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Using a Plug System to Produce Hygienic Vegetables
Wen-Shann Lee and Shaw-Rong Yang*
Department of Horticulture,
National Chung-Hsing University,
Taichung, Taiwan ROC
*Tainan District Agricultural
Improvement Station,
Tainan, Taiwan ROC, 1999-09-01

Abstract

The first part of this Bulletin describes how the plug system is used to produce short-term leafy vegetables in Taiwan. Conventional leafy vegetable production in Taiwan is based on direct seeding. In the plug system, plug seedlings are transplanted into 18-cell flats, which are then laid out in the field under a protective nethouse. This type of production has several advantages. The plants grow faster and can be harvested sooner, so they occupy land for a shorter period of time. There are also fewer pest problems. In the second part of the Bulletin, some research data are presented. Vegetables grown in a more porous medium had better root and shoot growth, and thus higher yields. Seedling age had a negative effect on subsequent plant growth and final yield. Vegetables produced in flats had a respiration rate 7.5% to 60% lower, and a storage life of 2 to 6 days longer, than conventionally grown vegetables, as observed on lettuce, Chinese kale and water convolvulus.
Abstracts in Other Languages: 中文(790), 日本語(747), 한국어(739)

Introduction

Most people in Taiwan like to include in their daily diet fresh leafy vegetables harvested an early growth stage. Young, leafy vege-tables are highly perishable, so that postharvest handling and long-distance transport are difficult. For this reason, it is considered that the production of leafy vegetables is one part of the vegetable industry that will not have to face competition from imports in the future. About three million metric tons of vegetables were produced on 180,209 ha of cultivated land in Taiwan in 1997. Of this land area, about 22,081 ha was used for short-term leafy vegetable production. This is about 12.2% of the total vegetable production area (Taiwan Agricultural Yearbook 1998).

Taiwan is a subtropical island, and often suffers from typhoons, heavy rain and thunderstorms in summer and fall. These usually cause severe losses of leafy vegetables in terms of both their quantity and their quality. Cultivating vegetables under structures is an effective way of protecting them from such disasters (Liao et al. 1989). Currently, more than 3,000 ha of leafy vegetables are grown under various kinds of protective structures. The most popular structures used in Taiwan are flat-roofed nethouses and structures of metal piping covered in PE film (Sheen and Chen 1992).

Most leafy vegetables, including Pak choi and Chin-keng Pak choi (both are varieties of Chinese mustard), leafy lettuce, Chinese kale, leaf mustard, celery, water convolvulus etc. can be grown successfully under structures all year round. Only spinach and garland chrysanthemum are cool-season crops that are difficult to grow in Taiwan's hot summer.

A sufficient supply of leafy vegetables has long been the key to the price index in vegetable markets. Leafy vegetables are so vulnerable to environmental factors that shortages in markets after climatic disasters always causes a fluctuation in prices. Prices may soar to 3-5 times the normal level. In order to stabilize the market supply, two District Agricultural Improvement Stations in Taiwan, one in Taoyuan and one in Tainan, started trials on growing leafy vegetables in nursery flats without inner partitions, filled with local compost media, in the early 1990s (Chang Chien 1995, Yang and Cheng 1997). Although the results were very good, the production process was considered too laborious and costly. After several modifications, an operational model of leafy vegetable production using the plug system was developed.

Conventional Production of Leafy Vegetables under Protective Structures

The conventional production of leafy vegetables follow the seven-stage sequence:

  • Land preparation < Seed sowing < Plant thinning < Harvesting <
  • Sorting and cleaning < Packaging < Shipping or storing

If leafy vegetables are direct seeded, beds must be prepared for furrow irrigation and improved drainage. The beds are separated by narrow paths for working access. Usually 10 - 20% extra seed is sown, to ensure good establishment. Thinning is carried out when plants reach the 2- to 3-leaf stage in order to maintain good spacing. Thinning is such laborious work that growers spend about 400 hours on this per hectare, which is equal to US$1,200 in terms of labor costs. Occasionally, weeding is also needed. Most of the leafy vegetables grown in soil have a cropping period of 20 to 45 days before they are ready for harvest. Harvesting, including cleaning and packaging, are an even more labor-intensive step in vegetable production. As reported in earlier research, when leafy vegetables are grown under structures, the labor cost accounts for 68% of production total costs. Harvesting accounts for more than 60% of the labor cost (Chang Chien and Chang 1995). The vegetables produced by conventional methods are usually packed in large baskets made of bamboo or plastic. Most of them are shipped to traditional markets or to wholesale markets.

