Multiple Plants per Transplant Plug in Processing Tomatoes: Is There a Benefit? Gene Miyao 1, Michelle Le Strange 2 and Mike Murray 3 1 University of California, Cooperative Extension, 70 Cottonwood Street, Woodland, CA 95695, USA 2 University of California, Cooperative Extension, 4437 S. Laspina Street, Suite B, Tulare, CA 93274, USA 3 University of California, Cooperative Extension, P.O. Box 180, Colusa, CA 95932, USA Keywords: Lycopersicon esculentum, quality, yield, stand establishment, California Abstract California growers of processing tomatoes have long understood that a desirable target for a direct-seeded stand is multiple plants in a clump. Field studies have consistently demonstrated slightly higher yield with multiple-plant compared to single-plant configured stands. Before the development of vacuum planters, mechanical plate planters could commonly only deliver 2 to 4 seeds in a clump. The practice of multi-seeding was horticulturally advantageous as well as practical. In 1990, a few growers began switching to transplants as a response to increased seed and labor prices. Today, in California, an estimated 70% of the 113,000 hectares of canning tomato are transplanted. While the transplant industry examined multiple plants per cell, the practice did not become the norm. In 2002, applied field research in Colusa County indicated a benefit from multiple plants per plug. In 2004, testing at Fresno s Westside Station supported the Colusa findings. Plug population density studies were also conducted in Yolo County. The earliest study in Colusa, demonstrated substantial yield gains of 9 to 15% when planting 2 or 3 plants per plug (117 t/ha was the baseline with single plants). As further tests were conducted, the results were both encouraging and substantial. However, the results were also mixed, both within and as well among locations. INTRODUCTION Tomatoes consistently rank in the top 10 of agricultural commodities of California. The farm gate value of California tomatoes, fresh market and processing combined, is over $1 billion (California Department of Food and Agriculture). Since 1990, processing tomatoes were planted on 100,000 to 120,000 ha in California (National Agricultural Statistics Service) with an annual farm value of about $500 million (California Tomato Growers Association). In the 1950s, tomatoes were transplanted as bare rooted seedlings. This stand establishment method posed many challenges and the practice of direct seeding soon replaced the bare rooted transplants (Sims, 1980). Management practices for the establishment of the seedlings include coordinated efforts for bed preparation, weed control and irrigation. All these practices have been optimized to manage the critical plant establishment stage. Early direct seeding field studies indicated yield increases with multiple plants in a clump over single plants (Zahara, 1970). Clump planting or thinning high seedling populations to leave 2 to 3 plants per clump became the practice for direct seeding with 20-cm spacing between clumps. In 1990, growers began moving from direct seeding to transplanting. In 1995, some 30% of the California processing tomatoes was transplanted (Hartz and Miyao, 1996). Ten years later, 70% of the production is transplanted with greenhouse-grown seedling plugs produced in cells with potting soil. Following the standard practice for fresh market tomatoes, the norm has been to sow a single seed per cell in the transplant trays. Studies in two key areas for processing tomato production, southern Sacramento Valley (Colusa and Yolo counties) and Fresno County, examined the effects of multiple Proc. IV th IS on Seed, Transplant and Stand Establishment of Hort. Crops Ed.: D.I. Leskovar Acta Hort. 782, ISHS 2008 171
