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R. W. Henley*
University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC - Apopka Research Report, RH-91-20
During the past 10 years, many of Florida's foliage plant producers have adopted several new technologies to produce higher quality plants more efficiently. One such technology is root zone heating, frequently called bottom heating. This technology is employed during cool periods, which for tropicals can be when the production environment temperature drops below 65 to 68°F. Supplemental heat is provided under the plant to stimulate root development and subsequent top growth with minimal use of energy. Most bottom heating systems are adjusted to maintain a minimum temperature of 68 to 75°F. Slightly higher temperatures may be employed for special applications such as palm seed germination.
Many systems which heat a large amount of space around the plant before heating the root zone tend to be more heat-energy-wasteful. Some of the oldest root zone heating systems used for greenhouse pot plant production involved placement of a few relatively large caliper (1¼ to 2-inch) steam or hot water heating lines under raised benches to warm the surface supporting the potted plant material and the air around the plants. These systems are most effective for pot plant production when the bench tops are either partially open to convection currents of rising warm air or are excellent heat conductors. If a bench top does not fit one of these categories, the under-bench heating system is likely to be relatively inefficient.
An alternative approach to root zone heating is to place the heat directly in the bench or bed through the use of numerous small diameter distribution lines which carry warm or hot water. Much of the root zone heating technology adopted by Florida nurserymen falls in the latter category. Depending on the tubing or pipe diameter, heat transfer quality of the tube, water temperature, container size and pot spacing to be used, the growing area is equipped with a network of parallel heating lines ranging in diameter from ¼ to 1-inch or slightly larger, spaced 2 to 6 inches apart. These heating lines may be installed on the tops of raised benches or on or near the surface of ground beds. In a few cases, depending upon bench design, the heat distribution tubing may be attached directly under a bench top with good heat transfer characteristics, an option which leaves the upper surface with fewer obstructions to container placement.
Most root zone heating systems work rather well for closely spaced potted plants, flats of cuttings and various cavity trays used for plug plant production. However, there are a few situations which have evaded root zone heating technology as it is now applied. Large, widely spaced potted plants, usually set at floor level, and hanging baskets, normally arranged in elevated linear patterns are usually not equipped with root zone heating systems. An experiment was conducted at the Central Florida Research and Education Center - Apopka during the winter of 1990 and 1991 to address the challenge of more efficiently heating the root zone of widely spaced pots or other containers isolated from conventional root zone heating systems.
Materials and Methods
Four 8-inch, white hanging baskets (REB Plastics, Inc.) were modified by drilling two holes through the middle of the container side wall approximately 2 inches apart, one above the other, and inserting a length of ¼-inch EPDM tubing (Biotherm Engineering, Inc.) through the top hole, making two loops close to the side wall and drawing the lead end out through the bottom hole. The tubing inside the container was held close to the side wall with several fine thread ties which were looped around the EPDM tubing and through tiny holes drilled in the container. The input and return tubing leads outside the containers were 30 inches long. The modified containers served as water jackets to warm the root zone of experimental potted plants nested in them.
A warm water header was constructed from ½-inch, schedule 80 PVC pipe in a side-by- side loop configuration. The completed header assembly was approximately 10 feet long and tube connection fittings were installed approximately 2 feet apart along the input and return sections of the assembly. A gate valve inserted midway in the loop was partially closed to create a pressure differential between the input line and the return line. The 4 hanging baskets with installed tubing were attached to the header assembly positioned on a raised bench with a corrugated transite top. The header was then connected to a warm water supply submain. Warm water was supplied by a hydronic boiler (Raypak, Model 136).
A Honeywell controller with thermocouple was used to regulate the flow of warm water to the 4 containers equipped with water jackets. The controlling thermocouple was placed vertically midway between the container central axis and the sidewall and the controller was set to circulate warm water when the medium temperature dropped below 70°F.
Aglaonema 'Silver Queen' was selected for the experiment because its rooting response to supplemental heat in the root zone had been documented previously (1, 2). Three uniform freshly harvested cuttings 45-55 centimeters long and each weighing 95 to 100 grams (fresh weight) were stuck January 15, 1991 in each of eight 8-inch baskets (without hangers). The cuttings were positioned half way between the central axis and the sidewall of the container. Four of the eight planted containers were nested in the pots equipped with coiled tubing and the others were placed on the same bench without water jackets. The potting medium was a commercially prepared peatlite mix (Vergro Nursery Mix A). The cuttings were rooted and grown in a relatively humid greenhouse shaded to provide 1000-1300 foot-candles and maintained at 65°F minimum air temperature at bench level. The cuttings were watered manually only when the medium surface became slightly dry. No mist was used during propagation.
Three thermocouples were positioned midway down in the potting medium 1/4 inch from the sidewall, midway between the sidewall and the central axis and on the central axis, to monitor temperatures in the root zone. Two additional thermocouples were placed halfway between the sidewall and the central axis inch from the upper surface and inch from the bottom of the container. The copper/constantan type thermocouples were connected to a multi station electronic thermometer which was switched manually between stations.
February 26th and 27th were selected to make root zone and greenhouse temperature measurements because the temperature outdoors was expected to be cool. Measurements began 8 a.m. on February 26th and continued on 4-hour intervals through 8 a.m. the following morning. Five different measurements were made from each thermocouple position during each measurement period.
