Impact of Growing Method and Irrigation Regime on the Establishment of Arrowwood Viburnum (Viburnum dentatum) Two Months After Landscape Planting
by Derek Snyder

Abstract
The purpose of this research was to compare top growth of arrowwood viburnum (Viburnum dentatum) two months after landscape planting through the factors of two growing methods and two irrigation regimes. The two growing methods investigated were shrubs grown from seed in air root-pruning containers (ARPCs) and shrubs grown from stem cuttings in conventional black plastic containers (CBPCs), and the irrigation regimes consisted of both frequent and infrequent watering. Shrubs grown from seed in ARPCs had significantly more top growth volume than those grown from stem cuttings in CBPCs. The effects of irrigation regime on top growth volume were insignificant, as were any interactions between growing method and irrigation regime. Areas of future research are suggested and related to both the study of additional growing method options and the investigation of root growth as an additional indicator of shrub establishment and long-term success in the landscape.

Introduction
When selecting trees and shrubs for planting in the landscape, homeowners, municipalities, private organizations and public agencies alike look for plants that will rapidly establish themselves, be most cost effective to install, and will quickly increase in size with minimal maintenance. Top growth of the plant after installation is often considered to be the feature of most importance, since it is the part of the plant that is seen and showcased in the landscape setting. In addition, the preferred ease of installation makes comparably lightweight containerized plants usually more desirable than their heavy field-grown counterparts (Whitcomb 1988). As several studies indicate, the most critical factors in both maximizing top growth at time of establishment and minimizing maintenance, aside from soil type and topography of the planting site, include the growing method prior to planting and irrigation of the plant following planting (Whitcomb 1991; Harris and Gilman 1993; Gilman 2001; Gilman et al. 2003).

Currently, there are numerous systems available to produce containerized plant material, based mainly on the four components of container type, propagation method, soil mix, and irrigation regimen (Whitcomb 1988). Container types range widely from conventional black plastic containers (CBPCs) and fabric containers to copper-treated plastic containers and air root-pruning plastic containers (ARPCs), each having unique advantages and disadvantages to plant growth (Whitcomb 1988; Harris and Gilman 1993; Gilman 2001; Gilman et al. 2003). Propagation is typically performed either from seed or from stem cuttings, and the two methods produce either similar or dissimilar growth results depending on plant species (Whitcomb 1988). Numerous types of soil mixes and combinations of fertilizers in those mixes result in a wide range of growth potentials for plants depending on the particular selection of substrate and nutrients (Whitcomb 1988). In addition, selection of water volume and frequency of irrigation each play an important role in the growth of containerized plants, both during production and after landscape planting (Whitcomb 1988; Whitcomb 1991; Gilman 2001; Gilman et al. 2003).

The specific objective of this study was to determine if growing method and/or irrigation regime had a significant effect on top growth of arrowwood viburnum (Viburnum dentatum) two months after planting. The approach involved planting a controlled layout of the shrubs grown from two different production methods, subjecting an equal amount of shrubs grown by each method to frequent irrigation as to infrequent irrigation, and measuring the average heights and diameters of the shrubs at the end of the study period.

Materials and Methods
In June 2004, thirty two arrowwood viburnum (V. dentatum) 5-Gallon shrubs were obtained for the study and placed on pallets in a holding location at Possibility Place Nursery (Monee, Illinois) for two weeks, where they each received 5 gallons of water daily. Sixteen of these shrubs (purchased from Rosehill Creek Nursery in Manhattan, Illinois) had been grown from stem cuttings in 5-Gallon conventional black plastic containers (CBPC). The other sixteen shrubs (donated by Possibility Place Nursery) had been grown from seed in Rootmaker I ® 5-Gallon Grounder air root-pruning containers (ARPC) (see Figure 1). The shrubs were selected to have approximately the same volume of top growth, with the anticipation that there would not be a significant difference in shrub size between the two growing methods.

