What to consider when seeking standards for advanced tree stock
On mega‑projects, budgets for mature nursery tree stock can reach seven‑figure sums—but the true cost goes far beyond the trees themselves.
Installation of underground infrastructure, years of irrigation and maintenance, and the risk of future replacement all hinge on the quality of the original stock. Consultants should specify batch testing for root quality, measurable criteria for rootball integrity, canopy–root balance. We recommend specifying advanced nursery tree stock to meet AS 2303 in full to protect long-term investments.
Two industry standards compared
| Aspect | BS 3936‑1 (UK, 1992) | AS 2303 (Australia, 2018) |
| Underlying idea | Prescriptive, trade‑style size classes keyed to height or stem girth. | Performance‑based. Uses a size‑index (height [m] × calliper [mm]) and rootball/container dimensions; applies the same criteria to any size. |
| Largest stock for which numeric guidance is given | Extra‑heavy Standard – girth 14‑16 cm at 1 m, overall height “350 cm min.”. No guidance beyond that. | Container‑grown trees up to 3 000 L with preferred size‑index ~1 535 – 2 142 and an absolute ceiling of 2 758; ex‑ground trees with rootballs 2 500 mm Ø and size‑index up to 3 075. |
| How size is expressed | • Height bands (e.g. whips 100‑200 cm).
• Stem girth bands (e.g. 4‑6 cm, 6‑8 cm …). • Clear‑stem height bands. |
• Size‑index ties height to calliper for any stock.
• Container volume or rootball Ø establishes minimum root mass. |
| Stock types recognised | Seedlings, transplants, whips, feathered, half/extra‑light/light/standard/selected/heavy/extra‑heavy standards, multi-stem. | Container‑grown, containerised, bare‑rooted and ex‑ground (balled‑and‑burlapped) stock at all stages of growth. |
| Upper limits imposed by the standard | None beyond the extra‑heavy standard class; not intended for very large semi‑mature or ex‑ground trees. | Practical upper limits come from tables/graphs (3 000 L containers; 2.5 m rootballs) but method is scalable—larger stock can still be assessed by the same formula. |
| Girth/height vs. container/rootball linkage | Not addressed. A 16 cm‑girth tree could be in a 35 L pot or a field rootball with no check on root : shoot balance. | Mandatory rootball / container size bands derived from research; e.g. a 200 L container tree should have size‑index 142 – 247 (25‑75 %ile). |
Key take‑aways for specifiers
Breadth of coverage
- BS 3936‑1 tops out at trees about 3.5 m tall or 16 cm girth—typical of UK bare‑root or small root‑balled stock for street and amenity planting.
- AS 2303 runs through to very large semi‑mature container or ex‑ground specimens (>2 m rootballs).
Metric used
- BS 3936 requires you to pick a class first, then measure girth / height.
- AS 2303 lets you measure whatever you have, calculate size‑index and check it against the appropriate container or rootball table—so it scales naturally as specimen sizes grow.
Root–shoot balance
- AS 2303’s container‑volume and rootball‑diameter bands give explicit minimum root mass for every size of top‑growth.
- BS 3936 contains no such linkage, so very large container trees could still be “legal” under BS 3936 yet badly under‑rooted for Gulf heat and wind loads.
What this means in practice
- For small nursery lines (seedlings, 10–15 cm high) either standard works.
- For semi‑mature stock (≥200 L containers or field‑lifted with >400 mm rootballs) AS 2303 is the only one that tells you whether the rootball, calliper and height are in proportion.
- BS 3936 size classes can still be quoted for familiarity (e.g. Heavy Standard, 12‑14 cm girth) but should be backed up by AS 2303 rootball/size‑index criteria to avoid importing under‑sized root systems.
Bottom line:
BS 3936‑1 gives you discrete size classes up to small/medium landscape trees; AS 2303 offers a sliding scale that covers everything from tubestock to multi‑tonne, crane‑lifted feature trees—making it the more suitable benchmark when specifying advanced tree stock.