Results and Discussion
Figure 1 illustrates the changes over a 24-hour period which occurred in outdoor temperature, greenhouse temperature at crop level and the root zone temperatures of the heated pots and the non-heated pots. The root zone temperatures are means of 3 sensors positioned vertically midway between the center axis and the sidewall in the heated and non-heated pots. The values for each of the 3 sensors are means of 5 measurements made approximately 2 minutes apart every 4 hours during the 24-hour period. The root zone of heated pots was approximately 8 degrees or more warmer than the same zone in the non-heated pots between 8:00 p.m. February 26th and 8:00 a.m. February 27th. By 8:00 a.m. the root zone temperatures of both treatments began to rise from solar radiation which warmed the medium on a daily cycle. During the period when heating was required, the root zone temperature of heated containers was maintained within a desirable range of 68 to 76°F.
The temperature in the non-heated pots dropped 1 to 3 degrees below the greenhouse ambient temperature from approximately 2:00 a.m. February 27th through 8:00 a.m. the same day. Chilling of the medium below the ambient greenhouse temperature in non-heated pots is partially attributed to evaporative cooling at the pot surface during the dark hours when radiant energy from the sun does not contribute heat to the potting medium. As solar radiation continued to heat the greenhouse and the potting medium during the daylight hours, the temperature spread between the treatments narrowed and both rose above the range to which the controller would respond.
To better understand the temperature gradients which occur in a container system heated by a warm water jacket, two times were selected to display the root zone temperatures measured during a warm afternoon (4:00 p.m., February 26th) and a cool early morning (4:00 a.m., February 27th). The measurements taken at 4:00 p.m. revealed that there was little variation in temperature at the mid-level and bottom of the pots either treatment (Figure 2). It was apparent that most of the heat in the two container systems was being acquired from radiant energy from the sun because the container in the water jacket was not receiving supplemental heat. The potting medium temperature near the upper surface in both treatments was cooler during both measurement periods, primarily the result of water evaporation from the soil surface. The strongest vertical gradient was observed at 4:00 a.m. in the heated containers where the difference in root zone temperature from the middle of the pot to the top was 3 degrees while the difference in the non-heated pot was 1 degree. The temperature inch below the surface in non-heated pots was 64°F at 4:00 a.m., while the ambient greenhouse temperature ranged between 64 and 69°F, as the forced air heating system cycled on and off.
The horizontal temperature gradients were greatest during periods when heat was supplied to the water jacket equipped containers (Figure 2). Approximately inch inside the side wall the medium temperature fluctuated considerably as the warm water in the jacket cycled in response to the controls. The mean temperature at 4:00 a.m. in the outside inch, during a 10 to 15-minute period, was 94°F while the temperature near the center of the pot was 71.0°F, a gradient of approximately 23 degrees in 3 inches. The mean temperature from the 5 sensors taken at 4:00 a.m. were 65°F and 77°F for the non-heated and heated pots, respectively.
Growth responses of Aglaonema 'Silver Queen' to the treatments, without bottom heat and with the warm water jacket, are shown in Table 1. The values presented are means of measurements taken from 12 plants in each treatment on April 17, 1991, 90 days after sticking freshly harvested cuttings. The increase in number of new leaves was not significant at the time of measurement, but this would change within a month or two, as there were more than five times as many leaf bearing basal shoots developing on the bottom heated plants compared to those without heat. The larger number of swollen basal buds on the non-bottom heated plants indicate side shoots were forming, but at a rate slower than the bottom heated plants. The difference in plant growth weight reflects the additional tissue (leaves, shoots and roots) that developed under the two treatments. The growth responses of Aglaonema 'Silver Queen' to bottom heating is consistent with other reports (1, 2).
This study has demonstrated that widely spaced 8-inch hanging baskets or other containers can be supplied with root-zone heating using the water jacket technique described herein. Because of the need for an additional container equipped with warm water heating coils and, ideally, some insulation to minimize heat loss from the outside of the water jacket, it is unlikely the water jacket technique of root zone heating will be adopted commercially due to the high cost of the system. However, there may be some special applications of the water jacket container heating system which would justify its cost.
*professor, Environmental Horticulture, Central Florida Research and Education Center - Apopka, 2807 Binion Road, Apopka, FL 32703-8504.
2. Henley, R.W., Robert Mellen, Jr., William H. Bodnaruk, Jr. and Dewayne L. Ingram. 1982. Root-zone heating innovations in Florida. Combined Proc. International Plant Propagators' Soc. 33:583-589.
Figure 1. Root-zone temperatures in non-heated and warm-water-jacket-heated 8-inch containers in a greenhouse over a 24-hour period.
INFLUENCED BY A WARM WATER JACKET AROUND THE
TEMPERATURE (DECREES °F)
Click image for larger view.
Figure 2a&b. Potting medium temperature gradients in 8-inch hanging basket containers supplied with supplemental heat furnished with a water jacket and in containers without supplemental heat.
4:00 P.M., February 26, 1991
Outside temperature: 72°F
Greenhouse temperature: 78°F
Click image for larger view.
4:00 AM., February 27, 1991
Outside temperature: 48°F
Greenhouse temperature: 64-69°F
Click image for larger view.