On June 26, 2004, the thirty two shrubs were planted at Possibility Place Nursery in a field of s omewhat poorly drained Frankfort silty clay loam, as described in the USDA Soil Survey of Will County, Illinois (2004). The study area was rototilled to a depth of 8 inches, the shrubs were planted by hand, and mulch was spread evenly over the entire study area to a depth of two inches to reduce the occurrence of weeds and to help retain soil moisture (see Figure 2). The layout of the shrubs in the study area was determined according to a two-factor, two-level factorial design with replication (Montgomery 1984). As such, eight replication blocks, each 6 feet square measured from the centers of the four shrubs in each block, were spaced 6 feet from the shrub centers of the adjacent block in a line running down a uniform 4 percent slope from west to east. The two levels for the factor of pre-planting growing method were (A) shrub grown from seed in ARPC and (B) shrub grown from stem cuttings in CBPC. The two levels for the factor of post-planting irrigation regime were (1) frequent irrigation and (2) infrequent irrigation. In e ach of the eight replication blocks, the four treatment combinations (A1, A2, B1, and B1) were arranged as follows: A1 in the southwest corner, B1 southeast corner, B2 in the northwest corner, and A2 in the northeast corner. This arrangement allowed for a ½ inch diameter, 1.0 GPH pressure compensating dripline with 24 inch spacing between emitters to be placed along the entire south row of shrubs, as all of them were on the frequent irrigation regime. Since the dripline applied water in a 2 foot maximum radius from each emitter, the desired amount of water was provided to the south row shrubs without affecting the soil moisture content of the north row shrubs, which were all on the infrequent irrigation regime.

At time of planting, the average heights and diameters of every shrub were measured and recorded. Assuming the shrubs to have a cylindrical shape, the diameters were converted to areas, which were then multiplied by the respective heights to obtain the initial top growth volumes for the shrubs. A t-test was performed using Excel (Microsoft Corporation 2002) to statistically verify whether or not the difference in initial top growth volume of the shrubs was significant between the two growing methods.

During the first week after planting, all shrubs were watered individually to saturation once daily using a spray nozzle hose attachment. For shrubs on the frequent irrigation regime, they received 5 gallons of water daily during weeks 2 through 8 by means of the dripline. This daily irrigation was not applied, however, if more than 1.5 inches of rain fell since the previous scheduled irrigation, and this occurred once in each of weeks 3, 4, and 6. For shrubs on the infrequent irrigation regime, they received 5 gallons of water on any day in weeks 2 through 8 when more than 50% of the shrubs on the regime showed excessive wilting, and this occurred once in week 3 only, when high temperatures peaked above 90 oF. Over the course of the entire study period, the average daily high temperature was 79 oF, with a maximum of 93 oF and a minimum of 61 oF. Light conditions were mostly sunny for 47% of the study period, mixed sun and clouds for 35% of the study period, and mostly cloudy for 18% of the study period.

After 8 weeks from the time of planting, the average heights and diameters of every shrub were measured again, recorded, and converted to final volumes of top growth in the same manner as before. A two-way analysis of variance (ANOVA) was performed on Excel (Microsoft Corporation 2002) , with growing method and irrigation regime as the two factors, to determine the significance of treatment effects. In addition, a t-test was performed using Excel to determine if there was a statistically significant difference in final top growth volume of the shrubs between the two growing methods.

Results
As indicated by a t-test for the two pre-planting growing methods, there was no significant difference (P > 0.05) in the initial top growth volumes of the shrubs between those grown from stem cuttings in CBPCs and those grown from seed in ARPCs (see Figure 3). After the two month study period, however, the final top growth volumes of the shrubs were significantly different (P < 0.05) between the two pre-planting growing methods (see Figure 3). As indicated by ANOVA, there was a significant main effect (P < 0.05) of growing method on top growth volume two months after planting (F = 28.943; F crit = 4.196). Shrubs grown from seed in ARPCs had larger average top growth volumes than shrubs grown from stem cuttings in CBPCs for both irrigation regimes (see Figure 4). The main effect of irrigation treatments on top growth volume, however, was not significant (P > 0.05; F = 1.274; F crit = 4.196). Average top growth volumes of the shrubs did not vary significantly between frequent and infrequent irrigation regimes for either pre-planting growing method (see Figure 4). Also, there was no significant interaction (P > 0.05) between growing method and irrigation regime (F = 2.849; F crit = 4.196).

