DEGRADABLE MULCH AS AN ALTERNATIVE FOR WEED CONTROL IN LETTUCE PRODUCED ON ORGANIC SOILS

Abstract

Lettuce in Quebec is primarily grown on organic soils, where weed control accounts for 30% of the pre-harvest production cost. As no efficient post-emergence herbicide is available, weeding is done mechanically and manually. Using mulch could be a possible alternative to current practices. Degradable paper mulches (pale, pale/black, black/pale) were compared with polyethylene mulches (white/black or black) and a manually weeded unmulched control. The experiment designed as a randomized complete block with 4 blocks was carried out in 1999 and 2000. Polyethylene or paper mulches with at least one black side were effective in controlling weed growth in both seasons. During a warm season in 1999, air and soil temperatures of the paper-mulched plots were similar to those of the control. Day temperatures for the white/black polyethylene mulch were reduced by 2.8°C for air and 0.9°C for soil compared with the control. Soil humidity was highest under the polyethylene followed by the paper and finally the unmulched control. Lettuce grown on paper or polyethylene mulches had 25% greater marketable yield, with heads significantly heavier than those of unmulched lettuce. In 2000, the air temperatures for all mulches were similar to and 1°C above those of the control. When the black side of mulches was exposed to the sun, the soil temperature increased by 2.8°C compared with treatments where the pale side of the mulch was exposed. Lettuce grown in treatments with higher soil temperatures had longer stems. Soil moisture was similar in mulched and unmulched plots during the cool and rainy 2000 season. Growing lettuce on paper or polyethylene mulches increased marketable yield by 7% and resulted in significantly heavier heads compared with the control. Although similar yields were obtained with both paper and polyethylene mulches, the former has the environmental and practical advantage of being able to degrade in the soil at the end of the season. INTRODUCTION Quebec lettuce, valued at CAN $34 million and accounting for 77% of Canada's total production, is the province's leading vegetable crop (Statistics Canada, 2002). Lettuce is mainly grown on organic soils in the southwestern part of the province. A major challenge in producing lettuce on organic soil is weed control, since no efficient post-plant herbicide is available. Mechanical and hand weeding may represent up to 40% of pre-harvest production costs (Naegely and Greenleaf, 1999). Paper mulch has been a well-known method of weed control for close to a century (Flint, 1928; Smith 1931). The discovery of plastics brought a new era in the horticulture industry. Emmert (1955) developed many principles of plasticulture with his research on mulches and row covers. The benefits of using polyethylene in vegetable crop production include easy mechanical installation, conservation of soil moisture, reduction of soil compaction, as well as weed control if the plastic is opaque to photosynthetically active radiation (Waggoner et al., 1960). These many benefits resulted in the rapid adoption of polyethylene by growers. Clear and infrared transmitting types of plastic mulch have been adapted to promote Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 112 earliness in warm-season crops (Loy and Wells, 1990; Jenni et al., 1998). A highly reflective mulch such as coextruded white/black polyethylene may be better suited to cool-season crops such as lettuce, since the upper surface of the mulch reflects incoming light, thereby reducing surface temperatures (Carnell, 1996). Although there are many benefits associated with the use of plastic mulches, increasing costs and concerns about plastic removal and disposal has sparked a renewed interest in biodegradable paper mulches (Anderson et al., 1995; Cappelletto et al., 1998; Schonbeck and Evanylo, 1998; Shogren, 2000). Paper mulches have not yet been widely adopted for commercial field production, primarily because of their rapid degradation in an unprotected environment (Anderson et al., 1995; Schonbeck and Evanylo, 1998). New paper mulches with polymer coating have been developed to improve the resistance of the material to breakdown. Previous papers mulches were either pale or black in color (Brault et al., 2002b). Black (paper or polyethylene) mulches offer complete weed control and increase the temperature of the air and soil environment close to the plant as a result of the mulch's absorption of solar radiation. Brault et al. (2002a) found weed growth under pale paper mulch. They attributed this to light transmitted in the PAR range, which increased from 4% at mulch placement to 10% after 42 days of field exposure. The development of a paper mulch with a black side for weed control and a pale side to reduce solar radiation absorption may offer the best overall combination to enhance growth of this cool-season crop. The general objective of this work was to compare the agronomic performance of paper mulches with a black side and a pale side to that of a coextruded white-over-black polyethylene mulch in head lettuce crops grown on organic soil. More specific objectives were: (1) to evaluate whether having the black side or the pale side of the paper mulch as the upper surface affects the air and soil temperature around the plant, and hence the yield and maturity of cripshead lettuce; (2) to compare soil moisture levels under paper and polyethylene mulches; and (3) to evaluate the field degradation of paper mulches with a polymer coating on one or two sides. MATERIALS AND METHODS The experiments were carried out in Napierville, Quebec, Canada (lat. 45°11’N, long. 73°25’W) in a well-decomposed organic soil (humisol). The paper mulch used in the treatments was a kraft paper (83 g⋅m, Cascades Multi-Pro Inc., Drummondville, Quebec), either pale on both sides or black on one side and pale on the other, with a polymer coating on one side (coating in contact with the soil) or two sides. In 1999, the mulches consisted of a pale-colored (P) paper with a coating (C1) on one side only, paleover-black (P/B) papers with three types of polymer coatings (C2, C3, C4), and a whiteover-black (W/B) coextruded polyethylene (0.028 mm, Plastitech Culture, Saint-Remi, Quebec). Since the paper mulches rapidly degraded in 1999, a new series of three papers with a polymer coating on both sides were tried in 2000. The mulches included a palecolored (PC1) paper, pale-over-black (P/B) paper with three types of polymer coatings (C5, C6, C7), two black-over-pale (B/P) paper mulches with two types of polymer coatings (C5, C6), white-over-black (W/B) polyethylene and black (B) polyethylene (0.025 mm, Plastitech Culture, Saint-Remi, Quebec). Weeded and non-weeded controls were included in both years to evaluate weed populations. Treatments were replicated four times in a complete randomized block design. Plots consisted of raised beds 14.6 m long, 0.15 m high and 0.76 m wide. A pre-plant herbicide (paraquat) was applied in 1999, but none was used in 2000. Mulches were laid manually onto wet soil on June 10, 1999 and June 5, 2000. Planting holes (5.0 cm diameter) were made manually at 35.5 cm intervals in two rows spaced 30.5 cm apart. ‘Ithaca’ crisphead lettuce was seeded in a greenhouse on May 20, 1999 and May 11, 2000 and transplanted in the field on June 15, 1999 and June 14, 2000. Plants were irrigated once after transplanting and then depended on natural rainfall. Air and soil temperatures were measured in one block using copper-constantan thermocouples connected to an AM-416 multiplexer and a CR-10 data logger (Campbell

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