Conifers – Root collar diameters or Girth

Root collar diameters may be given in specifications of conifers, especially seedlings and transplants used in forestry planting, since they assist in quantifying the ‘substance’ of the plant.

Girth measures are seldom used, but are a useful measure especially for Pinus spp. which can be grown to semi-mature size with a clear stem.

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 Root Collar Diameter may be specified for seedlings (especially cell grown trees) and transplants, particularly where these are used as rootstocks or for forestry purposes.

Cell grown conifers

For cell grown trees, minimum Root Collar Diameters vary from 3 to 8 mm for Conifers, depending on overall height and size of cell.

Root collar diameter Overall height (cm) Volume (cc)
3 10-20 50
4
20-40 100
6 40-60 150
8 60-90 N/A

Seedlings and Transplants – open-ground

For Seedlings and Transplants, Root Collar Diameters will be a minimum of 3 mm and be graded in one or two mm bands, increasing with height and species, e.g. 3/4, 4/6, 6/8, 8/10 or 10/12.  This table indicates some typical root collar diameters for open ground forestry conifers:

Height (cm) no less than
7.5 10 15 20 25 30 40 50 60
Species Minimum root collar diameter (mm)
Abies grandis n/a n/a n/a 5 6 6.5 8 9 8.5
Chamaecyparis lawsoniana, Thuja plicata n/a n/a n/a 4 4.5 5 6.5 7.5 6.5
Larix decidua, L. eurolepsis, L. kaempferi n/a n/a n/a 2.5 3.5 4 5 6 9.5
Picea abies n/a n/a 3 4 4.5 5 6.5 8 n/a
Pinus nigra maritima 3 3 4 4.5 5.5 6.5 8 9.5 n/a
Pseudotsuga menziesii n/a n/a n/a 3 3.5 4 5 6 5.5
Tsuga heterophylla n/a n/a n/a 2.5 3 3.5 4 5 n/a

Seedlings and transplants – container-grown

Increasingly, coniferous forestry seedlings are grown in cells.  However, if they are grown in other containers, the following root collar diameter grades apply.

Minimum height Min. root collar diameter Minimum container size
 Species cm mm
ml
Picea abies 20 4 100
Pinus nigra maritima 12 2 100
Pseudotsuga menziesii
20 3 100

Girth

The girth of a tree is “the circumference of the stem measured at 1 metre above ground level.”  For Semi-Mature/Specimen conifers with clear stems, girth measures are indicated in the same ranges as for broadleaf trees, but it is important to note that height grades are different, namely:  18-20cm; 20-25cm; 25-30cm; 30-35cm; 35-40cm; 40-45cm; 45-50cm; 50-60cm.