2019 Volume 88 Issue 1 Pages 67-75
The effect of non-woven fabric floating row covers on the growth and yield of spring-sown, direct-seeded onions was investigated in Hokkaido, a subarctic island of Japan. Field experiments were carried out in 2014, 2015, and 2017. The seeds were sown in late April in each year. Floating row covers were installed a few days after seeding and kept in place until the end of May. During the treatment period, average daily maximum and minimum soil temperatures were approximately 4–5°C and 1–3°C higher, respectively, under covered compared with uncovered treatment. The effect of row covers on soil moisture varied annually. Emergence was obviously accelerated by floating row covers, as evidenced by an emergence date that was 1 to 4 days earlier than that of the uncovered treatment. Growth during early stages was promoted by floating row covers; however, the difference in plant growth between covered and uncovered treatments gradually decreased, with an almost non-significant difference observed by the beginning of bulb formation. The effect on growth earliness differed between years and an earlier lodging time was observed in 2015, in which the sunshine duration during the covered period of about 40 days reached 350 hours and the soil temperature difference between covered and uncovered treatments was larger than the other 2 years. Floating row covers had little effect on bulb yield. As for other growth aspects, decreased survival under floating row covers due to heat injury was observed when high temperature and drought simultaneously occurred. Floating row covers also tended to decrease onion maggot feeding damage. Non-woven fabric floating row covers effectively promote the emergence and early growth of spring-sown, direct-seeded onions mainly by elevating soil temperature. Although the degree differs depending on the year, earlier bulb formation and lodging, and a decrease in onion maggot feeding damage are also expected. These results suggest that non-woven fabric floating row covers may be a viable option to reduce the risks of delay in emergence, growth suppression caused by low temperatures, delay in lodging time and feeding damage due to onion maggot.
In Japan, onions (Allium cepa) are usually grown from transplanted seedlings, with the current production system tailored to this method (Yakuwa, 2006). Hokkaido, the northernmost of the four main islands of Japan, is the largest onion-producing area in the country. Hokkaido is situated in a subarctic climate zone and is covered with snow for approximately 4 months every year. In contrast to warmer areas, where autumn seeding is the norm, spring-seeded onion types are grown in Hokkaido (Yakuwa, 2006).
In Hokkaido, the practicality of direct seeding has recently been examined with the aim of reducing onion production costs. Although accurate statistical data that distinguish direct-seeded and transplanted onion production are unavailable, the cultivation area of direct-seeded onions is currently estimated to be 300 ha in Hokkaido out of a total onion cultivation area of approximately 14,000 ha. The need for labor-saving production technology has been strengthened in recent years by a decrease in the number of farmers, a shortage of successors, and an expansion in the scale of management. The frequency of direct seeding is thus projected to increase. Consequently, improvements and stabilization of productivity with the direct-seeding method are extremely important. Boyhan et al. (2008) reported that direct seeding of onions can reduce the transplantation-associated costs and labor requirements of autumn seeding; however, bulbs from direct-seeded onions tend to be smaller than those grown from transplanted seedlings (Leskovar et al., 2004). In Hokkaido, in particular, fields are covered with snow until early April, which prevents earlier sowing, and, in contrast to autumn sowing, a sufficient growth period cannot be ensured. Furthermore, autumn temperatures decrease earlier in Hokkaido than in other onion-growing areas. The period available for growing direct-seeded onions is therefore shorter than that needed for transplanted onions, which can be sown in a protected nursery as early as February to March. Compared with warmer areas, the direct-seeded onion production method is even more unreliable in Hokkaido, as the growth period cannot be extended by advancing the spring-restricted sowing date. Under such conditions, the promotion of early growth to extend the growing period and ensure bulb enlargement is extremely important to improve direct-seeded onion production in Hokkaido. One measure to promote early growth, the use of floating row covers, has been trialed on various direct-seeded vegetable crops with varying degrees of success (Gordon et al., 2010; Hemphill and Mansour, 1986; Loy and Wells, 1982; Mansour and Hemphill, 1987; Wells and Loy, 1985). In the present study, I investigated the use of floating row covers and evaluated their effects on the growth and yield of direct-seeded onions in Hokkaido.