Production of Leafy Vegetables Using the Plug System

The flow chart in Fig. 1(2) shows the process of growing leafy vegetables in a plug system. It begins with young seedlings in plugs, and ends with the plants in 18-cell flats. The whole system is rather complicated, including the medium supply, nutrient control, pest management, and chemical residue inspection in crop production, and also marketing and customer services. Inspection for pesticide residues is an important part of the production of hygienic vegetables. It should be strictly enforced on all crops which are sprayed with pesticides, and there should be strict standards of quality and safety. The production process can be roughly divided into four steps.

Auto Seeding and Plug Nursing

Basically, leaf vegetables are propagated by seeds, sown by a mechanized seeder. The exception is water convolvulus, which is sown directly into 18-cell flats by hand. Growers use 288-cell plug trays for seedling propagation. As well as the mechanized seeders, growers use a mechanized system for filling the flats with growing medium ( Fig. 2(1)). Two people working together can take care of one medium filling machine and one mechanized seeder, preparing 240 - 330 plug trays per hour. The seeded plug trays are then stacked on carts and transported to greenhouses for germination and early growth. While the young plants are in the plug nursery, experienced workers should be taking care of watering and nutrition applications, because the success of the system relies on high-quality plants.

Plug seedlings are ready for transplanting when they are at the 2- to 3-leaf stage, with a well developed root system. For most leafy vegetables, this takes less than 26 days ( Table 1(1) and Fig. 3(1)).

Hardening off plugs, by keeping them fairly dry for a few days before transplanting, is important in summer. Seedlings without hardening are delicate, and easily wilt in Taiwan's hot summer. Once the young seedlings have wilted and are lying flat on the surface of the growing medium, they are easily killed off by high temperatures caused by solar radiation. However, hardening young plugs too much is a common problem. Roots of seedlings subjected to too much stress grow more slowly. This prolongs the cropping time and reduces the yield.

Transplanting

During transplanting, workers take each seedling out of its plug tray and plant it into a cell in an empty flat moving along a conveyer belt ( Fig. 4(1)). The flat is 55 cm long and 28 cm wide, and has 18 cells arranged in three rows. Each cell in the flat is a cone shape measuring 4 x 4 cm, and 4.2 cm in depth. The transplanted flats then move through a machine which fills them with medium. They then move over a vibrating bench which shakes off any excess medium on the seedlings ( Fig. 5(1)). Finally, the flats are watered. They are then stacked on carts and transported to the field. There, they are spread out under a protective netting ( Fig. 6(0) and Fig. 7(0)).

The transplanting of seedlings from the plugs into the 18-cell flats is the most labor-intensive step, because so far it must still be done by hand. A team of five workers, four of them doing the transplanting and one stacking the flats, can transplant about 400 flats in one hour. At this rate, it would take 112.5 hours to transplant 45,000 flats, the quantity needed for one hectare.

Cultural Management

The first step in leafy vegetable production is land preparation. Soil to which fertilizer has been applied is plowed and leveled, before the flats are spread out across the field. There is no need to pile up ridges, but even so it takes 120 hours of labor to spread out 45,000 flats over one hectare ( Fig. 8(0)). Care should be taken that the bottom of every cell in the flats has good contact with field soil, so as to enhance root growth ( Fig. 9(0)).

As soon as the flats have been spread out in the field, there is no further need for intensive labor. Weeding is unnecessary, and there is usually no need to spray pesticides. Crops are irrigated with perforated PE tubes ( Fig. 13(0)) or by overhead mist, both of which can be automated. Nutrients needed by crops are added to irrigation water through fertilizer injectors. One problem during Taiwan's hot summer is the high temperature of flats and the root medium, caused by strong solar radiation. Although frequent irrigation can lower the temperature, slow growth of roots because of waterlogged medium is often observed in summer. More porous medium with coarser particles is recommended for summer production.

Harvesting and Packaging

Most vegetables produced by a plug system are not sold in wholesale markets, but go direct to supermarkets, or to individual families who have placed an order for them. All vegetables sent to supermarkets are cut off at the base of the stem, and put into transparent plastic bags marked with a brand name. Each bag contains about 250g of vegetables ( Fig. 11(0)). Plants sent to individual families are put in paper bags with their root systems intact, with about 300 g of vegetables per bag. Plants which still have their roots intact have the advantage of a longer storage life.

