A Guide to Nursery Production Practices
which Impact Post Planting Survival and
Growth of Landscape Plants
Jim Sellmer, Ornamental Horticulture Extension Specialist
Larry Kuhns, Professor of Ornamental Horticulture

As difficult as it is, we must separate ourselves from poor plant health due to improper nursery management practices and transplanting activities. Determining the cause of plant problems in the landscape begins by examining the conditions under which it was grown, planted, and maintained.

A common question heard by horticulturists is "We planted several trees or shrubs earlier this year and they just aren't growing as we expected. What is wrong with them?" The key statement here is "earlier this year". Transplant shock refers to a number of stresses occurring in recently transplanted trees and shrubs. It involves failure of the plant to root well; consequently, the plant becomes poorly established in the landscape. New transplants do not have extensive root systems, and they are frequently stressed by lack of sufficient water. Plants suffering from water stress may be more susceptible to injury from other causes such as the weather, insects, or diseases. When several stresses are being experienced, the plant may no longer be able to function properly. Plants vary in their ability to recover from transplant shock based on their growth momentum prior to transplanting and overall structural organization. Consider the difference between poplar and forsythia both of which are rapid growers with several flushes of growth a year and tend to require less recovery time versus red oak and rhododendron which grow are slower to grow producing determinant shoots and often requires more time to recover and reestablish. Of course, all of this is dependent on proper handling, planting, and maintenance during the transplanting process.

The following discussion highlights some nursery management practices that can adversely affect the survival and growth of landscape plants. The hope is that this list will be used constructively and proactively to help avoid unnecessary plant loss in the landscape rather than destructively to place the blame of a poor planting and maintenance job onto others. For landscapers, this list of impacts should be used in carefully selecting and purchasing nursery stock. For nursery operators this list of impacts should be taken into consideration when evaluating the propagation, planting, growing, managing, and digging of nursery stock for sale.

Deformed Root Systems
Plants grown in containers for too long may develop circling root systems that are permanently hardened into that shape. As the plant grows in future years the roots can expand and girdle each other, resulting in a plant that is unstable, and may die or blow down due to an underdeveloped root system. More likely, the plant may die due to the slow strangulation caused by one or more "girdling roots" which are growing in diameter as the plant grows. Girdling root has been a common diagnosis for declining Norway Maples in the landscape especially after the tree reaches fifteen to twenty years of age. Deformed or girdled roots can also be caused by twisting bareroot liner trees at the time of planting in the nursery to force long roots into a hole that is not large enough. This simple twist of the wrist can result in circling roots in the future.

LANDSCAPE TIP: Visit the nursery and see first hand the handling methods for each phase of the operation. Tag the trees and shrubs for the landscape site. Check the plants after delivery. For container grown plants randomly select plants and knock them out of the pots in order to check for circling roots prior to planting. Score or cut the circling roots to promote new branch root initials before planting. Note that scoring the roots will be an additional cause of transplant shock. The other option is to reject the plants for the planting project and return them to the nursery. For balled in burlap plants, find the root collar, excavate to find the tops of the structural roots and check the direction in which the roots radiate out from the trunk.

NURSERY TIP: For container plants, use a root pruning solution (ex. Spinout) to prevent root circling and promote new root initials. Avoid crowning the soil up around the trunk of the plants in the nursery and root prune in the field in the late summer or early fall before a spring sale to promote fibrous roots. This is especially helpful for trees that have been in the ground for 5 or more years. Dig plants with proper size balls for the size of the plant based on the American Standard for Nursery Stock.

Limited Root Systems & Planting Too Deeply
There are two nursery practices that can lead to a soil ball with no roots in the upper part of the ball; 'hilling up' soil around the base of the tree, and planting too deep in the nursery.

The practice of hilling up soil around the base of trees was begun many years ago as a method of weed control. When trees were dug by hand, experienced diggers knew they had to take the time to move the 'hilled up' soil away from the base of the plant before digging it.

