Effect of Bottom Heating Regimes on Growth of Three Foliage Plants

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R.T. Poole and C.A. Conover*

University of Florida, IFAS
Central Florida Research and Education Center-Apopka
CFREC-Apopka Research Report, RH-90-22

Tropical foliage plants are among the most energy consuming of all greenhouse crops to produce. Even in Florida, with its sub-tropical climate, growers are looking for ways to reduce energy costs. One method of energy conservation is utilization of bottom heat. In greenhouses where bottom heat has been employed in the production of some flowering plants and woody ornamentals, air temperature was reduced without increasing crop turnover time because bottom heating elevated the root zone temperature. This increased the rate of uptake of nutrients from the potting medium and the resulting plant growth was comparable to plants grown in higher air temperatures without bottom heat. Very little research has been conducted on bottom heating regimes in foliage plant production or on the scheduling of such bottom heat regimes. The following study was designed to examine the growth of three species of foliage plants subjected to various air temperatures and bottom heat regimes.

A 4 x 4 factorial experiment was initiated 20 December 1989, using liners of Dieffenbachia maculata 'Camille' (Camille dieffenbachia), Epipremnum aureum (golden pothos) and Spathiphyllum 'Petite' ('Petite' or 'Bennett' Spathiphyllum) potted into 6-inch standard pots with Vergro Container Mix (Canadian sphagnum peat moss, coarse vermiculite and perlite, without superphosphate, Verlite Co., Tampa FL 33680). Pots were then placed in zoned forced-air chambers where plants received (1) control, no bottom heat, (2) 75°F minimum bottom heat from 6 a.m. to 6 p.m. daily, (3) 75°F minimum bottom heat from 6 p.m. to 6 a.m. nightly, or (4) 75°F constant minimum bottom heat. The forced-air chambers were located in glasshouses receiving a maximum 1500 ft-c light and plants were subjected to minimum air temperatures of 60, 65, 70 or 75°F. Plants received 5 grams/pot 19-6-12 Osmocote 3-month release rate fertilizer (Sierra Chemical Co., Milpitas, CA 95035) at time of placement in glasshouses and were irrigated 3 times per week.

Height or vine length was measured initially and monthly thereafter. The number of hours bottom heaters and air heaters operated to maintain the various minimum potting medium and air temperatures tested were recorded daily. Air temperature at bench level and potting medium temperature taken from pots growing golden pothos were recorded twice a week at 8:00 a.m. and 1:00 p.m. from (1), the control, receiving no bottom heating, for each of the four air temperatures tested. Plant grade, based on a scale of 1 = poor quality, unsalable, 3 = fair quality, salable and 5 = excellent quality plant material, was determined when experiment ended on 27 March 1990.

Increasing air temperature from 60°F to 75°F increased growth and plant grade of Camille dieffenbachia and golden pothos, but had no effect on Petite Spathiphyllum (Table 1). Neither the presence or absence of bottom heat, nor the hours regimes were implemented affected growth and quality of Petite Spathiphyllum. Best quality golden pothos were produced from the control group which received no bottom heating. Plant grades were slightly higher for Camille dieffenbachia when plants received 75°F minimum bottom heat for 12 hours during the day, from 6 a.m. until 6 p.m., but the increase in plant quality was too small to justify the extra expenditure. Results from this experiment show the bottom heating schedules used in the production of the three species tested; 1, had no affect on Petite Spathiphyllum, 2, 12 hours during the daytime slightly increased plant quality but was not cost effective for Camille dieffenbachia, and 3, actually proved detrimental to growth of golden pothos.

Hours air heaters were in operation almost doubled when minimum air temperature was maintained at 75°F as opposed to 70°F and about a 10 fold increase in hours of operation was needed to maintain 75°F instead of 60°F (Table 2). When minimum air temperature maintained was 70°F potting media temperatures recorded at 8:00 am. and 1:00 pm. were never more than 6F lower than air temperatures recorded at the same time (Table 3). Since there was only a slight increase in plant growth and grade for all species tested when air temperature was increased from 70°F to 75°F, we recommend utilizing the 70°F air temperature for these plants if energy conservation is a priority.

*Professor of Plant Physiology and Professor and Center Director, respectively. Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504.

Additional Reading

1. Bodnaruk, W. H., Jr., T. W. Mills, and D. L. Ingram. 1981. Response of four foliage plants to heated soil and reduced air temperatures. Proc. Fla. State Hort. Soc. 94:104-107.
2. Conover, C. A. and R. T. Poole. 1987. Growth of Dieffenbachia maculata 'Perfection' as affected by air and soil temperatures and fertilization. HortScience 22(5):893-895.
3. Jones, W. H. and R. McAvoy. 1982. Effect of root zone heating on growth of Poinsettia. J. Amer. Soc. Hort. Sci. 107:525-530.
4. Koller, D. C., L. K. Hiller, and R. W. Van Denburgh. 1980. A forced air system for controlling soil temperatures in plastic pots. HortScience 15:189-190.
5. Wang, Y. T. and A. N. Roberts. 1983. Influence of air and soil temperatures on the growth and development of Lilium longiflorum Thumb. during different growth phases. J. Amer. Soc. Hort. Sci. 108.810-815.
6. White, J. W. and J. A. Biernbaum. 1984. Effects of root-zone heating on growth and flowering of calceolaria. HortScience 19:289-290.

1. Table 1. Growth and plant grade of three foliage plants grown from 20 December 1989 through 27 March 1990 utilizing various air temperatures and bottom heating regimes.
   

1. ^Zplants were graded on a scale of 1 = poor quality, unsalable, 3 = average quality, salable, and 5 = excellent quality.
   ^Yns, **, Nonsignificant or significant at P = 0.01.
   ^XMean separation in columns by Duncan's multiple range test, 5%.
   

1. Table 2. Number of hours air heaters were in operation to maintain greenhouses at air temperatures tested from 1 January through 26 March 1989
   

1. Table 3. Air and potting media temperatures recorded at 8:00 am and 1:00 pm twice a week in greenhouses where test plants were grown from 4 January through 21 March 1989.
   

1. ^ZAir temperature data obtained from thermometers placed at bench level.
   ^YMedia temperature readings obtained from thermometers placed in control media, receiving no supplemental heat, in pots containing Epipremnum aureum.