Girth, girth size, height and pot volume: how to read plant measurements

The measurements used in the nursery make it possible to describe plants, prepare transport and compare offers. On their own they do not assess quality.

Girth, girth size, overall height, crown height, spread and pot volume refer to different characteristics. When a specification uses only one of them, it can bring together plants with very different presence, form and preparation.

Girth is the circumference of the trunk at breast height. It is measured with a tape around the trunk, at a reference height set by the trade, the standard or the specification.

In ornamental trees, the classes often appear as ranges, such as 20–25, 30–35 or 40–50 cm. A 30–35 specimen has a circumference that falls within that range.

The tape should sit perpendicular to the axis of the trunk and should not pass over a local irregularity that artificially increases the reading. When there are several stems, girth no longer represents the scale well and should be complemented by other measurements.

Girth is related to the maturity of the trunk, but it does not indicate the spread of the crown, the exact age or the quality of the root system.

‘Girth size’, or calibre, is a term used differently between countries and suppliers. It may mean circumference, diameter or a locally defined commercial class.

For that reason, it should be accompanied by the unit and the method. In international documents, writing just ‘calibre 20’ can lead to incompatible interpretations.

When the purchase involves several markets, it is worth confirming whether the measurement is a circumference or a diameter and at what height it was taken.

Height is measured from the base to the highest point. In regular crowns, this is relatively simple. In weeping, flexible or asymmetric trees, it can vary with the position of the branches.

A taller tree is not necessarily more mature, nor does it have a larger crown. It may have grown quickly in height and still have a thin trunk. Another, shorter one may have a greater girth and a more developed structure.

Height should be related to the species and the form. A columnar cultivar and a globe crown cannot be compared by this measurement alone.

Crown height gives the distance between the ground and the start of the main branching. It is decisive in streets, car parks, paths and seating areas.

A tree can meet the girth and the overall height and still start branching too low for the intended use. The opposite also happens: a very high crown can lose its relationship with a garden seen at close quarters.

The measurement should make clear whether it refers to the first branch, the predominant base of the crown or a minimum clear height. Photographs and drawings help to avoid ambiguity.

Spread makes it possible to anticipate space, shade, transport and distance from buildings. Irregular crowns should be measured in two directions.

In a mature specimen, this information can be as important as the girth. A tree six metres wide calls for different logistics and a different future space from a tree of the same species three metres wide.

The current spread is not the final spread. It should be checked against the expected development of the species or cultivar.

Pot volume, expressed in litres, indicates the space of growing medium available in production. It is not a direct measure of age, nor does it guarantee that the roots have filled that volume.

A plant may have been moved recently into a larger pot. In that case, the growing medium falls away and the mass is not yet consolidated. Another may have stayed too long and formed circling roots.

The assessment should confirm the distribution of the roots, the root collar and the stability of the mass. Comparing litres alone can favour the larger packaging rather than the better-grown plant.

In field trees, the size of the rootball relates to the species, the size of the tree and the transplanting history.

A larger rootball increases weight and logistical demands. It is not automatically better if it has little root or loses cohesion. Quality depends on the presence of roots, on the soil and on how it was prepared.

The measurement should be confirmed before the planting hole, the crane and the access are settled.

Age rarely appears as the main commercial measurement because, on its own, it does not explain quality. Two trees of the same age can be very different sizes.

At Diaplant, the mature range spends at least six years in our fields, receives a minimum of two transplantings and begins, on average, to be sold from ten years of age. These figures give the method context, but selection still depends on the specimen.

A class represents a range. Not every 20–25 cm tree should be expected to have exactly the same circumference.

In batches, it is useful to define which differences are acceptable and which attributes matter most. A small variation in girth may be less relevant than a large difference in crown height.

Tolerances should be realistic and verifiable.

A complete specification can include botanical name, cultivar, form, girth, height, crown height and spread, type of production and size of rootball or pot.

Not every project needs the same detail. The aim is to state enough for the material delivered to match the intended function.

The nursery visit adds what measurements cannot convey: proportion, orientation and character.

• Large tree or mature tree
• How to choose a specimen in the nursery
• What standard, half-standard and low standard mean

1. European Arboricultural Standards. European Tree Planting Standard.
2. European practices for grading nursery plant material.
3. Measurement and catalogue criteria used by Diaplant.