QBARS - v33n3 The Influence of Container Media and Transplanting Technique and the Establishment of Container Grown Rhododendron cv. Hershey Red in Landscape Plantings

The Influence of Container Media and Transplanting
Technique on the Establishment of Container Grown
Rhododendron cv. 'Hershey Red' in Landscape Plantings ¹
R. D. Wright
Department of Horticulture, Virginia Polytechnic Institute and State University, Blacksburg, VA
D. C. Milbocker
Virginia Truck and Ornamentals Research Station, Norfolk, VA

¹ This work was supported in part by a grant from the American Rhododendron Society.

Many studies have been conducted to formulate media which provides highly favorable growing conditions for specific container grown plants in terms of moisture holding capacity, drainage, gas exchange and nutrient availability. Container plants grown in such media grow rapidly when supplied with ample nutrients and water; however, difficulties occur when the plants are transplanted into the landscape where less favorable growing conditions exist.
The medium must absorb and retain sufficient moisture from periodic rains or irrigations to maintain plant vigor and encourage root growth. In studies dealing with water availability in soil mixes, Juncker (4) demonstrated that in sand-peat mixes, larger proportions of peat increased the water holding capacities and released this moisture more gradually to plants. Other research has revealed that aggregate size is an important parameter influencing soil moisture content (1, 3, 5) and the availability of soil water for plant growth (4, 5). These studies imply or have shown that soil additives such as peat, crushed bark, perlite, sand and vermiculite improve soil aggregation, water holding capacity, and in general improve conditions for plant growth.
After transplanting, penetration of roots into the surrounding soil is influenced by a number of factors. Taylor (8) has shown that elongation rates of cotton and peanut roots decrease as the physical resistance to penetration (measured with a penitrometer) increased. Also, in order to penetrate a soil mass, a plant root must exert a root growth pressure greater than the resistance of the soil through which it is growing (7). Other studies (6) demonstrated that adequate anchorage is necessary before a root can transmit the necessary root growth pressure for penetration of a soil mass. Root penetration, therefore, is influenced by three classes of variables; those affecting (a) root growth pressure, (b) root anchorage and (c) resistance of the soil. Usually the incorporation of soil into the container medium should improve root anchorage and create a resistance similar to the surrounding soil after transplanting.
Treatment of the root ball during transplanting can also influence survival of container plants after transplanting. Fulmer and Jones (2) have shown that mechanical disarrangement of roots of container grown holly plants resulted in better root development and top growth after transplanting.
The purpose of this research was to investigate the influence of increasing proportions of soil and peat in container media and different transplanting techniques on the establishment of container grown rhododendrons into the landscape.
On December 5, 1975, 24 rooted liners of Rhododendron cv. 'Hershey Red' were potted in 2.9 I plastic containers with each of the following media: 100% sphagnum peat; peat and sand 1: 1; peat, sand and soil 4:4:1; peat, sand and soil 2:2:1; and peat, sand and soil 1:11. Sand was coarse builders' sand and soil was a clay loam. Plants were subsequently grown in a greenhouse at 78°F (25°C) (day) and 65°F (18°C) (night) under natural photo-period until spring and in a container nursery in full sunlight until fall. Plants were fertilized every 2 wks. at a rate of 200 ppm N with a 20-20-20 (20.ON-8.7P-16.7K) soluble fertilizer. A randomized complete block design with 4 replications was used.
On October 8, 1975, plants from each replicate were divided into 3 groups and prepared for transplanting as follows: 1) root ball cut vertically 8 times with a knife to a depth of approximately 1/2 inch; 2) media partially washed away from roots with water hose; and 3) root ball undisturbed. Plants were than planted in a Norfolk sandy loam soil at the Virginia Truck and Ornamentals Research Station, Norfolk, VA. A split plot design of 4 replicates with 2 plants per replicate was used with media as main plot and transplant treatment as subplot. Plants were mulched with 2 inches of pine bark mulch to simulate landscape practices and irrigated during periods of low rainfall. Plant growth was measured using, index (height + width)/2 to determine any difference in size at transplanting time due to media.
On September 6, 1977, the termination date for the experiment, plant growth index, top fresh weight, and diameter of the root ball were measured. The root ball diameter was measured by cutting vertically through the center of the root ball and removing one-half. The extremities of root spread were then determined for each plant and measured.
Azaleas grown in 100 percent peat moss were significantly larger at the time of transplanting than those grown in other media (Table 1). As the amount of peat moss in the media decreased, there was a simultaneous decrease in growth of the plants. Azalea growers have noted that plants grown in 100 percent peat moss were superior to plants grown in the other media. One year after transplanting the same trend continued and top weights substantiated this trend. This result is probably due to their larger size at transplanting since all plants increased in size at about the same rate after transplanting. Root growth into the surrounding soil after transplanting was not significantly affected by the container media (Table 1).

The size of plant tops one year after transplanting was not significantly affected by the transplanting technique although plants from the undisturbed treatment appeared to be the largest and those from the washed root ball treatment, the smallest (Table 2). Fresh weights followed a similar trend, however, the washed root ball was significantly smaller than the other 2 treatments. Root ball widths among treatments were significantly different with the undisturbed root ball treatment yielding the largest root ball.
Mechanical disarrangement of the root ball at transplanting was not found to increase root and shoot development as had been reported for holly (2). Contrarily, the treatment with the least disruption of root system resulted in the largest top and root ball size. These results may be due to root systems which had not become extremely compacted. More severely compacted root systems may benefit from disruption of the root ball to encourage new root growth. The possibility also exists that the fibrous root system of an azalea does not respond as favorably to mechanical disruption as other plant species such as holly with less fibrous systems.
All the plants at the end of this experiment were of acceptable landscape quality thus indicating that if azaleas are adequately cared for after transplanting, neither the container medium nor the transplanting technique (as followed in this experiment) are limiting factors determining transplanting success.

Literature Cited

1. Amemiya, M., 1965, The influence of aggregate size on soil moisture content-capillary conductivity relations. Soil Sci. Soc. Proc., 29:744-748.
2. Fulmer, J. P. and E. V. Jones, 1974, The effect of four transplant treatments on root growth of container-grown Ilex cornuta burfordi nana, Proc. Southern Nur. Assoc. Res. Conf. Atlanta, Georgia, p. 27.
3. Grable, A. R. and E. G. Siemer, 1968, Effects of bulk density, aggregate size, and soil water suction on oxygen diffusion, redox-potentials,. and elongation of corn roots, Soil Sci. Soc. Amer. Proc., 32:180-186.
4. Juncker, P. H. and J. J. Madison, 1967, Soil moisture characteristics of sand-peat mixes, Soil Sci. Soc. Amer. Proc. 31:5-8.
5. Shaykewich, C. F. and B. P. Warkentin, 1970, Effect of clay content and aggregate size on availability of soil water to tomato plants, Can. J. Soil Sci., 50:205-217.
6. Taylor, H. M. and H. R. Gardner, 1960, Use of wax substrates in root penetration studies. Soil Sci. Soc. Amer. Proc., 24:79-81.
7. Taylor, H. M. and H. R. Gardner, 1963, Penetration of cotton seedling taproot as influenced by bulk density, moisture content and strength of soil., Soil Sci., 96:153-156.
8. Taylor, H. M. and L. F. Ratliff, 1969, Root elongation rates of cotton and peanuts as a function of soil strength and soil water content. Soil Sci., 108:113-119.