Field experiments were carried out in 2014, 2015, and 2017 in a light-colored andosol at the Tokachi Agricultural Experiment Station, Hokkaido Research Organization (HRO), Memuro, Hokkaido, Japan (42°53' N, 143°05' E). Onion cultivars ‘Okhotsk 222’ (Shippo Seed Co., Ltd., Kagawa, Japan) and ‘Wolf’ (primed seeds; Takii & Co., Ltd., Kyoto, Japan) were planted in 2014 and 2015, and ‘Okhotsk 222’ was planted in 2017. The seeds were directly sown (10 cm planting distance, 25 cm row spacing and 3 cm depth) on April 23, 22, and 25 in 2014, 2015, and 2017, respectively. Two herbicides were used in this experiment: pendimethalin 30% emulsion (4 L·ha−1), applied to the soil before emergence a few days after sowing, and ioxynil 30% emulsion (0.3 L·ha−1), applied once during the growing season in late June. After July, weeds were removed manually when necessary. No pesticide was applied except to control onion thrips. Disease management and all other cultural conditions and practices were consistent with local conventions.
The row cover material used in this study was a polypropylene non-woven fabric (Paopao 90; Mitsubishi Chemical Agri Dream Co., Ltd., Tokyo, Japan). Floating row covers were installed immediately after herbicide treatment and not removed until late May (dates of removal are given for each year in Table 2). Each experimental plot consisted of 10 rows (5 m long) in 2014 and 2015 and 8 rows (6 m long) in 2017. Two rows on either edge of the plot and 50 cm at both ends of each row were excluded from subsequent investigations to avoid edge effects.
Soil temperature during the covered period was measured at a depth of 3 cm (the same depth at which onion seeds were sown) using Thermochron G-type data loggers (KN Laboratories, Inc., Ibaraki, Osaka, Japan). Data loggers were buried in each plot more than 1 m from the edge. One logger was used per experimental plot replicate. The average soil temperature for each treatment was calculated.
To measure the soil moisture content of covered and uncovered plots during seedling emergence, soil at a depth of 0–10 cm was sampled in early May, when emergence was actively occurring. After drying soil samples in a 105°C oven for 48 h, the soil moisture content was calculated based on weights before and after drying.
As seedlings emerged, the number of emerged seedlings in total row lengths of 7, 6, and 8 m (in 2014, 2015, and 2017, respectively) was counted and used to calculate the emergence rate relative to the number of seeds sown. The day on which the emergence rate attained 40% was recorded as the emergence date. Before counting, plants that withered after emergence were extracted from the soil and examined to clarify the cause of wilting. If no trace of disease or pest was detected, the cause was ascribed to physiological injury.
To evaluate plant growth, the height, leaf number, leaf sheath diameter, and bulb diameter of 10 uniform plants in each plot were measured two or three times during the growing period. Measurements were carried out in early to mid-June, late June to early July, and late July. The second measurement was omitted in 2017. To evaluate growth earliness, dates of bulb formation and lodging were recorded. The first set of dates was equivalent to dates when plant bulb diameters of more than 10%, 40%, or 80% were twice those of the leaf sheaths, whereas the latter dates corresponded to dates when more than 10%, 40%, or 80% of plants had lodged.
Onions were harvested in September in accordance with local conventional practices, and bulb yield was measured. In each plot, for yield measurement all plants were harvested from 2.5 m2 (2014 and 2015) or 5.0 m2 (2017) areas with few missing plants and relatively uniform growth. Each bulb was classified as marketable, processable, or non-processable on the basis of size and external appearance in accordance with general Japanese vegetable market standards. Bulbs smaller than 50 mm in diameter were excluded from the processable yield. Bulbs for inclusion in the marketable yield were then selected from the processable yield on the basis of shape and external appearance.
Weather data were edited based on the daily observation data of the Automated Meteorological Data Acquisition System (AMeDAS, Memuro Station) provided by the Japan Meteorological Agency website (http://www.data.jma.go.jp/gmd/risk/obsdl/index.php).
Experiments were carried out using a split-plot design in 2014 and 2015, with row cover as the main plot and cultivar as the subplot, and using a randomized block design in 2017. Data were analyzed for statistical significance by ANOVA or Student’s t-test. Statistical analyses were performed using JMP version 13.0.0 (SAS Institute Inc., Cary, NC, USA).
