原著論文
Root-zone Heating in Winter Using N.RECS is Effective for Promoting Growth and Flowering of Pot Flowers and Reducing Energy Consumption for Heating
2025 年 94 巻 1 号 p. 92-99
詳細
2025 年 94 巻 1 号 p. 92-99
Winter heating energy saving is an important issue, not only for reducing production costs, but also for reducing CO2 emissions during cultivation. This study investigated the effect of root-zone heating using a new root-zone environmental control system (N.RECS) in winter on the growth and flowering of pot flowers and the reduction in heating costs. Bulbous begonia, geraniums, dwarf gerberas, and New Guinea impatiens, which require relatively high temperatures, were used in the present study. This experiment consisted of two plots: a conventional plot (control) with a minimum air temperature of 16°C and no root-zone heating, and a root-zone (RZ) heating plot with a minimum air temperature of 12°C and root-zone heating of 24°C. The results showed that RZ heating significantly promoted the growth and flowering of all tested pot flowers. Heating costs for bulbous begonia, geranium, and dwarf gerbera were reduced by 28–45% in the RZ heating plot compared to that of the control. In New Guinea impatiens, heating costs were not reduced by RZ heating because this experiment started late, in mid-March, and heating costs in the control were low. CO2 emissions were roughly proportional to energy costs. In conclusion, root-zone heating with the N.RECS enabled the early production of high-quality plants in winter cultivation of pot flowers with relatively high temperature requirements and could contribute significantly to the reduction of CO2 emissions during the cultivation period by reducing heating costs.
The most common heating technique used in protected horticulture is air heating by burning fossil fuels, which accounts for approximately 89% of the total heating area (The Ministry of Agriculture, Forestry, and Fisheries, 2021). The heating cost in winter accounts for a large proportion of the total production cost, and the price fluctuations in fossil fuels such as fuel oil are a major factor in the instability of producer management. Fuel costs account for a high proportion of horticulture management costs, with the price of A fuel oil rising by approximately 40 yen·L−1 between 2020 and 2023, putting significant pressure on the profitability of horticulture producers. Although the introduction of equipment with relatively low operating costs, such as heat pumps, is progressing, further energy savings must be promoted. Electrical heat pumps have rapidly gained popularity in recent years because of their high COP and excellent heating efficiency; however, their installed area is still small at only 5.6% of the total heating area in Japan (The Ministry of Agriculture, Forestry, and Fisheries, 2021). Heating with a heat pump can be used in combination with a hot-air heating system to reduce fossil fuel consumption.
The Ministry of Agriculture, Forestry, and Fisheries (MAFF) developed the ‘Strategy for Sustainable Food Systems MIDORI’ in 2021, with the aim of creating a food system based on sustainable agriculture. The MIDORI key performance indicator target for 2030 is to increase the penetration of hybrid heating, which combines oil-fired and heat-pump heating, by 50% in order to reduce greenhouse gases such as CO2 (The Ministry of Agriculture, Forestry, and Fisheries, 2022). However, air temperature control in greenhouses also controls the temperature of spaces not required for plant growth; therefore, the effect on plants is relatively low compared to the amount of energy input, and the energy-saving effect and reduction in CO2 emissions are limited.
Many energy-saving temperature-control technologies have been developed for the local heating of horticultural crops. Kawasaki et al. (2010, 2011) installed hot-air ducts near the apex of the stem and flower clusters of tomatoes to provide local heating. As a result, fruit quality was improved and fuel consumption was reduced compared to the conventional control. Moriyama et al. (2011) developed a system that used hot-air ducts to heat the basal part of eggplants as an energy-saving technique with low installation costs. Heating costs also put pressure on rose growers’ operations; therefore, a heating technique for the basal part of roses was developed to reduce heating costs (Kanagawa Agricultural Technology Center, 2012). This was done by installing two pipes at the basal parts of shoot-bending roses and circulating 30°C water through the pipes to heat the basal parts. The combination of lowering the air temperature in the greenhouse to 15°C and heating the basal part of the roses improved the productivity of cut flowers and reduced the heat consumption by approximately 20%. In gerbera cut flower production, root-zone heating increased the number of cut flowers and increased cut flower length (Goldsberry and Lang, 1987; Martinez-García et al., 1989). Furthermore, crown heating of gerbera increased the number of cut flowers and reduced heating costs (Inamoto et al., 2019).
