems resulting in weaker trees.

The Air pruning container is a reusable growing system that actively enhances the quality of the root systems of trees, produces non-spiralling root systems that are impossible to develop in any hard walled container. The Air pruning container is cylindrical in shape with a perforated sidewall, which is textured like an egg carton. There are no flat surfaces to deflect roots and start the spiralling process. The inward pointing cones direct the root towards the hole in the outward pointing cones where the air density in the soil is too great and therefore the apical cells at the very tip of the root dehydrate, or are air-pruned. The air pruning process results in the tree roots developing new fibrous roots to compensate for the loss. This compensation results in the development of a dense and fibrous radial root system in a relatively short period of time.

The real benefits of using air pruning are that they:

  • Eliminate root circling and.
  • Minimise transplant shock.
  • Improved success rate once transplanted - reducing costs for clients relating to stock losses.
  • Improved water and nutrient uptake when compared to stock grown in hard walled containers.
  • Reduced root defects.


Growing trees in air pruning containers develops a stronger root system with a dense matting of adventitious formative roots. The lack of root defects eliminates the problems associated with poor root systems generally found in hard walled pots. A stronger root system enables the tree to establish structural roots more effectively in the soil when transplanted. When the tree is planted, the fine, dense root system immediately starts to spread, it efficiently gathers an immediate source of nutrients to foster vigorous growth. The tree is able to quickly establish itself.  


A simple answer is Yes! You may not notice the impact of a girdled or deformed root system has on a tree initially, and in fact it may take many years for the severity of the problem to be known. At this time though to correct the problem is either impossible, resulting in the impending death of the tree, or if it is possible it would be extremely costly. Problems associated with deformed root systems include failure of the tree, whereby the tree can be blown over in high winds, reduced growth - due to the impact that the root system is having on the trunk, sufficient resources for the ongoing growth and survival of the tree are not able to be effectively transported throughout the trees vascular system resulting in reduced growth, dieback and eventually death of the tree.

Root defects that develop during nursery production can lead to poor vigor and tree failure.  Most defects, such as circling roots in the root ball interior, can be mostly eliminated with appropriate and timely management in the nursery. Strategies in the nursery should focus on producing trees that have straight main mother roots growing to the edge of the container. Straight roots growing from the trunk form a strong, wide root plate. Trees with a wide root plate are stable and require a large force to tip them over. Trees with deflected, kinked, or bent roots can develop a smaller root plate and represent lesser quality. When main woody roots are deflected and not straight, there may be no root plate and the tree can become unstable A tree’s instability can result from the curved or bent shape of the main roots combined with increasing crown size.  Some trees are able to remain stable despite root deformities, but they may lose vigor if circling roots meet the trunk and constrict sap flow . A reduced growth rate often precedes the other signs of reduced vigor such as chlorotic foliage and dieback.


Trees grow in a range of heights and widths creating a variety of shapes. Some trees grow in a dense form. This means that although they are not tall, they occupy precious space in a landscape environment.  There are other trees which grow in a more tall and open shape; they easily provide shade, while only the trunk occupies personal space in the landscape. Choosing a tree involves consideration of many factors including height, width, density and colour as well as the overall shape. Not to mention soil, drainage, proximity to buildings and importantly climatic conditions and their impact on the tree. Before going to the nursery to select a tree you should compose a list of questions such as:

  • Do I want a big tree or a little tree in that location? ie. Do you need a lot of shade or no shade?
  • How much water will I have available to water the tree? i.e. consider water restrictions, times of drought etc.
  • Species selection i.e. if your location is prone to drought conditions, certain species, such as Birch, would not survive adequately.
  • What are the soil conditions? i.e. clay/loam ratio, acidic/alkaline etc.
  • Once you have determined what you need, you then need to locate a quality tree nursery. When visiting a nursery before purchasing any stock you should:
  • remove the tree you are considering purchasing from its container to examine the root system, are the roots circled or girdled?
  • Do they appear to have been burnt or have died?
  • Does the tree have one dominant leader?
  • Does the tree have any damage to the trunk or branches?
  • Is there evidence of pest or disease damage?
  • Can the tree stand without anyone having to hold the upper region of the canopy?


In some instances staking a tree is acceptable, those occasions would be where the tree requires protection from damage i.e. close to a road, driveway

Staking for support or anchorage should not be necessary if the tree has been grown correctly in the nursery. A tree should be able to be self supportive of its structure without the need for staking. Trees require exposure to uniform stresses to facilitate their growth. Exposure to environmental conditions, particularly wind, is crucial to the formation of a high trunk caliper and trunk taper. Trees that are not staked may take slightly longer to achieve height, but the positive outcome would be the formation of a high caliper and taper providing a more structurally sound tree. Nurserys that expose their trees to the different uniform stresses develop trees that have:

  • a high trunk caliper and taper,
  • are structurally stable trees,
  • trees that are self supporting and don't require staking,
  • reduced transplant stress once planted.