Understanding Nursery Stock Sizes

It’s time to build your plant lists…. And there’s one thing to remember. Size matters! But understanding and deciphering standard nursery stock sizes can be like trying to find your way out of an Iowa cornfield. You’re bound to get lost a few times. Everyone calls out nursery stock sizes differently. Gallons, flat size, tree height, or caliper have all appeared in our plant lists. But depending on your familiarity with the nursery industry, you may not know that there is a continuously updated national standard on how to spec nursery stock sizes!

When writing this blog, I set out to include a nice handy, dandy all-purpose cheat sheet for our landscape professionals but it turns out that is easier said than done! Instead, this article provides a brief overview of the standard and can get you started on the right path to learning more for the next time you specify plants. We cannot promise that your suppliers will be following this standard to the number and letter, but if you stay familiar with the standards you will be much more likely to specify the nursery stock sizes that are available.

Specifying the Right Sizes Takes Knowledge

Specifying the Right Sizes Takes Knowledge

Called the American Standard for Nursery Stock, this national standard is published and maintained by AmericanHort, which is a somewhat recently formed consolidation of the American Nursery & Landscape Association and OFA—The Association of Horticultural Professionals.

According to AmericanHort, “The purpose of the American Standard for Nursery Stock … is to provide buyers and sellers of nursery stock with a common terminology, a ‘single language,’ in order to facilitate commercial transactions involving nursery stock. For instance, the Standard establishes common techniques for (a) measuring plants, (b) specifying and stating the size of plants, (c) determining the proper relationship between height and caliper, or height and width, and (d) determining whether a root ball or container is large enough for a particular size plant.”

The organization has been publishing this nursery standard since 1923, and it was adopted as a national standard in 1949 (ANSI Z60.1-2014). The most recent revision to the standard was published in 2014, and is available free from AmericanHort HERE. Use of this standard when growing and specifying plants is voluntary, but it is widely accepted and often referenced in standard specification sections. Using the standard has many benefits for designers and contractors, not the least of which is simplifying the procurement and acceptance processes by relying on clear pre-defined standards for quality and size of nursery stock.

The complete standard may be a daunting 109 pages but it is a surprisingly easy read, and provides in-depth, but digestible information. Below are a few key points from the American Standard for Nursery Stock you may find helpful (along with a cheat sheet to get you to the appropriate tables in the larger document). However, we would HIGHLY recommend reading through the whole standard and having a copy handy. It truly provides an amazing wealth of info (there’s a great section on identifying unacceptable co-dominant leaders – a problem I’ve had first hand issues with!). This resource also provides all the information necessary when accepting or rejecting plant material that shows up at your site!

 General Standards for Nursery Stock Sizes:

  • Specifications must include plant size by caliper, height or width appropriate to the plant type.
  • Certain plants may be specified by container size (see below). ALL others should be specified with caliper, height or width.
  • Typically, plant size specifications should include only the minimum allowable plant size in that interval. In other words, if you specify a 2.5 in. caliper Type 1 shade tree you may get a tree anywhere up to a 3 in caliper because it references the 2.5 to 3 inch range for that type of tree. As long as you specify the minimum, you know you will not get a SMALLER tree.
  • Caliper is always measured six inches above ground or soil level for trees less than 4.5” caliper, and 12 inches above the ground for 4.5” or larger.

General Standards for Containers:

  • These are generally the only plants to specify by container size
    • Herbaceous perennials
    • Non-winter-hardy shrubs
    • Ornamental grasses
    • Groundcovers
    • Vines
  • Containers marketed and sold with a # designation must have volumes within the ranges shown in the ANSI standard in order to comply
  • Container classes #1 through #100 include the volume of a container that, if such a container were manufactured, would hold the equivalent number of gallons as the container class number. #1 = one gallon; #5 = five gallon and so on.
  • Nursery stock specifications that reference only an imperial volume measurement, such as “quarts” or “gallons,” are not in accordance with the Standard. I can tell you that our plant lists have gotten that WRONG for years!!
  • The SP designation refers to ‘small plant’ containers. #SP4, for example, is a 4 inch container, or “quart” container. This designation goes from #SP1 – #SP5 and should reference square or round.
  • Container grown nursery stock shall have a well-established root system reaching the sides of the container to maintain a firm ball, but shall not have excessive root growth encircling the inside of the container (the same holds true for roots in ball and burlap!).

 

Root Bound

Container Growth Gone Wrong – Root Bound Tree

Tree B&B

When Done Right – A Thing of Beauty

 

Shade and Flowering Trees:

  • There are FOUR types of shade and flowering trees identified in the standard with the following associated specifications tables
    • Type 1 – Page 15-16
    • Type 2 – Page 17
    • Type 3 – Page 19
    • Type 4 – Page 21
    • Multi-stem and Shrub Form Trees – Page 24
  • Multi-stem tree specifications should include a minimum number of stems. If none is specified, 3 will be assumed.

Deciduous Shrubs:

  • There are THREE types of deciduous shrubs identified in the standard with associated specifications
    • Type 0 – Page 28
    • Type 1 – Page 28
    • Type 2 – Page 29
    • Type 3 – Page 30
  • Plants may not meet plant size specification or minimum number of canes at time of shipment at certain times of the year, but would be expected to reach the plant size specification and minimum number of canes during the first growing season after shipment.

Coniferous Evergreens:

  • There are SIX types of Coniferous Evergreens identified in the standard with associated specifications
    • Type 1 – Page 32
    • Type 2 – Page 33
    • Type 3 – Page 34
    • Type 4 – Page 36
    • Type 5 – Page 38
    • Type 6 – Page 40
  • Coniferous evergreens will also be described by their shearing:
    • Natural: (showing the form natural for the species)
    • Semi Sheared (sheared when plant is young to maintain symmetrical shape)
    • Sheared (pruned regularly to retain a symmetrical shape)
    • Altered Form (Can you say POODLED!!)

Broadleaf Evergreens:

  • There are SIX types of Coniferous Evergreens identified in the standard with associated specifications
    • Type 1 – Page 42
    • Type 2 – Page 44
    • Type 3 – Page 46
    • Type 4 – Page 48
    • Type 5 – Page 50
    • Type 6 – Page 52

Additional information in the standard provides specs for young plants, roses, palms (and other bare root stock), fruit trees, bulbs, corms/ tubers, and understock/seedling trees and shrubs.

Now, armed with this great information, you can confidently go forth and specify!

Also, check out these other great articles regarding considerations on transplant size:

https://www.michigan.gov/documents/dnr/Influence_of_Tree_Size_on_Transplant_Establishment_277656_7.pdf

http://www.ncufc.org/uploads/Tree_Establishment_A_Review_of_Some_of_the_Factors_(Struve_2009).pdf

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