Improving Root Systems On Container-grown Azaleas
Bonnie L. Appleton
Susan C. French
Virginia Polytechnic Institute and State University
Virginia Beach, Virginia

Synopsis
One factor that contributes to the death of many transplanted azaleas is poor root growth into the surrounding soil because of the formation of matted roots during container production. The use of copper-treated containers reduces matted root formation without significantly reducing azalea root or shoot growth and should, therefore, reduce the mortality rate for newly transplanted azaleas.

Introduction

Kurume azaleas are used extensively in the landscape as foundation plants, specimens, shrub borders, and massed plantings. They are highly valued for their showy spring bloom. Despite their popularity, however, the mortality rate for newly planted Kurume azaleas in mid-Atlantic region landscapes is very high.

One of the major reasons for decline and death of many newly planted azaleas is the failure of their roots to grow out from the root ball into the surrounding soil. Cramped planting holes, planting holes dug too deep, and heavy clay soils are also factors that can contribute to this problem. The most significant factor in transplanting azaleas that are container-grown, however, may be the thick mat of roots that often forms on the outside of root balls during production.

Azalea roots are fine and fibrous. When container-grown, the roots have a tendency to mat at the root ball/container interface (the outside of the root ball). One method that has proven successful for container-grown tree root modification involves the use of copper-based coatings on container interior surfaces (1, 2, 3). Cupric hydroxide in a liquid carrier (SpinOut ^® , Griffin Corporation, Valdosta, GA) can be highly effective in redirecting root growth at the root ball/container interface. The objective of this study was to determine whether containers treated with SpinOut ^® would reduce or eliminate root matting on the balls of container-grown azaleas.

Materials and Methods

On April 25, 1995, uniform liners of the Kurume azalea 'Hershey's Red' were potted into one gallon containers in a 5:1 (v:v) pine bark: sand medium. Two container types were used: smooth wall plastic (Nursery Supply, Fairless Hills, PA) and compressed fiber (Henry Molded Products, Inc., Lebanon, PA). Two container wall treatments were used for each container type: container with no copper coating or full interior coating of SpinOut ^® (sprayed on plastic container [Figure 1]; mixed with fiber prior to molding for compressed fiber container). Plants were top dressed with 18 g (0.63 oz) Osmocote 17-7-12 (Grace/Sierra, Milpitas, CA) and grown under overhead irrigation. Each of the four treatment combinations was replicated five times in a randomized complete block design.

Treatments were harvested on Sept. 25, 1995, and dried root and shoot weights taken. Results were analyzed using analysis of variance and Duncan's Multiple Range Test.

Figure 1. Plastic containers treated with copper for root modification.

Results and Discussion

After five months, azaleas grown in either fiber or plastic containers without SpinOut ^® had densely matted roots on the outside of the root balls. Little root growth was visible on the outside of azalea root balls grown in either SpinOut ^® -treated container (Figure 2). Based on visual examination, SpinOut ^® eliminated root matting equally well at the root ball/container interface of azaleas grown in either treated fiber or plastic containers.

For dried root and shoot weights there were no significant differences among any treatments. Although use of copper-treated containers can alter the root growth pattern of azaleas, copper appears to neither significantly increase or decrease plant biomass.

Figure 2. Matted roots on the root ball of an azalea from an untreated plastic container (left) vs. no matted roots on the root ball of an azalea from copper-treated plastic containers (right).

In an earlier preliminary study, azaleas from these same four container treatment combinations were transplanted to the landscape. When excavated one year after planting, azaleas with significantly matted roots when transplanted still had matted roots evident, with little root elongation into the surrounding soil. For those from SpinOut ^® -treated containers, considerable soil clung to the root ball indicating that roots had successfully grown out into the surrounding soil (Figure 3).

Figure 3. Little soil clings to the root ball of an azalea from an untreated container (right) vs. considerable soil clinging to the root ball of an azalea from a copper-treated container (left).

Our results show that containers treated with SpinOut ^® are highly effective in combating the problem of matted root systems on azaleas. Increased use of copper-treated containers by the nursery industry could reduce the mortality rate for newly planted Kurume azaleas in the landscape.

Literature Cited
1.  Struve, D. K.; Arnold, M. A.; Beeson, R.; Ruter, J. M.; Svenson, S.; Witte, W. T. The copper connection. Amer. Nurs. 179:52-54, 56-61; 1994.
2.  Appleton, B. L. Nursery production alternatives for reduction or elimination of circling tree roots. J. of Arboric. 19:383-388; 1993.
3.  Appleton, B. L. Nursery production methods for improving tree roots-an update. J. of Arboric. 21:265-270; 1995.

Dr. Bonnie Appleton is an Extension Nursery Specialist and Associate Professor of Horticulture, and Susan French is a Research Specialist.