Most local heating technologies have been adapted for soil cultivation and hydroponics and there are few local heating technologies for potted plants. Muramatsu et al. (2017) conducted root-zone (RZ) heating cultivation of impatiens using a root-zone environmental control system (RECS) and found that RZ heating promoted growth and flowering in addition to being energy-saving. RECS is a system in which porous pots with plants are placed in a pool of circulating warm water to heat the RZ, which is impractical. Therefore, Kubota (2020) developed a new root-zone environmental control system (N.RECS) compatible with the plastic pots commonly used in potted plant production and aimed at the practical application of RZ temperature control. The root-zone temperature (RZT) can be maintained at approximately 25°C in the N.RECS heating mode, even when the air temperature drops to approximately 5°C. The RZT can be cooled to approximately 23°C in the cooling mode, even when the air temperature reaches approximately 35°C. The effect of RZ heating on the growth and flowering of dwarf dahlia in winter was investigated using this system (Kubota et al., 2018). The number of leaves, leaf area, number of branches, and dry matter weight were increased by heating the RZT to 24°C, and the energy consumption was reduced by more than 34% in monetary terms compared to the control.
In this study, we aimed to expand the application of RZ heating using N.RECS and investigate the effects of RZ heating on the growth and flowering of four species of pot flowers: bulbous begonia, geranium, dwarf gerbera, and New Guinea impatiens.
Bulbous begonia (Begonia × tuberhybrida Voss ‘Fortune Mix’), geranium (Pelargonium × hortorum L. H. Bailey ‘Apple 2000 Deep Scarlet’), dwarf gerbera (Gerbera jamsonii Bol. ex Adlum ‘Revolution Bicolor Rose Shade’), and New Guinea impatiens (Impatiens hawkeri W. Bull ‘Divine Scarlet Red’) were used in this study. Seeds of geraniums, dwarf gerberas, and New Guinea impatiens were purchased from Sakata Seed Corporation (Yokohama, Japan), sown in 200-cell plastic trays on January 12, 2017 (Table 1), and maintained in a greenhouse set at a minimum temperature of 16°C. Bulbous begonia seedlings with 4–5 leaves were purchased from commercial growers (Hakusan International Co., Ltd., Kumagaya, Japan). A 1:1 mixture of Akadama soil and Metro-Mix 360 (Hyponex Japan Co., Ltd., Osaka, Japan) was used as the substrate for transplanting, and all experimental plants were transplanted into 10.5 cm plastic pots on the dates indicated in Table 1 using substrates with 3 g of slow-release fertilizer (MAGAMP K, N-P2O5-K2O = 6-40-6; Hyponex Japan Co., Ltd.).
Cultivation, treatments and sampling schedules of the experiment.
This experiment consisted of two plots: a conventional plot (control) with a minimum air temperature of 16°C and no RZ heating, and an RZ heating plot with a minimum air temperature of 12°C and a RZ heating temperature of 24°C. Five replicates were set up in each plot, and eight pots of each species were grown in each replicate. All plants were grown under natural daylength.
The experiment was conducted in a Venlo-type plastic greenhouse (16 m wide, 24 m long, 4.5 m high; north-south oriented) at the College of Bioresource Sciences, Nihon University (Fujisawa, Kanagawa, Japan). The greenhouse was divided into two sections (8 m wide and 24 m long) using double plastic curtains in the north-south direction at the center, and the east and west sections were assigned to control and RZ heating, respectively. The minimum air temperature was controlled using a kerosene-burning heater (KA-125E; Nepon Inc., Tokyo, Japan) and a heat-pump air conditioner (PA-P180AG3H; E’s Inc., Tokyo, Japan) in each plot. The control plants were cultivated on metal culture benches. A large-scale N.RECS (Kubota et al., 2018) was installed on the culture benches (Fig. 1), and RZ heating was performed using the N.RECS. Treatments for bulbous begonias, geraniums, dwarf gerberas, and New Guinea impatiens were conducted from February 18 to April 29, February 18 to March 30, February 24 to May 16, and March 18 to May 22, respectively (Table 1). The geraniums grown under RZ heating were moved to the control plot on March 31, and all plants were grown under the same conditions until May 9.
Structure of the New Root-zone Environmental Control System (N.RECS) (Muramatsu and Kubota, 2021). EPS, Expanded polystyrene form; PEP, Cross-linked polyethylene pipe; S, Pt100 temperature sensor; V, Electric three-way valve.