The most noteworthy difference in the weather conditions of the 3 experimental years was the length of sunshine duration during the covered period. The sunshine duration during the covered period in 2015 exceeded 350 h, and this was markedly longer than that in the other years (Table 1). The average daily maximum soil temperature during the covered period was approximately 4–5°C higher under covered than uncovered conditions (Table 2). The average daily minimum soil temperature in the covered treatment was 1–3°C higher than that of the uncovered control. The differences in soil temperature were largest in 2015.
Weather data during the covered periodz.
Effect of floating row covers on soil temperature during the covered period.z
In 2014, no significant difference in soil moisture content was detected between the covered and uncovered treatments (Table 3), whereas a significant difference was observed in 2015.
Soil moisture content during emergence of direct-seeded onionsz.
Seedling emergence was clearly accelerated under floating row covers, regardless of the experimental year, with an emergence date 1–4 days earlier than that of uncovered plots (Fig. 1). In contrast, floating row covers had no effect on the final emergence rate. In 2015, the final emergence rate of ‘Wolf’ was lower than that of the other years and of ‘Okhotsk 222’ in the same year and did not attain 70%.
Effect of floating row covers on emergence of direct-seeded onions. Bars indicate SE (n = 3).
As evidenced by growth parameter measurements, a trend for promotion of early plant growth was exhibited in the covered treatment in each year. A significant difference in growth parameters between covered and uncovered treatments was generally observed until early July in 2014 and 2015. This difference gradually decreased, however, with no statistically significant difference detected in late July, when bulb formation begins (Table 4). In addition, an interaction between floating row covers and cultivars was observed on the second measurement date in 2014 and 2015, and floating row covers were indicated to have a greater effect on the growth of ‘Wolf’ than of ‘Okhotsk 222’.
Effect of floating row covers on growth parameters of direct-seeded onions.
The effect of floating row covers on growth earliness differed among years. Although floating row covers had no significant effect on the date of bulb formation and lodging in 2014 and 2017, bulb formation was almost 1 week earlier, and lodging more than 1 week earlier, under covered vs uncovered conditions in 2015 (Table 5).
Effect of floating row covers on the timing of bulb formation and lodging of direct-seeded onions.
In 2014, survival rates of seedlings decreased during the last 9 days of the covered period under covered conditions compared with those of the uncovered treatment (Fig. 2). The hypocotyls of withered seedlings had narrow necks, which suggested that the withering was caused by heat injury. Withered seedlings showing the same symptom were rare in the uncovered treatment, which resulted in differences in survival rate. This type of damage was not observed during the other 2 years. In 2015, a strongly significant difference was observed in the incidence of onion maggot (Delia antiqua (Meigen)) feeding damage on ‘Wolf’ between treatments, with much less severe damage occurring under covered conditions (Fig. 3). In contrast, no difference was detected between treatments for ‘Okhotsk 222’. Although much less feeding damage was apparent in the other 2 years than in 2015, the damage tended to be less severe in covered plots irrespective of the cultivar (data not shown).
Effect of floating row covers on survival rate of direct-seeded onions in 2014 (n = 3). ‘+’ and ‘−’ respectively refer to covered and uncovered treatments. Bars indicate SE (n = 3). Significant differences based on Student’s t-test (n = 3) are indicated as follows: *, significant at P < 0.05; NS, non-significant.
Effect of floating row covers on onion maggot feeding damage in 2015. ‘+’ and ‘−’ respectively refer to covered and uncovered treatments. Bars indicate SE (n = 3). Significant differences based on Student’s t-test (n = 3) are indicated as follows: **, significant at P < 0.01; NS, non-significant.
No significant difference in bulb yield was detected between the covered and uncovered treatments in any year (Table 6). In 2015, an interaction between row cover and cultivar was observed; although only a slight difference was observed in the bulb yield of ‘Okhotsk 222’ between treatments, the yield of ‘Wolf’ was especially low in the uncovered treatment. In regard to bulb weight, no significant difference between the covered and uncovered treatments was observed throughout the years of the experiment.
Effect of floating row covers on the yield of direct-seeded onionsz.
