AS 2303:2018 Tree stock for landscape use
5.1  General
Tree stock balance is used for guidance to assess the proportional balance of tree stock grown in containers ≥ 20 L or ex-ground tree stock with a minimum rootball diameter of ≥ 400 mm. It describes the proportional relationship between the above-ground and below-ground portions of tree stock and can have a significant impact on the development of quality tree stock. Tree stock height and calliper (expressed as size index) and rootball volume are interlinked and form the key elements of tree stock balance.
Sufficient rootball volume contributes to the ability of tree stock to support itself in the landscape. It results in a reduction of artificial supports and structures and enables tree stock to be grown at wider spacings. Tree stock balance is also useful to ensure that tree stock has not outgrown its container or been dug ex-ground with too small a rootball. It can also be used to ensure that tree stock has sufficiently grown into its container (see Clause 4.3.6), inform irrigation regimes, and potting processes within production nurseries.
The ratio of tree stock height to calliper directly contributes to the self-supporting nature of the tree stock. For example, tree stock that are grown too close together can exhibit a poor height to calliper ratio and are often disproportionately tall. Conversely, good height to calliper ratio can often lead to well-developed stem taper (see Clause 4.2.6). It is important to note that tree stock are living products and size index of closely specified tree stock will vary within a batch at dispatch. Therefore, tree stock balance should not be used in isolation from other parameters detailed in Section 4.
NOTE  It is recognized that species, climatic conditions and production processes may influence the height- calliper proportions. Therefore, size index should not be used in isolation when specifying tree stock for landscape use.