At harvesting, much labor can be saved if a conveyer system is used. Instead of workers going to the plants, the plants are brought to them. The workers can harvest and pack the crops, empty the flats, and recycle the used medium, all at the same place and in sequence. The spent medium containing the root residues is well composted and solar sterilized before reuse.

The high labor cost of transplanting has already been improved. Nowadays, most growers are using 32 four-connecting-cell units to fit in a 128-cell plug tray for seedling propagation ( Fig. 12(0)). At transplanting, four young seedlings in a four-cell unit are planted directly into the soil and left to grow.

This method has a similar cropping time and final yield as the system using 288-cell plugs. However, it reduces the labor cost by more than 30%.

Comparison of Plants Grown in Flats and in Soil

Vegetables grown in flats had plant characteristics and quality similar, or even superior to those grown conventionally in soil. The leaf area of Pak choi, Chin-keng Pak choi and leaf mustard grown in flats was found to be 22.7%, 11.8% and 33.6% higher, respectively, than that of the same varieties grown in soil ( Table 2(1)). Plants in flats appeared larger and more uniform, although only the difference in fresh weight of leaf mustard was statistically significant. It is suggested that vegetables grown in flats have faster growth at an early stage, better plant spacing for light interception, and less weed competition than those grown in soil. There were no significant differences in flavor, nitrate content, vitamin C content, or total soluble solids.

Effect of Seedling Age

Plug seedlings of Pak choi and Chin- king Pak choi at the age of 8, 9, 10, 11, 12, or 13 days were transplanted into 18-cell flats, in order to compare their growth with that of direct seeded plants. Plant height, leaf number, and leaf area of Chin-keng pak choi were very similar in all treatments. However, the yield per flat decreased as seedling age increased ( Table 3(0)). Plants transplanted at the age of 8 days had the highest yield of 1070g, which is comparable to that of direct seeded plants (1069.2g). Plants transplanted at 10 or 12 days had progressively lower yields. This effect was even more significant effect in Pak choi ( Table 4(0)).

All factors measured — plant height, leaf number, leaf area, fresh weight per plant and yield per flat — decreased as the age of transplanted seedlings increased. Direct seeded plants had the highest fresh weight (18.9g) and the highest weight per flat (340g). A higher growth rate of plug seedlings transplanted early has also been observed in tomato (Jand et al. 1996; Hsu et al. 1998) and cauliflower (Wurr et al. 1986). The increases in the yield of Pak choi was apparently due to an increase in plant height, leaf number, and leaf area, while the increases in the yield and average fresh weight of Chin- keng Pak choi seemed to be because of the thicker petioles. Although the plug seedlings transplanted earlier had higher yields, their root system was not yet developed enough to make it easy to pull the seedlings out of the plug cells. Therefore, we suggest that the seedlings should be kept in their plug cells at least 10 to 12 days, for better root development.

Medium Amendment for Better Growth

Maintaining a proper moisture content in the medium is quite difficult, especially in very hot weather. Newly transplanted seedlings wilt easily if they lose excessive water through transpiration. Seedlings subjected to water stress are reported to have a lower rate of assimilation and slower growth (Ko 1993). Although frequent watering is a good way to increase the moisture content and reduce wilting, too much water in the medium reduces air filled porosity and affects root growth. A high air content in medium is necessary for fast-growing plants, because roots need oxygen for respiration. The minimum air-filled porosity is reported to be 20% or more of container capacity (De Boodty 1972; Heiskanen 1997). However, frequent watering in summer to prevent plants from wilting results in oxygen deficiency in the waterlogged growing medium. In fact, actively growing roots need more oxygen under high medium temperature conditions. Our data showed that there is only 11.3% air-filled porosity in BVB No. 4 (a peat-based medium imported from the Netherlands) when the container is filled to capacity ( Table 5(0)) while the medium temperature is newly transplanted flats can be as high as 36.2°C. Adding 15% or 30% of perlite to the peat-based medium increased air-filled porosity to 14.7% and 19.5%, respectively. This increased the yield of Pak choi, and shortened the cropping time for both Pak choi and leaf lettuce ( Table 6(0)).

Respiration Rate and Storage Life of Vegetables Grown in Flats and in Soil

Leaf vegetables are very perishable and have only a short storage life. They have a high water content, a high respiration rate, and are tender and easily damaged in handling (Wang 1993). Since a faster respiration rate means more rapid deterioration, it is a good index of the potential postharvest life of vegetables (AVRDC 1992). Results indicated that leafy lettuce, Chinese kale, and water convolvulus produced from flat culture had a storage life 3, 6, and 2 days longer, respectively, than those grown in soil ( Table 7(0)). Further studies showed that the respiration rates of leafy lettuce and Chinese kale grown in soil were 7.5% and 65% higher. Although many factors may affect the deterioration of leafy vegetables, a higher respiration rate would at least contribute to a shorter storage life for soil-grown vegetables.

