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Selecting Nursery Stock

Park Cultural Landscapes Program

Updated Cultural Landscapes

When ordering plant material from a nursery or creating specifications for a planting service contract, identification of plant size and production method are crucial, both can have a large impact on successful plant establishment in the cultural landscape.

Production Method and Size

Nursery Production Method

Nursery stock is sold with bare roots, in containers, or with rootballs wrapped in burlap (“balled and burlapped” or “B & B”). Bare root plants grow in the ground, then lifted and sold in the dormant season without soil attached. These are common for fruit trees and restoration plantings, are low cost and transportation is easy. However, bare root stock must be planted within a short period in early spring and larger plant specimens are generally not available.

 

Examples of Nursery Production Method: One is bare root, one shows young tree in container, and the final shows a young tree with burlap around it's roots
Examples of Nursery Production Method

Bare root, container, and balled and burlapped nursery stock (Arborday.org). 

 

Containerized nursery stock grow in pots. Shrubs and small trees are most commonly available in containers. Of the three production methods, containerized plants are the most easily stored and are planted at any time of year. However, as they’ve received plentiful water in the nursery, containerized plants are easily shocked by a change in conditions after transplanting. Mitigate transplanting shock by planting in the dormant season. The roots of containerized plants may have circled the inside of the pot, potentially inhibiting establishment after transplanting. Mitigate encircled roots by making vertical cuts with a sharp blade up the root mass, to encourage the roots to spread out after transplanting.

Balled and burlapped plants grow in the ground, dug with a hydraulic tree spade, and transported with burlap wrapped in wire or twine around the root ball. Their roots are the most stable of the three types of nursery stock and are generally quicker to establish than containerized plants. However, B & B plants can be more difficult to transport, and their burlap and wires should be removed at planting. The root mass of B & B stock is more susceptible to drying out than containerized stock, and a best practice is to transplant during the dormant season.

Tree Size

For small trees under seven feet, specify the desired size by height. Specify the desired size of trees over seven feet by caliper. Caliper is the diameter of the tree near the base. Nursery tree calipers range from under an inch to either inches. Fruit tree calipers are measure at the root collar, trees with calipers four inches and under are measured six inches from the soil, and trees with calipers over four inches are measure 12 inches above the soil. Upon receipt, check tree size to verify accuracy of the order.

Diagram showing two trees and how to measure the height and root ball for specification in nursery stock ordering.
Diagram showing how to measure the height and root ball

Balled and burlap tree measurements (American Standards for Nursery Stock. ANSI Z60.1-2014).

 

The American Horticulture Industry Association’s American Standard for Nursery Stock (ANSI Z60.1) provides guidelines for recommended root area in relation to tree caliper and height. For deciduous and coniferous trees and shrubs, these guidelines ensure the caliper to root ball ratio is sufficient to support establishment. (A plant is established when its roots support shoot growth at a similar rate to a non-transplanted plant.)

Table showing examples of tree size specifications when ordering nursery stock
Table showing examples of tree size specifications

Specifying large nursery stock can be tempting when replacing a historic tree, to fill the void left behind. But research has shown that smaller nursery stock actually establishes faster than larger stock. In many cases, smaller stock will outpace the size of larger stock in several years. This is because the roots of small and large stock extend into the soil at approximately the same rate per year. In a race for establishment, smaller nursery stock has the advantage as it requires less root area. Larger trees take more time to establish as their roots must expand further to acquire sufficient area for anchorage and water supply.

Diagram showing roots of a small and large tree and how much the roots will grow per year.
Diagram how much roots will grow per year

Equivalent yearly root growth of small (A) and large (B) tree (Watson, W. Todd. Influence of Tree Size on Transplant Establishment and Growth ).

 

As a rule of thumb, in temperate climates, trees need one year to establish for every inch of caliper (3 months per inch in subtropical climates). The difference in height between a 2-inch caliper and a 4-inch caliper tree is typically 3-6 feet. This small size deficit is easily overcome when a 2-inch caliper tree becomes established two years earlier than a 4-inch caliper tree.

Image of two tree trunk sections showing tree rings. One tree section is much smaller because transplant stress
Two tree trunk sections showing tree rings

These spruce trees were transplanted when the one on the left was smaller than the right (indicated by outline ‘1’). The smaller tree at transplanting grew much more rapidly as the larger tree experienced transplant stress (Watson, Gary W. Tree Transplanting and Establishment). 