plants per transplant plug on fruit yield. MATERIALS AND METHODS Field trials were conducted in three geographic locations in the Central Valley of California as independent tests over a 5-year period beginning in 2002. Trial design was primarily a randomized complete block with smallest plot size 46.5 m 2. Bed width was 1.5 m center to center of bed. Tests were primarily located in commercial fields and managed by the grower according to local commercial practice. In the commercial fields, mechanical harvesters picked fruit, which were weighed in specialized portable highcapacity tubs outfitted with scales. Fruit quality was measured by the Processing Tomato Advisory Board (PTAB), which operates commercial inspection stations to grade fruit. PTAB measurements included Brix, ph and color. Five common tomato cultivars ( H 9492, Halley, AB2, AB5 and APT 410 ) were selected. Transplants were all grown in commercial greenhouses and seeded either as single or multiple plants per cell. Planting mix and growing conditions were uniform for each test. Common commercial transplant trays were used with cell dimensions of 2.5 2.5 5 6 cm deep with a volume around 20 cc. Transplants were mechanically planted by a commercial crew, with the exception of a few tests that were hand planted. Within row spacing was between 30 and 40 cm, except in the Fresno tests that compared within row spacing as a factor and included an extreme spacing of 71 cm. RESULTS The number of plants per plug had a mixed effect on fruit yield. The initial test conducted in Colusa indicated substantial yield increase of 9% with two plants per plug compared to a single plant and further gains with 3 plants per plug (Table 1). Tests in Fresno County were not consistent in yield response, but were mostly positive. In Fresno, the improvement in yield with multiple plants per plug occurred especially when stands were sparse as reflected in the 71 cm in-row space treatment (Table 2). In these Fresno tests, yield response was more common across the years of tests when plug population was low (Table 3). Common in-row spacing is 36 cm between plugs, which results in a density of 17,300 plants/ha. Tests over three years in Yolo County, which used more common in-row spacing, have never shown a yield increase with double plants per plug over singles (Table 4). Considering the average yield, across all locations and years, but removing the extreme wide spacing in Fresno tests, yields were increased 3.1 t/ha with double plants compared to singles (95.2 compared to 92.1 t/ha). Yields were not further increased, on average, by using triple plants per plug compared to doubles. Fruit quality was generally not affected when measuring Brix, ph or color. DISCUSSION The seed component of transplants is approaching $10 per 1,000 seed (hybrid processing tomato seed). A common transplant rate is about 17,300 plugs per hectare. At this planting rate, a double seeded plug would increase the seed cost by $173 per hectare. With a 3.1 t/ha yield increase, the breakeven price per ton is $56. Year 2006 price per ton in California was $63.92/metric ton. Therefore, double seeding can be profitable for the grower, if the yield improvement can be achieved. If multiple plants per plug were widely used, greenhouse efficiency would be increased as a result of fewer blank cells per tray. Greenhouse management may need to increase as plants will likely increase in height as well as decrease in stem girth. The higher density may also result in increased foliar plant disease. CONCLUSIONS Since 1990 in California, stand establishment for processing tomatoes has shifted from direct seeding to transplants. Fresh market tomatoes have long been transplanted, with the standard being a single seed in each plug. In these studies, multiple plants per 172