The N.RECS comprised four parts: a heat source unit, heat exchange devices, an RZ temperature controller, and insulated pot trays. An air-source heat-pump cold/hot water system (VEH-712HCC-K; Mitsubishi Electric Co., Tokyo, Japan) was used as the heat source unit. The capacity of this heat pump was 11.8 kW in the heating mode, and the heating power consumption was 2.95 kW.
The heat-exchange devices were structured as follows (Muramatsu and Kubota, 2021; Fig. 1). Eight expanded polystyrene (EPS) cultivation tanks for hydroponics (129 cm long, 105 cm wide, and 16 cm high, EK Tank Type I; Kaneko Seeds Co., Ltd., Maebashi, Japan) were placed on a metal cultivation bench. Styrofoam IB (DuPont Styro Co., Tokyo, Japan) 2 cm thick was laid inside the tanks in the form of a screen, and 28 heat exchange panels (60 cm × 60 cm, HM-600-7A12; Seamless Floor Heating Co., Ltd., Yokohama, Japan) were used for residential floor heating. Cross-linked polyethylene pipes (φ6.8 mm) were inserted into the grooves of these heat exchange panels and fixed with aluminum tape. These pipes were connected to the heat source unit, and 40°C hot water was supplied from the heat source unit to the heat exchange panel. A plastic sheet (Kaneko Seeds Co., Ltd.), capillary mat (LN250GRO; Unitika Ltd., Osaka, Japan), and root prevention sheet (20507BKD; Unitika Ltd.) were placed on top of the heat exchange panel from the bottom.
An electric three-way valve was installed in the pipe between the heat pump unit and heat exchange devices. The RZT of each pot was measured using a Pt100 temperature sensor (SPT-04; Tateyama Kagaku Co., Ltd., Toyama, Japan) placed 3 cm from the bottom of the pot. The hot water supplied to the heat exchange panel was regulated by opening and closing an electric three-way valve controlled using a temperature controller (WFC-MV-6; Akitsu Measurement Co., Ltd., Tokyo, Japan) to achieve a set value.
The insulated pot trays were made of EPS and were 60 cm long, 30 cm wide, and 10 cm high. Eight holes with the same shape as a 10.5 cm pot were drilled through the top and bottom of the pot tray (Fig. 2). Aluminum tape (0.2 mm thickness) was attached to the bottom of the pot tray and to the lower one-third of the pot tray from the inside of the holes to increase thermal conductivity. The pot trays were placed on heat exchange panels, and 10.5 cm pots planted with experimental plants were placed in the holes of the trays for RZ heating.
Structure of insulated pot tray.
Four uniform plants were selected per replicate, and the selected plants of bulbous begonia, geranium, dwarf gerbera, and New Guinea impatiens were collected on April 29, May 9, May 16, and May 22, respectively (Table 1). The collected plants were divided into leaves, stems, flower buds, and flowers, and the fresh weights of these parts were measured. Leaf area was measured using a leaf area meter (LI-3100; LI-COR, Inc., Lincoln, NE, USA). Dry matter weight was measured after each part was air-dried in a dry oven (WFO-1001SD; Tokyo Rikakikai Co., Ltd., Tokyo, Japan) at 80°C for two days. Analysis of variance (ANOVA) of the experimental data was performed using the aov function in R 4.0.5 on a posit cloud (https://posit.cloud).
Estimation of energy costs and CO2 emissionsThe amounts of electricity and kerosene used for air heating and root-zone heating are shown in Table 3 as values per experimental greenhouse (192 m2). The heating costs were calculated using a kerosene price of 90 yen/L and an electricity price of 20 yen/kWh. CO2 emissions were calculated by using a CO2 emission factor of 2.5 kg CO2·L−1 for kerosene (Ministry of the Environment, 2023) and a CO2 emission factor of 0.45 kg CO2·kWh−1 for electricity supplied by TEPCO Energy Partner, Inc. (Ministry of the Environment, 2018).
A representative example of the daily changes in air temperature and RZT from March 3 to March 5, 2017, is shown in Figure 3. The air temperature at night was 15–16°C in the control and approximately 12°C in the RZ heating plot, while the lowest temperature was almost the same as the set temperature. Day air temperatures rose to 25–30°C in both plots. The daily change control RZT was almost the same as the change in air temperature, dropping to approximately 15°C at night and rising to approximately 26°C during the day. However, the RZT for RZ heating was approximately 23°C at night and approximately 25°C during the day, which was almost the same as the set temperature.