Hilling up soil at the base of the trees no longer needs to be done because there are herbicides available that provide safe and effective weed control. However, some nurseries still to do it. This results in the original root system of the plant being buried with four to ten inches of soil.

Another nursery practice that results in the original root system of the plant being buried is planting too deep. Some plants have been planted up to 12 inches too deep in the nursery. This is done to stabilize bareroot tree liners. Plant the crown around two inches deep is sufficient.

Most trees today are dug with mechanical tree spades. Soil balls dug with tree spades are wider at the top than the bottom. If a tree has been planted too deep in the nursery, or if soil that has been hilled up around a tree is not removed prior to digging, the result is a soil ball that has no roots in the upper part of the ball. Since this is the widest part of the ball, it contains the highest proportion of the volume of the ball. Although the soil ball may be the correct size for the tree being moved, the actual root mass may be one-half or less what it should be. The other problem that occurs is at the planting site. It is difficult for those planting the tree to discover the problem, so if they plant the tree properly, with the top of the soil ball level with the existing grade, they are actually planting the root system four to twelve inches too deep. Trees with undersized root systems that are planted too deep rarely survive.

LANDSCAPE TIP: Check for the tree collar (root flare) in the field before tagging and get assurances that the soil above the tree collar will be removed prior to digging. In the landscape, check for the root collar prior to planting and remove excess soil to insure that the plants are planted at the proper depth.

NURSERY TIP: Don't crown the tree rows. If crowned, remove crowns prior to digging, and dig the plants at the proper depth based on the location of the root collar.

Damaged and Deformed Trunks
Postemergence herbicides are an essential part of a weed control program in the production of plants. However, if used improperly they can damage the trunks of trees around which they are applied. Young plants with thin bark and show pigment are most sensitive to misapplications.

Roundup Pro is the most commonly used herbicide around trees. If too much is sprayed directly onto the trunks of young, sensitive plants it can be absorbed through the bark and move upward. Injury can be difficult to diagnose because it may not show up for a year or two after application. When it does, it appears as a splitting of the bark as it separates away from the wood below it. Depending on the time and rate of application, and the sensitivity of the tree, the split may be small and barely noticeable, or it may develop to an inch or more wide and progress up into the canopy of the tree.

Damage to the bark by Finale and Reward results in a sunken canker that develops as the living tissue continues to grow and develop around the herbicide killed tissue. Injury from these herbicides is much less common than injury from Roundup Pro.

Poor pruning of side limbs, bark damage during transportation or nursery activities, planting too deep, and poor protection from sunscald and frost cracking can also lead to damaged and deformed trunks. Taking extra care in all aspects of nursery operations and looking for plant problems during a nursery visit can reduce the likelihood that damaged or stressed plants are placed in the landscape.

GENERAL TIP: Inspect plants for signs of bark splitting or sunken cankers. However, even if no injury is found, it is possible for latent injury from nursery applications to appear following planting into the landscape.

LANDSCAPE TIP: Visit the nursery, discuss weed management activities in the nursery, and personally select all plants for a specific job.

NURSERY TIP: Avoid oversprays of postemergence herbicides in cleaning up nursery rows. Prevent unnecessary wounding and poor basal pruning techniques that can lead to splitting and cracking during the winter. Protect tender barked tree trunks from sunscalding and frost cracking.

Poor Overall Growth
Plants that are grown properly develop a growth momentum. When a plant is grown in good soil with limited competition from weeds, and is provided adequate amounts of water and fertilizer; it develops a dense, dark green canopy of foliage that is highly photosynthetically active. It is in the process of photosynthesis that energy for additional growth of the plant is produced. The longer the good growing conditions for a plant are maintained, the more energy is produced, and the longer the growth momentum continues.

A plant that is grown under continuously favorable conditions will contain high-energy reserves that will be available to support the production of new roots, branches, and leaves. This plant will survive and develop much better than a plant that was grown under conditions that would limit growth and photosynthesis. Growth momentum does transfer from the nursery to the planting site if trees are dug, shipped, and handled properly.

