Jeff L. Sibley, John M. Ruter, and D. Joseph Eakes

Container production of shade and ornamental trees is increasingly important to the horticulture industry. Aboveground and pot-in-pot production methods are less expensive and more profitable than traditional field-grown methods. Container production of trees offers numerous advantages over traditional field production methods including year-round availability of plant material. Little information about container production of red maple cultivars is available, with no previous reports of multiple cultivar by location studies. About 55 distinct cultivars of red maple are available in the nursery industry, many of which have become popular in field production but have not been evaluated in container production. Climatological data give an indication of differences in rainfall, length of growing season, and maximum/minimum temperatures with USDA hardiness zones ranging from 8b to 6a across Georgia and Alabama (Table 1a,b, and c). Studies have demonstrated effects of different climates on growth of field- and container-grown plants other than red maple. Therefore the objective of this study was to evaluate the growth of eight container-grown red maple selections at four locations in Georgia and Alabama with different growing environmental conditions and irrigation practices.

METHODS
Rooted cuttings and tissue cultured plantlets (2-10 inches tall) of Acer rubrum and A. × freemanii, obtained in April 1995 were transplanted on the same day in Auburn, Alabama, to 3-gallon containers in a pinebark:sand (6:1 by volume) substrate amended with 18 pounds per cubic yard of 17-7-12 Osmocote, 1.5 pounds per cubic yard Micromax, and 5.0 pounds per cubic yard dolomitic lime in May 1995. Trees were grown in full sun under standard nursery practices for 1 month in Auburn prior to transport to the four study locations in the second week of June 1995. Locations were Blairsville, Georgia, 34^o53’N x 83^o58’W, elevation 1,892 ft; Muscle Shoals, Alabama, 34^o43’N x 87^o37’W, elevation 516 ft; Auburn, Alabama, 32^o 36’N x 85^o29’W, elevation 709 ft; and Tifton, Georgia, 31^o27’N x 83^o31’W, elevation 357 ft, in USDA hardiness zones 6b, 7a, 8a, and 8a, respectively. Trees were arranged at each location on landscape fabric-covered beds pot to pot. All trees were pruned to a central leader the last week of July 1995. Trees were overhead irrigated, using standard production practices for each given location, at 0.5 inch per day in Blairsville and Tifton; and 1.5 inches per day in Muscle Shoals and Auburn as needed in 1995 and 1996. Dormant trees were transplanted to 10-gallon containers the second week of December 1995.

Daily maximum and minimum air temperatures and rainfall were averaged for each month and location in 1995 - 1996 (Table 1b and c). All trees were transported to Auburn for harvest at the end of December 1996. Final caliper (stem diameter, 6 inches above the medium) and plant height were measured prior to dry weight measurements for each tree.

RESULTS
Despite differences in temperature and moisture (irrigation and rainfall) among locations (Table 1b and c), growth response across cultivars was similar in Blairsville and Tifton, and in Muscle Shoals and Auburn (Tables 2-3). Number of rainfall events varied little across locations during the two growing seasons; however, total rainfall was greater in Blairsville, Auburn, and Muscle Shoals, than Tifton in 1995 (Table 1b and c). Total rainfall was greater in Blairsville and Muscle Shoals, than Auburn and Tifton in 1996. In a related container study, we found the greatest height growth for container-grown red maple cultivars occurred between May and July. The greater overall growth across cultivars at the Alabama locations, compared to the Georgia locations, is attributed to non-limiting irrigation at the Alabama locations during the greatest period of growth. Across all cultivars tree height was highest for trees grown in Muscle Shoals (Table 2). Auburn trees had greater height growth than Tifton trees, with the exception of ‘Olson’ (Northfire^TM). Four cultivars had similar height growth in Auburn and Blairsville.

Caliper increase was generally greater for the Alabama locations compared with the Georgia locations (Table 3). Caliper increase was the same for Blairsville and Tifton for all cultivars with the exception of ‘Celzam’ (Celebration^TM) and ‘Landsburg’ (Firedance^TM). Caliper increase was the same for Auburn and Muscle Shoals for all cultivars with the exception of ‘Landsburg’ and ‘Franksred’ (Red Sunset^TM). Based on the similarities in Blairsville and Tifton and in Muscle Shoals and Auburn, the overall differences in caliper increase are attributed to differences in irrigation and rainfall across locations more than differences in temperature. For most cultivars, differences in caliper growth in this study would be considered a marketable difference from a container production standpoint. Based on common practice in the nursery industry and the American Standard for Nursery Stock, for trees of a similar height, caliper increases are generally considered marketable in 0.25 inch increments up to 2 inches.

Much can be gained from this study regarding cultivar performance under dissimilar environmental conditions. Three cultivars—‘Celzam’, ‘Landsburg’, and ‘Olson’—are new introductions and have not been included in container or field studies prior to this report. ‘Franksred’ demonstrated the greatest adaptability to the varied environmental conditions across locations. For each location ‘Franksred’ had the greatest height and caliper growth, with the exception of caliper at one location (Table 3), pointing out why this selection is often included in container studies. ‘Olson’ and ‘Celzam’ appear to be well adapted to container production in growing conditions of the Southeast. Based on the results of this study, we can not recommend ‘Landsburg’ as a suitable choice for container producers in the Southeast. This Zone 3 selection had the least height, caliper, and root growth for each of the four locations.

This study shows that differences in temperature and irrigation have a greater impact on growth of container-grown red maple cultivars by the end of the second year. In an earlier report no differences in first year tree height were found for ‘October Glory’ at three locations with dissimilar climates in Georgia and Alabama. Studies at Auburn in the 1980s found height growth was unaffected by irrigation rate in the first growing season in container-grown A. rubrum seedlings, but height and caliper growth increased as irrigation increased in the second year.

This study provides useful information regarding the impact of a long growing season on the growth of container-grown trees. For example, although Tifton had almost 4 months more frost-free days than Blairsville, these days apparently did not contribute to a growth advantage for Tifton over Blairsville as might be expected with container-grown ornamental shrubs. With primary shoot extension occurring early in the growing season in Tifton, the extended growing season may have caused a depletion in reserves through extended maintenance respiration. However, greater growth might have been realized from the longer growing season in Tifton, compared with Blairsville, if irrigation rates had replaced 100% of evapotranspiration, as opposed to 0.5 inches per day.