Daily changes in air temperature (left) and root-zone temperature (right) from March 3 to March 5, 2017.
The changes in the daily mean air temperature and daily mean RZT from February 18 to May 16, 2017, are shown in Figure 4. The daily mean air temperature of the control remained at approximately 19°C until mid-March, while that of the RZ heating plot remained at approximately 17°C until mid-March. From mid-March to mid-April, the daily mean air temperature in the control was above 20°C on many days, and that during RZ heating was approximately 1°C lower than that in the control. After mid-April, the daily mean air temperature in both plots gradually increased, and the difference between the two plots almost disappeared. The daily mean RZT of the control fluctuated around 18°C until late March, and then increased as the air temperature increased. The RZ heating temperature remained almost constant at around 24°C during the experimental period.
Changes in daily mean air temperature (left) and daily mean root-zone temperature (right) from February 18 to May 16, 2017.
The effects of RZ heating on the growth of the four species of pot flowers at the end of the experiment are shown in Table 2. In bulbous begonia, geranium, and New Guinea impatiens, the number of leaves, leaf area, number of branches, and dry matter weight of above-ground parts increased with RZ heating. Dwarf gerbera showed no significant difference in the number of leaves between the treatments, whereas leaf area and dry matter weight of the above-ground parts increased with RZ heating. The flowering percentage did not differ among treatments in New Guinea impatiens, but was higher in the other three species with RZ heating. The number of flowers of the four species increased with RZ heating. The number of flower buds in bulbous begonia, geranium, and New Guinea impatiens was higher in the RZ heating treatment than in the control, whereas that of dwarf gerbera was greater in the control.
Effects of root-zone heating on growth and flowering of four species of pot flowers.
The changes in the flowering percentages of the four species of pot flowers are shown in Figure 5. The control plants of bulbous begonia, geranium, and dwarf gerbera hardly flowered during the experimental period, whereas the flowering period was significantly earlier with RZ heating. Although flowering was observed in New Guinea impatiens in the control, the flowering rate was low, and flowering was significantly accelerated by RZ heating.
Changes in flowering percentage of four species of pot flowers grown at different root-zone temperatures. Bars indicate standard error.
The electricity consumption of the heat-pump, kerosene consumption of the kerosene heaters, energy costs, cost reduction rates, CO2 emissions and reduction rate of CO2 emissions during the experiment are listed in Table 3. Due to the different growing seasons and durations of the four species of pot flowers, the amount of energy consumed by each species was also different. Electricity consumption, including RZ heating, for bulbous begonia, geranium, and dwarf gerbera was lower with RZ heating. However, in New Guinea impatiens, the total energy consumption was higher with RZ heating because the electricity consumption in the control group was lower with RZ heating. No kerosene was consumed at all in RZ heating, while 13.6–65.7 L were used in the control. Energy costs were lower with RZ heating than in the control and 28–45% lower with RZ heating for bulbous begonia, geranium, and dwarf begonia. CO2 emissions and energy costs were roughly proportional, and the rate of reduction in CO2 emissions was higher when the rate of cost saving was higher.
Energy consumption, heating costs, and CO2 emissions during the experiment, and percentage of cost saving and reduction in CO2 emissions by root-zone heating.
In this study, Begonia × tuberhybrida Voss (bulbous begonia), Pelargonium × hortorum L. H. Bailey (geranium), Gerbera jamsonii Bol. ex Adlum (dwarf gerbera) and Impatiens hawkeri W. Bull (New Guinea impatiens) were grown using a conventional method (control) with a minimum air temperature of 16°C and root-zone heating (RZ heating) in which the minimum air temperature was reduced to 12°C and the root-zone heated to 24°C to expand the application of RZ heating using N.RECS.
The results showed that flowering of the four flower species tested were significantly accelerated by RZ heating (Fig. 5). In addition, Vogelezang (1988, 1990, 1991, 1992) found that RZ heating promoted the growth and flowering of Saintpaulia, Begonia × hiemalis, Ficus benjamin, Schefflera arboricola, Spathiphyllum, and Guzmania. Therefore, the results suggest that RZ heating for pot flowers is effective for promoting growth and flowering.