 

Planting service contracts should provide a one year guarantee of survival, and two-years for larger trees under normal conditions. The contractor is responsible for replacement of plants that die during the warranty period at no cost to the government. Carefully considering the production method and size will better equip park staff to help new plantings survive beyond the warranty period. Stay-tuned for the summer issue to learn more about tree selection.

 

Plant Inspection

Just like individual pieces of fruit in the grocery store vary in quality, individual nursery plants of the same species, size, and production method may differ considerably. Defects in nursery stock can occur from the roots to the leaves, and plants need thorough examination before purchase.

Roots

The root collar (also known as root flare or trunk flare) is the zone between the trunk and the roots. In older trees this area is wider than the trunk, creating a flare. Roots emerge at the base of the root collar. In nursery stock the bottom of the root collar should be at or slightly above the level of the soil. Planting too deeply can significantly affect tree survival: the bark touching the soil is prone to rot and deeper roots may not receive sufficient oxygen. Inspect a plant by removing soil around the base until the first major root is visible. The top-most roots should be no deeper than one to two inches within the soil.

Two images showing examples of tree root girdle. Both show roots wrapping around the base of the tree.
Examples of tree root girdle

Circling and girdling roots (University of Florida Environmental Horticulture).

 

Inspecting roots of nursery stock may seem difficult because roots are hidden. However, the most severe root defects of circling and girdling roots are often seen in the top of the container or root ball. Circling roots encircle the outer edge of the container and girdling roots encircle the base of the tree. Most nurseries allow patrons to remove plants from containers or pull back the burlap to inspect a plant’s root systems.

For balled and burlapped plants, avoid trees with large roots cuts at the edge of the root ball. Instead, select trees with a combination of small to medium structural roots and finer roots at the root ball perimeter. For container plants, if roots form a mesh around the edge of the container, the plant is root bound. While not ideal, this is not a rejectable defect because cutting the edge of the roots before planting can encourage root distribution.

Trunk and Branches

When inspecting the trunk, ensure that the trunk emerges from the center of the container or root ball. Look for straight trunk without cankers, bore holds, or major wounds. Pruning cuts on the trunk should be healed or cut at the appropriate location to allow for healing (outside the branch collar). When purchasing a tree, generally, the tree should have one central leader. Avoid trees with codominant stems, because they are more prone to failure. (This is not relevant to trees that are bred to have multiple trunks such as witch hazel, jacaranda, and strawberry tree.)

Branch arrangement and attachment will also influence future tree growth. The largest branches should be less than two-thirds the width of the trunk one inch above the branch union. Select trees with a balanced shape (branches growing around the entire stem at consistent vertical spacing). In addition, look at the angle branches emerge from the stem and avoid trees with tight branch angles (<46 degrees). (Not relevant for species of tree with narrow branching structure and vertical habit.) Avoid trees with included bark at the branch/trunk intersections (this is bark that has merged and fused in the space between the branch and trunk).

Diagram showing two trees and how to determine what is acceptable and rejectable for nursery stock
Diagram for acceptable and reject-able nursery stock
Crown inspection guidance for high branched trees (Urban Tree Foundation).

Leaves

Leaves can be the most obvious indicator of tree health. The presence of new growth is a general indicator of the health of the plant. Top die-back indicates that the tree is planted too deeply or the presence of disease. Leaf color can indicate a nutrient deficiency, stress, and disease. Look for damage to leaves from pests and avoid trees with more than very minor leaf damage.

Image of maple tree leaves that are scorched
Maple tree leaves that are scorched
Leaf scorch, a sign of water stress (West Virginia University Extension).

Plant Inspection Checklist

The Urban Tree Foundation developed open source detail drawings for plant inspections that can be added to landscape contracts. For non-contract work, consider checking local wholesale nurseries to see if they sell directly to government entities, as they will have larger stock and lower prices than a retail nursery. When purchasing nursery stock, use the following basic inspection checklist:

  • Roots emerge no more than two inches deep within the soil
  • Roots do not circle or girdle the base of the plant
  • Trunk is straight and free of defects
  • Tree has a single central leader or can easily be pruned to create one
  • Branches are distributed evenly at wide angles with no included bark
  • Main branches are less than two-thirds the width of the trunk
  • New growth is evident and leaves are free of insect damage

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