Trees: Root Collar Diameter, Stem Diameter or Girth
General
These dimensions are given in specifications of trees since they assist in quantifying the ‘substance’ of the plant. Through some cultural methods, trees can gain in height but without thickening and developing the preferred characteristics of a well grown tree. For this reason standard measures of diameter or circumference of the main stem of trees have been developed so that these can be added to plant specifications where appropriate.
Root Collar Diameter
The Root Collar Diameter is defined as ‘‘the diameter of the main stem measured at or within a specified distance from the root collar.’’ The root collar is between the base of the aerial part of the plant and the top of the root system.
The collar is generally a lighter colour than the stem, except for trees of the genus Juglans, where the opposite is true.
The Root Collar Diameter may be specified for Seedlings (especially Cell grown trees), and cuttings and transplants, particularly where these are used as rootstocks.
Cell grown trees
For Cell grown trees, minimum Root Collar Diameters vary from 4 to 8 mm for Broadleaf trees and 4 to 8 mm for Conifers, depending on overall height and size of cell.
Seedlings, Cuttings and Transplants
For Seedlings, Cuttings and Transplants, Root Collar Diameters will be minimum of 3mm and be graded in one or two bands, increasing with height and must be typical of the species e.g. 3/4, 4/6, 6/8, 8/10 or 10/12.
More details can be found under the Summary of Specification Criteria and also in European Technical and Quality Standards of Hardy Nurserystock (European Nurserystock Association E.N.A., Oct.1996) and Cell Grown Plants: A Guide to the Use and Specifications of Cell Grown Trees and Shrubs produced by Members of the Horticultural Trades Association Forestry Group.
Girth
The girth of a tree is ‘‘the circumference of the stem measured at 1 metre above ground level.’’ Usually this measure applies to standard trees, but girth grades can be given for large Feathered trees.
GIRTHS FOR CLEAR STEM STANDARD TREES
Figure 1: MEASURING THE GIRTH OF
A STANDARD TREE
| cm |
Form |
Marking colour |
|
| 6-8 |
Light standard |
blue |
|
| 8-10 |
Standard |
yellow |
|
| 10-12 |
Selected Standard |
red |
|
| 12-14 |
Heavy |
white |
|
| 14-16 |
Extra heavy |
blue |
|
| 16-18 |
Extra heavy |
yellow |
|
| 18-20 |
Extra heavy |
red |
|
| 20-25 |
Semi mature |
white |
|
| 25-30 |
Semi mature |
blue |
|
| 30-35 |
Semi mature |
yellow |
|
| 35-40 |
Semi mature |
red |
|
| 40-45 |
Semi mature |
white |
|
| 45-50 |
Semi mature |
blue |
|
For trees above 50 cm girth, the measure increases in 10 cm steps, 50-60, 60-70, etc.
The colours indicate the European colour system for marking trees.
Figure: 2
MULTI STEMMED (Alnus glutinosa)
Girth for Feathered trees
When the girth size is greater than 6 cm it should be specified as for clear stem standard trees, e.g. 6-8, 10-12, etc.
Multi Stemmed trees
For these trees, the number of stems required must be specified and the height of the tree.
Bushy Trees
Due to the many stems of bushy trees, these trees should only be specified by height.