Abstract
Indoor greening is becoming popular because it provides many benefits for people. However, plant selection for indoor greening is limited to shade-tolerant tropical plants internationally, and little research has been performed to expand the use of native herb plant species. The aim of this research was to study growth characteristics of Japanese native herbs under common light intensity regimes in office buildings. Eight species of Japanese native herbs (Acorus gramineus, Cameilla sinensis, Farfugium japonicum, Gynostemma pentaphyllum, Perilla frutescens var. crispa f. viridis, Petasites japonicus, Sasa veitchii, and Saxifraga stolonifera) were examined in an open growth chamber with light-emitting diode light tape under three light intensities [photosynthetic photon flux densities of 3 µmol·m−2·s−1 (250 lx), 6 µmol·m−2·s−1 (500 lx), and 12 µmol·m−2·s−1 (750 lx)] from 8 AM to 10 PM every day from Jun 2021 to Oct 2021 in an office. Farfugium japonicum and Saxifraga stolonifera were as shade-tolerant as typical tropical plants and grew well under all light intensities. Sasa veitchii performed well under high and medium light intensity. The other plant species require supplemental lightning to achieve sufficient growth indoors.
Indoor greening has been receiving attention because it provides many benefits such as the improvement of task performance and attention (Raanaas et al. 2011; Shibata and Suzuki 2004) and general increases in well-being as a result of stress reduction (Rhee et al. 2023) and cleaner air (Rabito et al. 2024). Humans have a close relationship with nature; therefore, integrating nature into indoor space could effectively increase their engagement with nature, which may benefit their health and comfort. Because people spend 80% to 90% of their time indoors, the indoor environment is very important to their health (Deng and Deng 2018). Currently, tropical foliage plants, such as Ficus benjamina and Pachira aquatica, are widely used for indoor greening because they are shade-tolerant and can adapt well to the indoor environment (Li et al. 2009). They are native to tropical rain forests, where light levels are low. In their habitat, they experience high humidity, heavy rainfall, and high temperatures (on average, 20 to 25 °C). The growth performance of tropical foliage plants under low light intensity has been studied (Chen et al. 2005; Kubatsch et al. 2007). However, little research has been performed to expand the use of species native to more temperate environments.
Recently, the use of native plant species to perform greening of urban areas has been recommended. Performing greening using native plants can provide several benefits for nature (e.g., biodiversity, creating habitats) and humans (e.g., environmental education, fulfilling ethical responsibilities) (Dearborn and Kark 2010). Indoor greening can connect people with native plant species; however, it is not clear which native species can survive in the indoor environment. Some plants are good candidates for indoor greening because their habitats are shady. The most serious problem for indoor plants is low light intensity. Two common symptoms of low light for indoor plants are leaf abscission and accelerated internode elongation (Li et al. 2009). Very little research of the use of native species indoors has been performed; therefore, a limited set of indoor plant species has been used repeatedly. Currently, lighting requirements for common ornamental plants tend to be loosely categorized as full-sun, semi-shade, and full-shade plants (Boo et al. 2014). Such classifications are useful for outdoor landscape practitioners and the average layperson; however, they do not provide sufficient information for indoor plant lighting design (Tan et al. 2017). Growth and chlorophyll fluorescence reactions of native fern species in Korea have been studied (Jang et al. 2021). Another study examined the suitability of ornamental native plants for indoor greening in Japan (Akihiro et al. 1997). These studies helped to establish that some plant species are able to grow under low lighting conditions.
This study investigated the growth characteristics of Japanese native herbs in an indoor environment. Herbs can be defined as plants with specific uses (flavoring, fragrance, medicine, cosmetics, and insecticides) by humans (Jones 1996). Japanese native plants are defined as plants that has been grown since before the Edo period (1603–1868), when Japan’s trade with the rest of the world markedly increased and there was a rapid increase in introduced alien species (Mizutani and Goka 2010). Some Japanese native plants grow in shady areas and may be suitable for indoor environments. Therefore, this study investigated which Japanese herbs can be suitably grown under common lightning in office buildings.