Conclusion

In conclusion, growing leaf vegetables in flats has eight advantages. Firstly, young seedlings are grown in compact 288-cell plugs, which occupy a much smaller area at an early stage than field grown vegetables. There are fewer weed problems, and the cropping time is shorter because of faster and better growth in artificial media. Fourth, crop production is divided into two stages at two separate locations, and the crop can be harvested before insects and disease become a problem. Fifthly, there are fewer problems from continuous cropping, because plants have grown fairly large in clean medium before their roots reach the soil. The cropping index is very high, with an average of 16 to 24 crops per year, thus increasing the yield year per unit area. Most of the production steps can be mechanized or automated. Finally, covering the soil surface with flats gives better control of the striped flea beetle ( Phyllotreta striolata), a serious pest in leafy vegetable production in Taiwan.

Since leafy vegetables grown in flats need little or no chemical pesticide for pest control, this can be seen as a model of hygienic vegetable production in tropical Taiwan. Further studies should include the use of flats made of some biodegradable or photodegradable material that is more friendly to the environment. Secondly, the possibility should be examined of using colored flats which reflect light, thus helping plant growth and reducing insect problems. Thirdly, labor saving by future mechanization and automation of the system is worthy of further study, since the main drawback of growing vegetables in flats is the high labor cost of transplanting plugs into the flats, and moving the flats out into the field.

References

  • AVRDC. 1990. Vegetable Production Training Manual. Asian Vegetable Research and Development Center. Shanhua, 447 pp. Reprinted 1992.
  • Chang Chien, H.J. 1995. Improvements of cultural practices in leafy vegetables by using plug seedlings. In: Automation for Organic Vegetable Production, S.K. Chang, (Ed.). Tao-Yuan District Agricultural Improvement Station, Tao-Yuan, Taiwan. (In Chinese).
  • Chang Chien, H.J. and T.R. Chang. 1995. Production of short-term leafy vegetables in Taiwan. Proceedings of Symposium on the Improvement of Vegetable Industry in Taiwan, Taichung District Agricultural Improvement Station, Taiwan, pp. 153-166. (In Chinese).
  • De Boodt, M. and O. Verdonck. 1972. The physical properties of substrates in horticulture. Acta Horticultura 26: 37-41.
  • Department of Agriculture and Forestry. 1998. Taiwan Agricultural Yearbook, 1998. Department of Agriculture and Forestry, Taiwan.
  • Heiskanen, J. 1997. Air-filled porosity of eight growing media based on sphagnum peat during drying from container capacity. Acta Horticultura. 450:277-286.
  • Hsu, C.C., W.S. Lee, and W.N. Chang. 1998. Effects of seedling ages and low temperature treatments on growth and yields in tomatoes. Hort. NCHU (National Chung Hsing University, Taichung, Taiwan). (In press).
  • Jang, S.W., J.H. Ku, J.N. Lee, J.T.L., W.B. Kim, B.H. Kim, and J.K. Kin. 1996. Effect of plug cell size and age of transplanted seedling on the growth and yield of tomatoes on alpine area. RDA (Korea), Jour. Agric. Sci. Hort. 38,1: 573-581.
  • Ko, S.S. 1993. The Influence of Water Lock on Three Vegetable Seedling Growth in Plugs. Unpub. M.sc. Thesis, National Chung Hsing University, Taichung, Taiwan.
  • Liao, F.S., T.R. Chang, Y.H. Chen, and H.C. Wang. 1989. Investigation of a production and marketing system for vegetables cultivated in a pipehouse in the suburbs. Proceedings of the Second Congress on Horticultural Production under Structures. Fenshan Tropical Horticultural Experiment Station, Fenshan, Taiwan. pp. 192-208. (In Chinese).
  • Sheen, T.F. and K.S. Chen. 1992. Production of vegetables under protective structures in Taiwan. Symposium on the Improvements of Crop production in Taiwan. National Chung Hsing University, Taichung, Taiwan, pp. 137-162. (In Chinese).
  • Wang, T.T. 1993. Postharvest handling of leafy vegetable. Proceedings of Symposium on Vegetable Production and Development in Taiwan. Taiwan Agricultural Research Institute, Taichung, Taiwan, pp. 209-219.
  • Yang, S.R. and J.J. Cheng. 1997. Production of Vegetables using Plug System. Extension Circular No. 68, Tainan District Agricultural Improvement Station, Tainan, Taiwan. (In Chinese).

