1997 Ornamentals Research Report - AUrora - Auburn University

2
Styrene Lining and Container
Size Affect Substrate
Temperature
ALABAMA AGRICULTURAL EXPERIMENT STATION
ALABAMA AGRICULTURAL EXPERIMENT STATION
WOODY LANDSCAPE PLANTS
THOMAS J. BRASS, GARY J. KEEVER, CHARLES H. GILLIAM,
AND D. JOSEPH EAKES
Root injury of woody plants may occur when rootzone
or substrate temperatures fall below 230F or rise above
1040F. During the summer, container medium temperatures in
the South have exceeded 1220F, while in the winter months
ambient air temperatures can fluctuate rapidly or drop below
80F for short periods of time. Thus, production of containergrown
plants requires cultural practices that regulate root-zone
temperatures during the summer growing season and during
overwintering to ensure adequate growth and plant survival.
One approach for controlling root-zone temperatures is
using styrene lining in individual pots. Potential benefits include
minimizing the cost and time of applying various production
practices to protect plants from extreme temperatures. The
objective of this study was to evaluate the influence of container
size and styrene lining on substrate temperature patterns in
various ambient air temperature regimes.
METHODS
In June 1993, one- and three-gallon containers were
filled with a pinebark:sand medium and placed on black
polyethylene in full sun. Prior to filling the containers with
substrate, 0.1-inch thick, white styrene lining was inserted into
half of the containers of each size. Substrate temperatures
were recorded hourly from June to August 0.08 inch from the
west sidewall. Five clear days with maximum ambient air
temperatures of approximately 90OF for a minimum of three
hours per day were selected to analyze container substrate
temperature patterns during the summer growing season.
Temperatures also were recorded from December 1993 to
February 1994. Data were analyzed under two conditions
when winter injury may occur: days with low ambient
temperature throughout the day, and days when wide
fluctuations in diurnal temperature occurred. Five days having
similar minimum and similar maximum ambient air
temperatures were used in each analysis.
RESULTS
SUMMER TEMPERATURES. Substrate temperatures
for al treatments showed a similar general response to changes
in ambient air temperature. Heating of the substrate in pots of all
treatments continued until well after midday when air
temperatures began to drop and reached a maximum about 5
p.m. before decreasing (Figure 1). Overall, containers lined with
styrene had lower substrate temperatures than unlined
containers under ambient conditions in this study. The larger
container provided greater buffering of temperatures at the rootzone/container
interface than the one-gallon container,
regardless of lining treatment, possibly due to the greater
substrate volume or thicker sidewalls.
FLUCTUATNG WINTER TEMPERATURES. On
winter days when wide fluctuations in ambient air temperature
occurred over a diurnal cycle, substrate temperature patterns
varied with both styrene treatment and container size (Figure 2).
When low ambient temperatures occurred, the substrate
temperature in styrene-lined containers exceeded that in unlined
ones. Also, styrene-lined containers of both sizes had less
temperature fluctuation during diurnal cycles compared to
unlined containers. These results are significant when one
considers that cold hardiness can be lost by brief exposure to
mild or high temperatures, rapid changes in temperature can
cause more injury than slow changes of a similar magnitude, and
roots are more subject to injury from rapid changes in
temperature than either stems or leaves.
WINTER TEMPERATURES WITHOUT RAPID
FLUCTUATION. When ambient air temperatures lacked wide
fluctuations during diurnal cycles, extreme variations in
substrate temperatures were not present, and container size had
no effect on substrate temperatures (Figure 3). Overall, when
low, nonfluctuating temperatures persisted, substrate in lined
containers was more insulated against temperature change than
that in unlined containers, and substrate temperatures in unlined
containers tended to be lower than ambient temperatures.
This study demonstrates potential modification of
container substrate temperatures by styrene lining, a material that
is lightweight, inexpensive, and easy to install. Styrene lining
inserted into black plastic containers insulated the substrate from
solar radiation striking the container sidewalls; benefits were
most pronounced in smaller-sized containers. Because substrate
temperatures in styrene-lined containers were kept from greatly
exceeding summer temperatures, higher quality plants and
greater growth may be achieved. Lining the container with
styrene also reduces heat loss from the container as ambient air
temperatures fall, and it insulates against wide fluctuations in
substrate temperatures that occur in winter months due to rapidly
changing ambient temperatures. Increasing minimum substrate
temperatures and reducing temperature fluctuation in styrenelined
containers during winter should decrease low-temperature
injury or death of plants. From an applied perspective, inserting
a styrene lining at potting may provide much, if not ail, of the
winter protection needed in the deep South.