Materials and Methods
Plant selection and cultivation.
We tested Japanese herbs that fit the following criteria: usually grown in shady areas in their natural habitat; unique textures and structures that may be attractive to indoor greening designers; usually grown in nurseries and easy to obtain; and familiar to others. We chose the following eight plant species: Acorus gramineus, Cameilla sinensis, Farfugium japonicum, Gynostemma pentaphyllum, Perilla frutescens var. crispa f. viridis, Petasites japonicus, Sasa veitchii, and Saxifraga stolonifera (Table 1). These plants were obtained from parkERs (Minato-ku, Tokyo, Japan). Plants with natural soil around the roots were placed in 9-cm-diameter slit pots (Kaneya Co. Ltd., Aichi, Japan). This experiment was conducted in an office (27 m2) in Chiba City, Japan (35°N, 140°E) between 11 Jun and 29 Oct 2021. The slit pots were filled with substrate composed of porous clay material (SERAMIS, Germany) and pearlite with a ratio of 1:1 by volume. Three plant pots were randomized by species and placed in one tray (23.8 cm × 34.0 cm × 7.3 cm). Three replications were chosen because of space limitations. Water was supplied to a height of 3 cm above the bottom every week and so that the plants could absorb water from the bottom. We applied pesticides (Sumitomo chemical garden with Crossianidin and Fenpropatrin as the main components; Benica fine spray, Sumitomo Chemical, Tokyo, Japan) to all plants once per week beginning from 6 weeks after the start of the experiment.
Characteristics of planted species used during this study (Kitamura 1961, 1962, 1964; Kitamura et al. 1981; Tsuyuzaki 2013).
Plant growth environment.
The plant growth environment was created so that it was similar to that of typical office spaces. The plants were grown in two open growth chambers with light-emitting diode (LED) light tape at different light level shelf (76 cm × 36 cm × 156 cm) (IRIS OHYAMA, Miyagi, Japan) (Figs. 1 and 2). With LEDs, light levels comparable to incandescent lamps can be provided with significantly lower radiant heat output, and they have been shown to be adequate for ensuring plant growth (Tan et al. 2017). In this study, cool color LEDs were used. In the light spectrum of the LED, there is a sharp peak in the blue wavelength range of approximately 460 to 470 nm and a moderate peak in the yellow wavelength range of 570 to 580 nm. The light intensities were chosen according to the general rules of recommended lighting levels provided by Japanese Industrial Standards (Japanese Industrial Standards Committee 2010). In offices, 750 lx is recommended for design and drawing workspaces, 200 lx is recommended for office lounges, 300 lx is recommended for lift lobbies, and 500 lx is recommended for printing rooms. Therefore, we chose 250 lx, 500 lx, and 750 lx, which are equivalent to the following photosynthetic photon flux densities (PPFDs): PPFD of 3 µmol·m−2·s−1 for 250 lx; PPFD of 6 µmol·m−2·s−1 for 500 lx; and PPFD of 12 µmol·m−2·s−1 for 750 lx. The PPFD represents the number of photons with a wavelength of 400 to 700 nm passing through a unit area per unit time and is strongly correlated with photosynthesis. The light utilization efficiency of photosynthesis is relatively high, near wavelengths of 400 to 680 nm, and those at wavelengths below 400 nm and above 700 nm are significantly low (Sugano et al. 2021). Previous studies have summarized the light saturation and compensation points for tropical foliage plants and crops. Light compensation points for plants varied; those for tropical plants were 3.5 to 10.1 µmol·m−2·s−1 and those for crops were 17.0 to 34.0 µmol·m−2·s−1. Light levels at each shelf were assessed at the base of each of the three plants and adjusted so that the average was the lux value set for that particular treatment. The light of the growth chamber was turned on for 14 h, from 8 AM to 10 PM, every day during the experiment. The room light was always turned off and all blinds were closed so that the plants only received the light of the growth chamber. The air conditioner was set at 27 °C throughout the experiment. The changes in temperature and humidity at the experimental site over time are shown in the Supplemental Material.