Plants grown on poor soils may survive in the nursery, but they donšt thrive. Their root systems will be limited, and they will not accumulate the high-energy reserves needed to develop the growth momentum that will help them become established in their new sites following transplanting. When dug with a tree spade the lower part of the soil ball may not have many, if any roots in it. If grown in wet soils, the root system may be infested with root-rotting organisms that will further restrict the re-establishment of the tree in its new site.

Plants grown too close together in the nursery have two major problems. They shade each other and reduce their level of photosynthesis, which results in lower energy reserves in the plants. A plant with low energy reserves is not capable of producing as many roots as quickly following transplanting as a plant that has higher energy reserves. Trees grown too close together develop too much height in relation to their caliper. They are weaker trees that may need considerable support following planting.

GENERAL TIP: Visit nurseries to determine their soil types, the level of their weed management, and their fertilization and irrigation practices. Evaluate the growth and foliage of the trees to determine if they are growing vigorously. Eliminate a nursery as a potential supplier if poor production practices are encountered.

Insect and Disease Problems
Some insect or disease problems have little effect on the survival and growth of trees in the landscape. A slight aphid or leaf spot infestation is of no concern. However, any plants that show signs of root rotting diseases, trunk cankers, scale insects, spider mites, or borer or weevil infestations should be rejected. These problems become progressively worse on plants that are undergoing the severe stress that is associated with transplanting.

NURSERY TIP: Scout the nursery or hire a scout to keep track of and provide recommendations for dealing with pest problems. Work with your local nursery inspector and Penn State University Cooperative Extension county agent to develop a strong integrated pest management (IPM) program. Carefully inspect incoming plant materials to reduce new insect and disease infestations. Maintain a strong weed management program within the nursery and on surrounding edges to reduce weed and associated pest problems. Recognize the difference between cyclic insect pests and endemic pests and become familiar with the environmental conditions that promote outbreaks.

Undersize Root Balls
As previously mentioned, the size of soil balls should meet the minimum standards presented in the American Standards for Nursery Stock. Undersized soil balls have less root mass and energy to grow the new roots that are essential for the survival and growth of newly planted trees.

Root Restriction and Girdling
Untreated natural burlap rots within six weeks following planting; treated natural burlap rots within six months. Plastic burlap does not decay in strength after it is buried and no longer exposed to sunlight. It restricts water movement into the soil ball, and root growth out of the ball. Roots that do penetrate the spaces between fibers are eventually girdled. Plants planted in plastic burlap establish slowly, and generally die within five years.

When ordering balled in burlap plants, specify that only untreated or treated natural burlap should be used to wrap the soil balls. Plants with soil balls wrapped in plastic burlap will be rejected. Removing the burlap entirely from a root ball prior to planting may result in damage to the soil ball. This is especially true if the burlap is covered with a wire basket and both the basket and burlap must be removed.

Though everyone in the industry should know that nylon twine must be removed at the time of planting, there are still far too many plants killed each year through girdling by nylon twine that was not removed from around the trunks of trees. Specifications should include a statement that soil balls must be tied with natural sisal twine; plants tied with nylon twine should be rejected. Natural twine can girdle trees if it is wrapped around the trunk of a tree too many times. Be sure to instruct planting crews to cut all twine that is wrapped around tree trunks.

Most of the trees and shrubs we plant into the landscape have the potential for long lives, some greater than 100 years. Unfortunately, today the average life span of a landscape plant is about 20-25 years. The shortened life of the plants in our landscapes is due to many factors that are can be controlled during the production of the plants in the nursery and during handling, transplanting, and maintenance.

Information provided by Rob Berghage and Jim Sellmer Penn State Department of Horticulture, Ornamental Horticulture Extension Specialist, 103 Tyson Building, University Park, PA 16802, (814) 863-2571

Back To Ornamental Horticulture Newsletter Page