Discussion
Based on the results of the t-test involving initial top growth volumes of the shrubs, there is a high degree of certainty that the shrubs of both growing methods were statistically similar in size to each other at time of planting. Thus, since the original volumes are essentially equivalent, the average final top growth volume of the shrubs grown from seed in ARPCs is further verified to be significantly larger, as determined by the respective t-test, than that of the shrubs grown from stem cuttings in CBPCs. Agreeing with the results of this t-test, two-way ANOVA indicates that the effect of the seed in ARPC growing method on top growth is significantly greater than that for the stem cuttings in CBPC growing method, while also suggesting that the effects of irrigation and any interaction between irrigation and growing method are both insignificant. This higher top growth rate for the shrubs grown from seed in ARPCs was noted also in a study by Gilman et al. (2003), and it illustrates the potential for shrubs grown in such a manner to best meet the wishes of nursery clientele. In addition, it is interesting to note how the two irrigation regimes resulted in little observable difference in top growth in each of the growing methods, which was similarly observed by Gilman (2001) with live oak Quercus virginiana. Although the poorly drained nature of the study site soil could account for some extended soil moisture retention that would aid the survival of the shrubs on the infrequent irrigation regime, the two rows nevertheless received widely differing amounts of water. Such findings suggest that shrubs grown from seed in ARPC would perform well whether watered regularly or only during extended high heat, allowing for flexibility in maintenance. Not only could the shrubs be watered regularly with an automated irrigation system, they could also be watered on an as needed basis to conserve water without detrimental effects on top growth.

During the course of the study, several areas have been identified for consideration in future research. First of all, it could be of benefit to examine the growing methods of seed in CBPCs and stem cuttings in ARPCs, as this might allow for the two variables of propagation type and container type to be separated for this small selection of containers. The reason these methods were not examined in this study, aside from limited available space, was that since there were no local nurseries that were selling V. dentatum grown either from seed in CBPCs or from stem cuttings in ARPCs, the two growing methods included in the study were the only industry employed options at the time. However, further investigation of all these growing method combinations may reveal a method that results in better top growth than seed in ARPCs. In addition to focusing on top growth of the shrubs, further research could be conducted on the differences in root growth after landscape planting between plants grown according to the discussed growing methods. As explained in Gilman et al. (2003) and Whitcomb (1991), the number and distribution of roots may be a more accurate indicator of long-term plant survival and success than top-growth alone.

Acknowledgements
Thanks to C. Shaw of Possibility Place Nursery in Monee, Illinois, for the donation of plant material and the necessary resources to setup and conduct the study. Thanks also to Professor L. Eberhardt for her advice and comments during the research and preparation of this report.


Literature Cited
Gilman, E. F. 2001 “Effect of Nursery Production Method, Irrigation, and Inoculation with Mycorrhizae-Forming Fungi on Establishment of Quercus virginiana.” Journal of Arboriculture 27: 30-39.

Gilman, E. F. et al. 2003. “Irrigation and Container Type Impact Red Maple (Acer Rubrum L.) 5 Years After Landscape Planting.” Journal of Arboriculture 29: 231-36.

Harris, J. R. and E. F. Gilman. 1993. “Production Method Affects Growth and Post-Transplant Establishment of 'East Palatka' Holly.” Journal of the American Society of Horticulture Science 118: 194-200.

Microsoft Corporation. 2002. Microsoft ® Excel 2002.

Montgomery, D. C. 1984. Design and Analysis of Experiments, 2 nd ed. New York, New York: John Wiley and Sons, Inc.

[USDA] United States Department of Agriculture. 2004. Soil Survey of Will County, Illinois. Available from <http://soils.usda.gov/survey/online_surveys/illinois/will/man.pdf>. Accessed 2004 December 14.

Whitcomb, C. E. 1988. Plant Production in Containers. Stillwater, Oklahoma: Lacebark, Inc.

Whitcomb, C. E. 1991. Establishment and Maintenance of Landscape Plants. Stillwater, Oklahoma: Lacebark, Inc.

Appendix

a A = Shrub grown from seed in air root-pruning container (ARPC) before landscape planting

b 1 = Frequent irrigation: 5 gal. applied daily for 8 weeks after landscape planting

c B = Shrub grown from stem cuttings in conventional black plastic container (CBPC) before landscape planting

d 2 = Infrequent irrigation: 5 gal. applied daily for 1 week after landscape planting, once in week 3