Thomas J. Brass, Gary Keever, Charles H. Gilliam, and D. Joseph Eakes

Two negative factors associated with container production of nursery crops are
high root-zone temperatures and root circling along the substrate-container interface. Temperatures can reach 122^oF in nursery containers in the southern United States, due to direct solar radiation striking the container sidewalls. Elevated root-zone temperatures in excess of 104^oF may lead to root injury or death, reduced shoot growth, and higher transpiration rates. Root malformation and circling in containers are common for trees with vigorously growing root systems. These conditions may lead to a less fibrous root system, a reduction in plant growth, or the formation of girdling roots which contribute to poor root regeneration and slow transplant establishment in the landscape. Copper compounds sprayed on the interior surface of containers control root malformation and circling. As a result, root malformation and circling are reduced, permitting increased root regeneration following transplanting.

Flowering dogwoods are naturally found in partial shade in well-drained, organic soils with roots maintained in a cool, moist environment. When dogwoods are grown in containers on beds exposed to full sun in the Southeast, root-zone temperature becomes an important factor. Several growers have expressed difficulty in producing container-grown dogwoods, presumably due to high substrate temperatures. This study examined the response of dogwood when grown in styrene-lined and/or copper-treated containers.

METHODS
An experiment which included all combinations of two dogwood cultivars, +/- styrene lining, and +/- copper coating was established on March 13, 1993. Styrene sheeting, 0.1 inch thick, was inserted into 3-gallon nursery containers covering the sidewalls but not the container bottom. Copper hydroxide at 13.4 ounces per gallon of latex base (Spin Out) was applied with a paint sprayer to the containers’ interior surfaces or directly to the styrene, which was later inserted into the container. Uniform liners of ‘Weaver’s White’ dogwood, an early bloomer in south Alabama, and ‘Barton’s White’ dogwood, a Tennessee selection that blooms about a week later, were planted into containers using amended pinebark:sand substrate and grown in full sun.

Data on shoot and root growth were collected in July and December 1993. Four plants receiving each treatment were repotted into their original treatment containers to determine residual effects of copper on root suppression and copper and styrene’s effects on growth of plants held for a second growing season. Six plants from each treatment were repotted into 7-gallon, non-treated black containers to determine treatment effects on new root growth outside the original rootball.

RESULTS
Treating containers with copper effectively reduced root circling at the substrate-container interface in both cultivars. However, plants grown in containers treated with copper had less root dry weight during the first year’s production and less growth in height during the second season compared to those grown the first season in non-copper-treated containers. Both cultivars repotted into 7-gallon containers had less trunk diameter growth and a lower percent surface root coverage when previously grown in containers treated with copper than in containers not treated with copper. Copper treatment also reduced root dry weights during the first year of production but had no effect on dry weights of newly generated roots outside the original rootball following repotting.

Dogwoods grown in styrene-lined containers had a greater percent surface root coverage and less root dieback during the first and second growing seasons in 3-gallon containers, in addition to greater growth in height during the second season. Plants originally grown in styrene-lined containers had more trunk diameter growth and a higher percent surface root coverage after being repotted into 7-gallon containers than those grown in non-lined containers. While substrate temperatures were not monitored in this study, the reduction in supraoptimal substrate temperatures by styrene lining, and the concomitant beneficial effects on plant growth from such a reduction have been previously reported. These results demonstrate potential beneficial effects of styrene-lining to dogwood, a species sensitive to supraoptimal substrate temperatures.