Leskovar et al. (2004) reported that direct-seeded onions produce smaller bulbs than transplanted seedlings, even when sown at similar times. Especially in Hokkaido, the period available for growing direct-seeded onions is shorter than that needed for transplanted onions because of the climatic conditions, and the transplanting method is advantageous for extending the growing period because seeds can be sown earlier in a protected environment. However, the need for labor-saving production technology has increased in recent years and increasing attention has been paid to the direct-seeding method. To succeed in the production of direct-seeded onions in Hokkaido, promotion of early growth to extend the growing period is extremely important.
In this study, the use of floating row covers effectively promoted seedling emergence and early growth during direct-seeded onion production. The emergence rate of onion seedlings is largely determined by temperature and soil moisture (Bierhuizen and Wagenvoort, 1974). In the present study, the soil moisture content varied between 2014 and 2015 (Table 3), but the difference in emergence date was small (Fig. 1). In addition, emergence in 2017, when the average soil temperature was relatively lower, was delayed compared with the other 2 years (Table 2; Fig. 1). These observations suggest that soil temperature, rather than soil moisture, was the factor limiting emergence in these experiments. Given that the floating row covers elevated the soil temperature, the period required to reach the effective accumulative temperature required for emergence might be shortened, resulting in earlier emergence. A general trend observed with the floating row cover treatment was the promotion of plant growth after emergence. In 2014, statistically significant differences between the covered and uncovered treatments were detected in leaf number on the first measurement date and bulb diameter on the second measurement date. In 2015, plant height, leaf number, and leaf sheath diameter differed significantly between the treatments at the first and second measurements (Table 4). In a phytotron experiment, Kato (1964) observed that onion leaf growth was promoted by high temperatures within the range of 10 to 25°C. Although the air temperature under the floating row covers was not measured in the present study, the elevation in air temperatures expected to have occurred can be assumed to have promoted leafing and leaf elongation in the early stages of direct-seeded onion growth.
An annual difference was observed in the effect of floating row covers on several growth aspects. A decrease in survival rates during the last 9 days of the covered period was observed under row covers in 2014 (Fig. 2), but not in the other 2 years. Analysis of weather data revealed that 2014, compared with the other 2 years, was characterized by fewer precipitation days from late April to May (Fig. 4). Considering that the covered periods of these 3 years were considerably hot compared to normal years (Table 1), seedlings in 2014 were thus exposed to both severe heat and drought immediately after emergence. Wolfe et al. (1989) pointed out that responses to row covers may differ depending on crop sensitivity to high temperature, and Mansour and Hemphill (1987) noted that excessive heat buildup under row covers may damage some crops. The soil moisture content data in the present study also indicated that the soil was drier in 2014 than in 2015 (Table 3). According to Oda et al. (1993), a high soil moisture content may protect against heat injury by hastening transpiration, thereby suppressing a rise in leaf temperatures. As shown in Fig. 4, precipitation was more uniformly distributed in 2015 and 2017 than 2014. The yearly difference in soil moisture observed in this study (Table 3) can thus be attributed to differences in natural precipitation. These results suggest that onion seedlings grown under floating row covers can be injured when simultaneously subjected to high temperature and drought. Importantly, however, no significant decrease, even with the occurrence of heat injury, was observed in bulb yield in 2014 (Table 6). This result can be explained by the fact that plants were also eliminated by other factors; consequently, there was no major difference in the stand at the time of harvest.
Weather conditions before and during emergence of direct-seeded onions. Onions were covered with floating row covers or uncovered.
