ANSI Z60.1: American Standard for Nursery Stock Sizing Rules

ANSI Z60.1, the American Standard for Nursery Stock, is a voluntary consensus standard that gives growers, landscape architects, and buyers a shared vocabulary for describing plants: how to measure trunk diameter, plant height, root ball size, and container volume so that a “two-inch caliper shade tree” means the same thing in Oregon as it does in Florida. It’s published by AmericanHort and approved by the American National Standards Institute.¹ Compliance isn’t required by federal law, but once a contract, purchase order, or government specification references the standard, its sizing rules become binding on the transaction.²

Z60.1 Has Been Superseded by Z60.2-2025

The document most people still call Z60.1 was approved by ANSI in April 2014 after a revision cycle that ran from 2005 through 2013. A lighter revision was approved on April 17, 2025, and the current designation is ANSI Z60.2-2025.² The core measurement methods and sizing tables carried forward, so the principles below apply to both editions. If you’re drafting a specification today, cite Z60.2-2025 to avoid ambiguity about which edition governs. Contracts that still name Z60.1 remain valid.

How Caliper Is Measured on Trees

For most deciduous shade trees, size is expressed as caliper, meaning the diameter of the trunk. Where you place the tape depends on how large the tree already is. For trees with a caliper of less than four-and-a-half inches, measurement is taken six inches above the ground line for field-grown stock, or six inches above the root flare for bare-root plants. Once caliper at that six-inch point reaches four-and-a-half inches or more, the measurement moves up to twelve inches above the ground to account for the natural flare at the base of larger trunks.³

The standard also pairs each caliper size with an expected height range, so a tree sold as two-inch caliper should fall within the corresponding height listed in the specification tables. A tree that meets caliper but not height, or vice versa, is out of spec.

How Height Is Measured on Shrubs, Evergreens, and Palms

Where trunk diameter isn’t a useful indicator, height becomes the primary measurement, and the method varies by plant type.

Deciduous shrubs are measured from the ground line to the top of the canes that meet the height specification, not to the tallest single branch. A shrub sold at two feet needs the qualifying number of canes reaching two feet, even if one or two canes happen to grow taller.²
Evergreens are measured to the midpoint between the uppermost whorl of branches and the tip of the leader, not to the tip itself.²
Palms can be specified by overall height or trunk height. Overall height runs from the ground to the top of the arc formed by the highest arching frond. Trunk height runs from the ground to the base of the heart leaf.²

The idea is to describe the plant’s functional canopy rather than a stray leader or wispy top branch that doesn’t represent what the buyer actually receives.

Minimum Root Ball Sizes for Field-Grown Trees

When a field-grown tree is dug for transplanting, the root ball must be large enough to preserve the roots the tree depends on. Minimum diameters scale with caliper. For Type 1 shade trees, several reference points from the standard’s table:²

1-inch caliper: 16-inch minimum root ball diameter
2-inch caliper: 24-inch minimum root ball diameter
3-inch caliper: 32-inch minimum root ball diameter
4-inch caliper: 42-inch minimum root ball diameter
5-inch caliper: 54-inch minimum root ball diameter
6-inch caliper: 60-inch minimum root ball diameter
8-inch caliper: 80-inch minimum root ball diameter

The progression isn’t linear. A five-inch caliper tree needs a root ball more than twice the diameter required for a two-inch tree, reflecting the larger root system that supports a bigger canopy. An undersized ball is one of the first things to check when a large transplant fails.

Container Size by Volume

Containerized plants are classed by volume rather than by pot diameter. Each container class has a defined volume range in cubic inches; a Number 5 container, for example, must hold between 785 and 1,242 cubic inches.² The range allows for variation in pot shape while ensuring adequate root space. A “#5” label on a pot that looks unusually small can be verified against the standard’s volume range.

Harvest and Packaging Terminology

The standard defines uniform terms for how a plant’s root system is prepared and delivered. The three most common designations:

Balled and burlapped (B&B): a field-grown plant dug with an intact ball of soil around the roots, wrapped in burlap or similar mesh fabric.²
Bare root (BR): a harvested plant with soil or growing medium removed from the roots entirely.²
Container grown: a plant grown and marketed in a container, including pot-in-pot production systems.²

Additional designations include field-potted plants (dug from the field and placed in a container for delivery) and in-ground fabric bag-grown stock. The harvest method affects planting season, transplant survival, and post-installation care, so the terminology on a purchase order has practical consequences.

Plant Type Classifications

The standard organizes nursery stock into distinct groups, each with its own measurement tables. The 2004 edition of Z60.1 identified thirteen: shade and flowering trees, deciduous shrubs, coniferous evergreens, broadleaf evergreens, roses, young plants, fruit trees, small fruits, understock, seedlings, bulbs and corms and tubers, perennials, and Christmas trees.⁴ The groupings matter because a spreading juniper and an upright arborvitae are both coniferous evergreens but require different measurements: spread for one, height for the other. Broadleaf evergreens such as hollies and rhododendrons have their own tables because their growth habits differ from both needled evergreens and deciduous plants.

What the Standard Does Not Cover

The standard is explicit that it does not guarantee the health or quality of nursery stock.⁵ A tree can meet every caliper, height, and root ball requirement while carrying disease, pest infestation, or girdling roots that will kill it years later. Health certification, pest inspection, and phytosanitary compliance are the province of state agriculture departments and separate contract language. A contract that references Z60.2 alone gives you a sizing warranty, not a health guarantee.

When the Standard Becomes Binding

Z60.2 is voluntary. The standard’s own preface notes that parties to a commercial transaction should expressly agree that it applies.² In practice it becomes binding whenever a contract, specification, or purchase order references it. State and municipal planting ordinances frequently incorporate it by reference, and federal landscape contracts almost universally cite it. Once written into an agreement, a failure to meet its sizing requirements becomes a breach of contract rather than a suggestion. The standard is available as a free PDF download from AmericanHort’s website, and anyone drafting or reviewing a nursery stock specification should have a current copy on hand.¹

1
AmericanHort. American Nursery Stock Standards and Best Practices
2
AmericanHort. American Standard for Nursery Stock
3
Volusia County Government. American Standard for Nursery Stock – Section: 1.2 Methods of Measurement
4
American Nursery & Landscape Association. American Standard for Nursery Stock ANSI Z60.1-2004
5
University of Florida IFAS Extension. American Standard for Nursery Stock (ANSI Z60.1)