plugs were compared to the more standard single plant per plug. In cases where stands are thin (8,600 plants/ha), multiple plants offer a clear economic advantage. In our experience, economic gains are achieved with double plants per plug. However, while yield gains have been achieved with double plants per plug compared to singles, yield improvement was more consistently observed in only the extreme in-row spacing (7 of the 8 cases). With normal in-row spacing, statistical improvement in yield was observed for only 6 of 22 cases. Fortunately, yield declines have not resulted from the multiple plant method. We remain cautious in suggesting that yield improvement occurs with multiple plants per plug over singles. ACKNOWLEDGEMENTS Field studies would not be possible without the collaboration of growers: J.H. Meek and Sons, D. Rominger & Sons, Button and Turkovich Ranches, Jim Borchard, Harris Ranch, Farming D and Five Star Ranch. Assistance was provided by Mark Kochi, field assistant, Yolo County, and summer students Fredrick Schulten-Baumer, Dan Sweet, Matt Rooney, Wes Bates, Ariel Rivers, Lane Dickson, and Julie Reimers. Generous support was provided by Westside Transplants, Timothy, Stewart & Lekos Seeds, the Processing Tomato Advisory Board and the California Tomato Research Institute. Literature Cited CDFA. California Agricultural Highlights. 2005. (PDF Brochure). http://www.cdfa.ca.gov/. California Tomato Growers Association. 2006. http://www.ctga.org/. Hartz, T. and Miyao, G. 1996. Processing Tomato Production in California. University of California. Publ. 7228. National Agricultural Statistics Service. USDA. http://usda.mannlib.cornell.edu/usda/ers/92010/tab007.xls. Sims, W.L. 1980. History of the production of tomatoes for processing in the USA. Acta Hort. 100:27-30. Zahara, M. 1970 Influence of plant density on yield of process tomatoes for mechanical harvest. Journal American Society Hort. Sci. 95:510-12. 173
Tables Table 1. Effect of multiple plants per transplant plug on processing tomato yield and fruit quality, cultivar H 9492, Colusa, 2002. # plants Yield Yield per plug ph Color Brix (t/ha) increase 1 4.42 22.5 4.4 117-2 4.46 22.8 4.6 128 9% 3 4.39 23.5 4.6 134 15% LSD.05 NS NS NS 6.7 Table 2. Effect of plant spacing x cultivar x plants per plug on yield (t/ha) of processing tomatoes, Fresno, 2006. AB 2 AB 2 Halley Halley at 36 cm at 71 cm at 36 cm at 71 cm Plants/plug Yield Yield Yield Yield Average 1 117 96 117 100 107 2 117 109 118 111 114 3 117 110 122 111 115 Average 117 105 119 107 statistical significance at p= 0.05 for: #plants per plug; within-row spacing; variety x spacing; spacing x #plants Table 3. Influence of #plants per plug on yield with low plant populations, Fresno, 2004-2006. Yield (t/ha) Statistical Rows/ Plugs/ha Plants per plug significance Location Year Cultivar bed ( 1000) single double triple at 0.05 1 Fresno 2004 T1 Halley 1 8.6 46 63 58 yes 2 Fresno 2004 T1 AB 2 1 8.6 39 50 57 yes 3 Fresno 2005 T2 Halley 1 9.2 93 91 88 NS 4 Fresno 2005 T2 AB 2 1 9.2 101 111 108 yes 5 Fresno 2006 T3 Halley 1 9.2 100 109 110 yes 6 Fresno 2006 T3 AB 2 1 9.2 96 109 110 yes 7 Fresno 2006 T4 Halley 1 9.2 63 70 - yes 8 Fresno 2006 T4 AB 2 1 9.2 65 75 - yes Average 75.2 84.7 174
Table 4. Compilation of yield (t/ha) from 3, county-located tests comparing multiple plants per plug, 2002-2006. Yield (t/ha) Statistical Rows/ Plugs/ha Plants per plug significance Location Year Variety bed ( 1000) single double triple at 0.05 1 Colusa 2002 T1 H 9492 1 17.2 117 128 134 Yes 2 Colusa 2003 T3 H 9492 1 17.2 66 74 79 NS 3 Colusa 2003 T3 Halley 1 17.2 60 71 63 NS 4 Fresno 2004 T1 Halley 1 17.2 56 66 69 Yes 5 Fresno 2004 T1 AB 2 1 17.2 55 63 61 Yes 6 Fresno 2005 T2 Halley 1 18.5 99 96 90 NS 7 Fresno 2005 T2 AB 2 1 18.5 97 113 112 Yes 8 Fresno 2006 T3 Halley 1 18.5 117 118 122 NS 9 Fresno 2006 T3 AB 2 1 18.5 117 117 117 NS 10 Fresno 2006 T4 Halley 1 18.5 82 89 - Yes 11 Fresno 2006 T4 AB 2 1 18.5 81 89 - yes 12 Yolo 2003 T1 H 9492 1 18.5 73 76 - NS 13 Yolo 2003 T1 Halley 1 18.5 69 70 - NS 14 Yolo 2003 T2 AB 2 1 17.2 124 118 - NS 15 Yolo 2003 T2 AB 5 1 17.2 120 121 - NS 16 Yolo 2005 T3 Halley 1 17.2 104 101 - NS 17 Yolo 2005 T3 AB 2 1 17.2 97 101 - NS 18 Yolo 2006 T4 APT 410 2 25.8 118 113 - NS 19 Yolo 2006 T4 H 9280 2 25.8 110 111 - NS 20 Yolo 2006 T5 HyPeel 45 2 25.8 93 97 - NS 21 Yolo 2006 T5 AB 2 2 22.5 89 86 - NS 22 Yolo 2006 T5 H 9780 2 22.5 83 78 - NS Average 92.1 95.2 94.0 T1 = trial 1, T2 = trial 2, etc. NS = non significant. 175
176