The effect of floating row covers on growth earliness also differed annually. In 2015, unlike the other 2 years, significant differences in bulb formation and lodging times were observed between the covered and uncovered treatments (Table 5). As a feature of the weather conditions in 2015, the sunshine duration was markedly longer than that in the other years. The long sunshine duration during the covered period in 2015 enhanced the temperature difference between the covered and uncovered treatments, and the mean of the daily average soil temperatures for each treatment differed by almost 4°C (Table 2). The enhanced difference in the mean of the daily average soil temperatures between the treatments resulted in larger differences in the emergence time and the early growth of direct-seeded onions. Furthermore, the results show that there is a possibility that markedly promoted early growth under the floating row covers may have led to an earlier lodging time (Tables 4 and 5). In Hokkaido, onions are lifted several weeks after 80% lodging, bulbs are field cured, harvested after curing, and the foliage is removed (topped) after harvest. The period of curing is important because it is known that topping before curing or insufficient drying of the foliage section increases the incidence of rot during storage (Tanaka, 1991; Wright, 1997). Given that the timing of lifting is decided based on the lodging time, curing tends to be insufficient in the direct-seeding method compared with that of the transplanting method owing to the decrease in temperature, and thus the risk of rot during storage increases. Therefore, acceleration of the lodging time would be effective to ensure a sufficient curing period, which in turn ensures the storability of the direct-seeded onions. Given that stability of supply and storability are extremely important considerations for Hokkaido onions, the possibility of promoting earlier growth and lodging times with floating row covers may be preferable for the producers of Hokkaido.
In regard to the two cultivars used in the present study, ‘Wolf’ seedlings tended to emerge sooner and grow more vigorously during early stages than ‘Okhotsk 222’ seedlings, with or without floating row covers (Fig. 1; Table 4). In addition, significant interactions were observed between floating row covers and cultivars with respect to plant height and leaf sheath diameter in late June and early July (Table 4). It must be noted that primed seeds were only used for ‘Wolf’ in this study. Brocklehurst and Dearman (1983) have reported that the emergence time of primed onion seeds was 3–9 days earlier and plant weight was heavier 12 weeks after sowing compared with untreated seeds under field conditions. Whether these interactions were due to cultivar characteristics or seed priming treatment is not clear, but the present results suggest that the commercial seeds of ‘Wolf’ are more influenced by row cover installation than those of ‘Okhotsk 222’.
In contrast to the above-mentioned effects, floating row covers had no obvious influence on post-mid-stage growth, bulb yield or bulb weight (Tables 4 and 6). A considerable difference in bulb yield between covered and uncovered treatments was observed in ‘Wolf’ in 2015, but this reflected feeding damage caused by onion maggots. Feeding damage due to onion maggots was more severe in 2015 than in other years, and the uncovered ‘Wolf’ plot suffered extreme damage compared with the covered plot, whereas no differences were noted between the covered and uncovered treatments in ‘Okhotsk 222’ (Fig. 3). In other years, however, irrespective of cultivar, a tendency for less severe damage was observed in the covered treatment. Considering the present results, floating row covers may have some effect to decrease maggot feeding damage in the open field. Hoffmann et al. (2001) reported that non-woven fiber applied to the base of plants can significantly reduce the number of onion maggot eggs. Given that in the present study row covers were removed in late May and no physical border existed between plots, the cause of the difference in feeding damage levels is unclear.
The findings of this study indicate that the use of non-woven fabric floating row covers until May effectively promotes the emergence and early growth of spring-sown, direct-seeded onions on the subarctic island of Hokkaido, mainly by elevating soil temperature. This effect is more pronounced under sunny conditions such that the sunshine duration during the covered period of about 40 days reaches 350 hours. Under such conditions, a difference in early growth is accentuated and an advance in bulb formation and lodging time could be expected. Floating row covers also retain soil moisture when precipitation is relatively even. Furthermore, the possibility exists that onion maggot feeding damage can be decreased by floating row covers. These effects, however, do not significantly increase bulb yield, probably because weather conditions after removal of floating row covers may have a more pronounced effect on bulb yield. Weed growth was not analyzed in this study, but the floating row covers also seemed to promote weed growth. Weed management is thus another subject that needs to be studied to stabilize direct-seeding onion production in Hokkaido.
In conclusion, the present results suggest that non-woven fabric floating row covers may be a viable option to reduce the risks of delay in emergence, growth suppression caused by low temperatures, delay in harvest time and feeding damage due to onion maggots, and ensure the storability of direct-seeded onions by acceleration of the lodging time. Although the use of non-woven fabric floating row covers with direct-seeded onions in Hokkaido has yet to be evaluated due to the lack of a clear effect on bulb yield, this study clarified their effects and those of certain weather conditions on direct-seeded onions in a subarctic area.
I am grateful to the members of the research support team of the Tokachi Agricultural Experiment Station for their technical support regarding field and crop management. I also thank Dr. K. Kawagishi for proofreading this paper.