Flowering of bulbous begonia is accelerated in long-day conditions and dormancy is induced in short-day conditions, but even under short-day conditions, flower bud development is more vigorous than under long-day conditions when the minimum air temperature is 25°C (Koizumi, 1979). The number of bulbous begonia flowers increased linearly with increasing minimum air temperature from 12 to 24°C under short day conditions, and the number of flowers increased more under 24°C than that under long day conditions (Djurhuus, 1985). Therefore, bulbous begonias under short day conditions are characterized by earlier flowering when air temperatures rise. In this experiment, bulbous begonias were grown under natural daylength from winter to spring. The daylength at Fujisawa, including civil twilight, during this season was 12–13.5 h, which was near the critical daylength of 13 h for bulbous begonia (Koizumi, 1974). Furthermore, according to the growing guide for the bulbous begonia ‘Fortune’ used in the experiment, 14 hours of daylength is required to maintain continuous growth (Sakata Ornamentals Europe, 2020), and the daylength during the experiment was shorter than that required by ‘Fortune’. The flowering was accelerated in RZ heating, in which the minimum air temperature was lower than that of the control (Figs. 3 and 5). Therefore, not only air temperature, but also RZT, may promote flowering near the critical daylength. The relationship between RZT and daylength in bulb begonia flowering should be studied in more detail in the future.
For bulbous begonia, geranium, and dwarf gerbera, the heating costs during the experiment were 28–45% lower with the RZ heating than in the control (Table 3). The experiment was continued from late April to mid-May because flowering was not fully observed in the control. However, because flowering occurred earlier with RZ heating, the electricity consumption for RZ heating could be further reduced when it was shut down at the start of flowering. No energy savings from RZ heating were observed in New Guinea impatiens, mainly because the experiment started later than in the other flowering plants, with higher ambient temperatures in mid-March resulting in less heating in the control. Therefore, in New Guinea impatiens, as with other pot flowers, energy savings can be achieved with RZ heating during the cold winter season. Olberg and Lopez (2017) examined the effects of RZ heating on the growth and flowering of petunias under low air temperatures. They set up a conventional method in which the root zone is not heated with a mean air temperature of 20°C and a root zone heated at 21–27°C with a mean air temperature of 15°C. Flowering of petunias was enhanced by RZ heating at 24–27°C, depending on the variety. In contrast, the effect of RZ heating on shoot dry matter weight was relatively small, and its effect on vegetative growth was limited. This result contrasts with the strong effects of RZ heating on both dry matter production and flowering in the four species of pot flowers in this experiment (Table 2).
The dry matter weight of the above-ground parts increased with RZ heating, indicating increased photosynthesis in the whole plant during the experiment. The amount of photosynthesis in a whole plant can be calculated as the product of the photosynthetic rate of a single leaf and the leaf area. In rice, low RZT has been shown to have no effect on the photosynthetic rate of a single leaf, but it did reduce the single leaf area, resulting in a reduction in whole plant dry matter production (Nagasuga et al., 2011). Summer RZ cooling has been shown to promote the growth and flowering of cyclamen, but also to reduce the single leaf area (Muramatsu et al., 2015). In this experiment, the total leaf area increased, as did the number of branches and leaves. The single leaf area was calculated by dividing the total leaf area by the number of leaves, and no differences were observed among treatments: 24–26 cm2/leaf for bulbous begonia, 11–13 cm2/leaf for geranium, 22–25 cm2/leaf for dwarf gerbera, and 4–5 cm2/leaf for New Guinea impatiens (Table 2). Therefore, the effect of RZT on the single leaf area is expected to vary according to differences in plant species and the relationship between RZT and air temperature. Although root growth and nutrient absorption were not investigated in this study, the effects of RZT on root growth, nutrient absorption, and photosynthesis should be investigated in the future, as RZT may influence aboveground growth due to these differences.
Winter RZ heating promoted the growth and flowering of verbena (Kubota et al., 2013), impatiens (Muramatsu et al., 2017), and dwarf dahlia (Kubota et al., 2018). It has also been shown that RZ cooling in summer promoted the flowering of cyclamen (Muramatsu et al., 2015) and the growth of fuchsia (Kubota et al., 2018). Current temperature control systems in protected horticulture mostly only control the air temperature in the greenhouse, but the positive effect of RZ temperature control was evident in a total of nine species of pot flowers, including the four species of pot flowers included in this study. Therefore, the incorporation of RZ temperature control and air temperature enables both energy-saving production in winter and protection against high temperatures in summer, at least in pot flower production.