Measurement indicators and methods.
All plants were measured once per week from 29 Jun to 16 Jul 2021, for a total of six times, and then once every 2 weeks from 30 Jul to 29 Oct 2021, for a total of 14 times. At the time of the first measurement, it was ensured that the starting parameters of each sample were consistent. The parameters measured were as follows: plant natural height (vertically measured from the bottom to top of the plant); total leaf number (including small leaves); and appearance rating (1 = severely stressed and completely dried out; 2 = stressed, with less than 20% of the leaves retaining green pigmentation; 3 = mildly stressed, with 60% of the leaves retaining green pigmentation; 4 = minor stress, with approximately 80% of the leaves appearing healthy; and 5 = unstressed, with all leaves appearing healthy) (Monterusso et al. 2005). Five leaves of each plant were chosen and measured according to soil plant analysis development (SPAD) values using a SPAD-502Plus meter. The SPAD-502Plus meter measures the transmittance of red (650 nm) and infrared (940 nm) radiation through the leaf and calculates a relative SPAD value that should correspond to the amount of chlorophyll present in the sample leaf (Konica Minolta 2009). At the end of the experiment, all plants were harvested and dried in a drying oven at 70 °C for 3 d; thereafter, the shoot dry weight was measured.
Statistical analysis.
Each species was analyzed separately. For dry shoot biomass, a one-way analysis of variance was applied to detect the effects of light intensity on each plant species. Post hoc comparisons were completed using Tukey’s honestly significant difference method. For all response variables, transformations were applied before the analysis if raw data were significantly not normal. Response variables were analyzed using a linear mixed model with light intensity as a fixed factor and block as a random factor. Post hoc comparisons were completed using Tukey’s honestly significant difference method. All statistical analyses were conducted using R 4.3.2. (R Core Team 2024).
Results
Dry shoot weight at the end of experiment
The mean dry shoot weight of each plant species at the end of experiment is shown in Fig. 3. The shoot weight differed greatly according to the plant species. G. pentaphyllum and S. veitchii had the most biomass. A. gramineus had an intermediate level of biomass. C. sinensis, G. pentaphyllum, P. frutescens var. crispa f. viridis, and S. stolonifera had relatively low levels of biomass. P. japonicus performed the least well, with close to zero shoot biomass by the end of the experiment. C. sinensis and P. frutescens var. crispa showed significant differences in light intensity; however, there were no significant differences between the light intensity values of the other species.
Mean dry shoot weight of each plant species (n = 3). Error bars represent standard error. Means with the same letter do not differ significantly from each other.
Citation: HortScience 59, 9; 10.21273/HORTSCI17869-24
Growth characteristics over time
The growth of each plant species in the different light intensity values over time is shown in Fig. 4.
Growth of plant species under different light intensities over time (n = 3). Error bars represent standard error. Means with the same letter do not differ significantly from each other. (A) Acorus gramineus. (B) Cameilla sinensis. (C) Farfugium japonicum. (D) Gynostemma pentaphyllum. (E) Perilla frutescens var. crispa f. viridis. (F) Petasites japonicus. (G) Sasa veitchii. (H) Saxifraga stolonifera.
Citation: HortScience 59, 9; 10.21273/HORTSCI17869-24
(A) Acorus gramineus.
A. gramineus tended to be taller, and elongation was observed with 3 µmol·m−2·s−1; however, the difference was not always significant. The SPAD values were generally lower during the second half of the experiment but showed no consistent pattern with light treatment. Appearance was highest at 12 µmol·m−2·s−1 during the last month of the experiment, and the number of leaves was generally highest in the treatment as well.