Index of Images

Figure 1 Operation Flow Chart of Growing Short-Term Leafy

Figure 1 Operation Flow Chart of Growing Short-Term Leafy

Figure 2 Vegetable Production in Flats Starts from 288-Cell Plugs

Figure 2 Vegetable Production in Flats Starts from 288-Cell PlugsFigure 3 Plugs Are Ready for Transplanting at 12 to 26 Days after Sowing

Figure 3 Plugs Are Ready for Transplanting at 12 to 26 Days after SowingFigure 4 Seedlings from 288-Plugs Are Transplanted into 18-Cell Flats by Hand

Figure 4 Seedlings from 288-Plugs Are Transplanted into 18-Cell Flats by HandFigure 5 Filling of Medium into Transplanted Flats Moving along a Conveyer

Figure 5 Filling of Medium into Transplanted Flats Moving along a ConveyerFigure 6 Watering of the Flats after They Have Been Filled with Peat-Based Medium

Figure 6 Watering of the Flats after They Have Been Filled with Peat-Based Medium

Figure 7 Transplanted Flats Stacked on a Cart Ready for Transporting to the Field

Figure 7 Transplanted Flats Stacked on a Cart Ready for Transporting to the Field

Figure 8 Spreading Out the Flats in the Field under a Protective Net House

Figure 8 Spreading Out the Flats in the Field under a Protective Net HouseFigure 9 Well Developed Roots of Leafy Lettuce Growing into Soil

Figure 9 Well Developed Roots of Leafy Lettuce Growing into SoilFigure 10 Pak Choi Grown in 18-Cell Flats Is Ready for Harvest 14 Days after Transplanting

Figure 10 Pak Choi Grown in 18-Cell Flats Is Ready for Harvest 14 Days after Transplanting

Figure 11 Workers Are Harvesting Leafy Lettuces and Putting Them into Retail Bags with a Brand Name.

Figure 11 Workers Are Harvesting Leafy Lettuces and Putting Them into Retail Bags with a Brand Name.

Figure 12 Four-Cell Units Fit into 128-Cell Plug Trays for Direct Seeding _ a Newly Developed System.

Figure 12 Four-Cell Units Fit into 128-Cell Plug Trays for Direct Seeding _ a Newly Developed System.

Figure 13 The 4-Cell Units with Young Seedling Are Transplanted into Field and Irrigated with Perforated Pe Tubing.

Figure 13 The 4-Cell Units with Young Seedling Are Transplanted into Field and Irrigated with Perforated Pe Tubing.

Table 1 Leafy Vegetables Produced in a Plug System

Table 1 Leafy Vegetables Produced in a Plug System

Table 2 Comparison of the Characteristics of Pak Choi, Chin-Keng Pak Choi, and Leaf Mustard Grown in 18-Cell Flats or in Soil at Harvest

Table 2 Comparison of the Characteristics of Pak Choi, Chin-Keng Pak Choi, and Leaf Mustard Grown in 18-Cell Flats or in Soil at Harvest

Table 3 Effect of Seedling Age on Growth and Yield of Chin Keng Pak Choi Grown in 18-Cell Flats

Table 3 Effect of Seedling Age on Growth and Yield of Chin Keng Pak Choi Grown in 18-Cell Flats

Table 4 Effect of Seedling Age on Growth and Yield of Pak Choi Grown in 18-Cell Flats

Table 4 Effect of Seedling Age on Growth and Yield of Pak Choi Grown in 18-Cell Flats

Table 5 Physical Properties of Peat-Based Media Amended with Perlite.

Table 5 Physical Properties of Peat-Based Media Amended with Perlite.Table 6 Changes in Growth Period and Yield of Pak Choi and Leafy Lettuce Grown in Media Amended with Perlite

Table 6 Changes in Growth Period and Yield of Pak Choi and Leafy Lettuce Grown in Media Amended with PerliteTable 7 Respiration Rate and Storage Life of Different Leaf Vegetables Grown in Soil or in 18-Cell Flats.

Table 7 Respiration Rate and Storage Life of Different Leaf Vegetables Grown in Soil or in 18-Cell Flats.

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