(B) Cameilla sinensis.
C. sinensis tended to perform best at 6 µmol·m−2·s−1; however, the differences were less obvious for height than for the other variables. The number of leaves dropped strongly by day 50 with all treatments but was still consistently highest at 6 µmol·m−2·s−1 until the end of the experiment.
(C) Farfugium japonicum.
Although not statistically significant, plants exposed to 3 µmol·m−2·s−1 tended to be taller by the last month of the experiment. The number of leaves increased over time with 12 µmol·m−2·s−1; however, given the variability within a treatment, the number was still equivalent to that of the other light treatments. F. japonicum had an increased number of leaves, and a high appearance score was maintained. The SPAD values differentiated among light treatments early during the experiment, with the highest SPAD observed with the 6 µmol·m−2·s−1 light treatment and the lowest SPAD observed with the 12 µmol·m−2·s−1 light treatment. The 12 µmol·m−2·s−1 values increased such that from day 22 onward, the SPAD values were equivalent among light treatments. F. japonicum showed only minor fluctuations in the appearance index over the course of the experiment, with mainly values of 5 observed.
(D) Gynostemma pentaphyllum.
Overall growth was gradually decreased, but the growth was acceptable for 1 month. G. pentaphyllum also tended to show higher responses with the 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 treatments, with declines observed with the three light treatment by the end of the experiment. The responses to 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 were equivalent.
(E) Perilla frutescens var. crispa f. viridis.
P. frutescens var. crispa f. viridis showed higher responses of all variables with the 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 treatments; by day 120, all four variables reached values of zero with the 3 µmol·m−2·s−1 treatment. Furthermore, the responses to 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 were equivalent. Overall, the growth of P. frutescens var. crispa f. viridis was reduced but remained acceptable with the 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 treatments. With 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1, the SPAD value decreased after 50 d and the appearance index decreased after 30 d. Those scores were stable thereafter.
(F) Petasites japonicus.
Overall growth was restricted, and only one leaf appeared and then disappeared continuously. P. japonicus showed variability in response to the light treatments over time. The SPAD value was the only variable that showed a clear response to the light treatments. Plants treated with 3 µmol·m−2·s−1 showed lower values during the last one-third of the experiment.
(G) Sasa veitchii.
S. veitchii also exhibited the poorest performance with 3 µmol·m−2·s−1, with differences observed later during the experiment. Regarding the appearance index and SPAD, the 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 treatments elicited similar responses; however, there was clear differentiation in the number of leaves during the second half of the experiment, with the 12 µmol·m−2·s−1 treatment resulting in double the number of leaves compared to that resulting from the 6 µmol·m−2·s−1 treatment.
(H) Saxifraga stolonifera.
S. stolonifera consistently had the best appearance index with the 12 µmol·m−2·s−1 treatment, and the number of leaves increased more rapidly with the 12 µmol·m−2·s−1 treatment than with the others. The SPAD increased over time with the 6 µmol·m−2·s−1 and 12 µmol·m−2·s−1 treatments; however, it was difficult to discern any trends in the height data, except for the consistently lowest heights with the 6 µmol·m−2·s−1 treatment.
The growth of each plant species with different light intensities are summarized in Table 2. F. japonicum and S. stolonifera were the most shade-tolerant plant species, and they were able to grow well with all light intensities. S. veitchii and A. gramineus showed medium shade tolerance and cannot grow well at the lowest light intensity. The other four species, C. sinensis, G. pentaphyllum, P. japonicus, and P. frutescens var. crispa f. viridis, were not sufficiently shade-tolerant and may be difficult to grow in indoor office rooms without supplemental lightning.
Summary of growth of each plant species under different light intensities.
Discussion
This research revealed the growth characteristics of Japanese herbs in different light intensities. It is important to note that the light environment may be formed by multiple types of light sources, such as a combination of daylight and artificial illumination in buildings. Sugano et al. (2021) showed that the PPFD quantified by using spectral irradiance simulation in an office varied spatially and temporally in response to proximity to a window, weather, and time of day. Tan et al. (2017) addressed the importance of determining actual lighting requirements for each plant and measuring daylight availability onsite before installing greenery in the indoor environment. In this study, the lighting intensity (250–750 lx; PPFD 3–12 µmol·m−2·s−1) was chosen to match the lightning intensity requirements in office spaces because planning a light environment where both humans and plants can coexist is crucial. The light enviroment which was used in this study was very low for foliage plants; however, some plants in this study, such as F. japonicum and S. stolonifera, can tolerate low light intensity as well as tropical foliage plants. Tazawa (1999) summarized the light compensation of multiple plant types. The PPFDs of 17 to 34 μmol·m−2·s−1 and 3.4 to 10.1 µmol·m−2·s−1 were the light compensation points for general crops and tropical foliage plants, respectively. The compensation point is the irradiance at which photosynthesis equals respiration. This is a useful estimate of the lowest irradiance that plants can maintain to reduce energy consumption (Stevenson et al. 1996). However, in this study, the light intensity was too low, and the light compensation was not clear for some plant species. Therefore, further research of higher light intensities is necessary to clarify how much supplemental lightning is required. The poor performance plants exhibited severe damage after 60 d. It is becoming common to use office plant rental and maintenance services in Japan. With such services, plants are usually changed every 1 to 2 months. If the plants can recover from damage under low intensity, then it may be possible to use various types of Japanese herbs.
F. japonicum and S. stolonifera were the most shade-tolerant plant species, and they were able to grow well under all light intensities during this study, indicating that they can grow in offices without supplemental lightning. Previous studies showed that they are shade-tolerant species. Jang et al. (2021) compared the growth of F. japonicum under different light intensities from 10 µmol·m−2·s−1 to 200 µmol·m−2·s−1 in a controlled chamber. They showed stem elongation at 10 µmol·m−2·s−1; however, the number of leaves and chlorophyll content did not show any significant differences between light intensities. F. japonicum can adapt well when moved from sun to shade; however, they grew the best at 50% of the natural outdoor light intensity (Takahashi and Kondo 1990). An (2012) showed that S. stolonifera requires shade; furthermore, shade with light transmittance of 40% is most suitable for its growth, whereas strong sunlight inhibits its growth, thus significantly limiting its large-scale cultivation. The other study showed that the growth of S. stolonifera was poor with all the light sources when light intensities were less than 200 lx; however, it improved at 700 lx and 1000 lx (Ju and Bang 2008).
S. veitchii and A. gramineus showed medium shade tolerance during this study. These species have been previously examined as shade-tolerant cover plants for outdoor urban landscaping. S. veitchii grew well during our study except under the lowest lighting intensity of 3 µmol·m−2·s−1. A previous study showed that S. veitchii grew in 1% of relative natural light outdoors from July to December in Tokyo, Japan; however, they grew well in semi-shaded areas with 10% to 50% relative natural light. In full sun, the plant exhibits a slight leaf scorch phenomenon (Takahashi et al. 1986). Leaves of S. veitchii have a life span of 2 years (Motomura et al. 2008), and it may be useful for indoor plants to maintain a certain amount of green. This study showed that A. gramineus were shade-tolerant. Similarly, a previous study showed that the number of leaves was lowest at 10 µmol·m−2·s−1 but did not show significant differences between 50 µmol·m−2·s−1 and 200 µmol·m−2·s−1. The SPAD did not show significant differences between light intensity treatments (Jang et al. 2021). S. veitchii, A. gramineus, and G. pentaphyllum showed a higher dry shoot weight; however, the appearance was low under low light intensity because the initial size of plants was bigger than that of the other species. Shade-tolerant plants tend to have a slow growth rate to maintain high light use efficiency (Valladares et al. 2002).
The other four species, C. sinensis, G. pentaphyllum, P. japonicus, and P. frutescens var. crispa f. viridis, were not sufficiently shade-tolerant and may be difficult to grow in indoor offices without supplemental lightning. Previous studies used different lighting intensities for the purpose of production. Shade management (dark treatment) of tea plants is an agronomic practice widely used for tea cultivation at the preharvest tea stage. Although prolonged shading/darkness can potentially lead to reduced tea leaf biomass (Fu et al. 2015), it can also effectively enhance free amino acids (Chen et al. 2018; Yang et al. 2012) and reduce catechin contents in preharvest tea leaves (Yang et al. 2012). Ohta and Harada (1996) attempted to culture tea plants in a glasshouse and estimated that the light compensation point for tea plants was 100 µmol·m−2·s−1. Suzuki et al. (2012) reported the shade–leaf reaction (gloss and epinasty) of tea leaves cultured under low light intensity (100 µmol·m−2·s−1) and found that the light intensity threshold under which tea plants exhibited the shade–leaf reaction and an improvement in the quality of leaves was ∼100 µmol·m−2·s−1. C. sinensis was the only woody plant in this study. Previous research showed that woody plants required more light than herbaceous plants (Akihiro et al. 1997; Guolin et al. 1991). C. sinensis also had problems with the Coccoidea and Tetranychidae pests during this study. When indoor plants are used, pest control is essential. The performance of P. japonicus was also limited during this study. At most, two leaves of P. japonicus opened and soon disappeared. It was assumed that the reason why they could not grow well was because they grow from rhizomes and may require larger pots to enlarge their roots. Another reason may have been low humidity. The low humidity in the indoor environment may have caused their poor growth because this plant usually grows in humid environment (Kitamura 1961). Previous studies showed that G. pentaphyllum prefers shady areas, with relative illumination of 40% to 80% and optimal relative illumination of 65% to 75%. Under these conditions, the output is the highest, the blossoming and bearing of fruit are increased, and the total saponin content is the highest (Guo and Wang 1993; Razmovski-Naumovski et al. 2005; Wang et al. 1996). G. pentaphyllum is a vine plant and can be used in a design that hangs from above; therefore, it may be able to receive more lighting from above. P. frutescens var. crispa f. viridis studied in plant factories had improved production efficiency and quality. An obligatory short-day plant, P. frutescens var. crispa f. viridis was induced to flower under long-day conditions when grown under low intensity light (30 µmol·m−2·s−1). The plant size was smaller under lower light intensity, indicating that the low-intensity light acted as a stress factor (Wada et al. 2010). The growth condition in previous studies tended to include high light intensity of approximately 100 to 300 µmol·m−2·s−1 (Lu et al. 2018). However, 200 µmol·m−2·s−1 is equivalent to 15,000 lx to 20,000 lx, which is too bright for an interior landscape; therefore, it is important to balance the light intensity required for human use and plant growth in the office environment. Egea et al. (2014) noted that lighting that can maximize crop yield is different from lighting required for indoor greenery because the latter has the added requirement of light that can give plants a perceived naturalistic appearance, light that can avoid excessive plant growth to minimize maintenance, and light that can cover a wide spectrum so that a large variety of plant species can be considered for use in the landscape design palette. As indoor greenery becomes more prevalent in highly urbanized areas, further studies of lighting requirements for ornamental plants are recommended (Tan et al. 2017).
Conclusion
This study showed that it is possible to grow some Japanese herbs in an office environment without supplemental lightning. Our study addressed the importance of selecting appropriate plants to achieve successful indoor greening, and it showed that some plant species such as F. japonicum and S. stolonifera are as shade-tolerant as typical tropical plants. Overall lighting intensities were low during this study; therefore, further research of higher light intensities is necessary to clarify how much supplemental lightning is required. Finally, it is important to balance the lighting intensity required for human use and plant growth. Further plant selection studies are required to extend the possibilities of designing with indoor pants.
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