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The functionality of biodegradable mulch can be evaluated in agricultural field settings by visually assessing mulch intactness over time (a measure of deterioration), but it is unclear if mulch deterioration is indicative of mulch degradation as measured by mechanical properties (like breaking force and elongation). This 3-year study (2010–12) examined mulch percent visual deterioration (PVD) during the summer growing season in open-field and high tunnel production systems, and compared these to mulch mechanical properties at mulch installation (12–30 May), midseason (22 July–9 Aug.), and season end (6–25 Oct.), to determine if the field-based measures reliably predict degradation as revealed by changes in mulch mechanical properties. Four different types of biodegradable mulches [two plastic filmmulches marketed as biodegradable (BioAgri and BioTelo); one fully biodegradable paper mulch (WeedGuardPlus); and, one experimental spunbonded plastic mulch designed to biodegrade (SBPLA)] were evaluated against a standard nonbiodegradable polyethylene (PE) mulch where tomato (Solanum lycopersicum L. cv. Celebrity) was planted as the model crop. Each year for the 3 years, PVD increased earlier for WeedGuardPlus than the other mulches in both the high tunnel and open field, and WeedGuardPlus had the greatest PVD in both high tunnels and the open field (6% and 48%, respectively). Mechanical strength of WeedGuardPlus also declined by the end of the season both in the high tunnel (up to 46% reduction) and in the open field (up to 81% reduction). PVD of BioAgri and BioTelo reached a maximum of 3% in the high tunnel and 28% in the open field by the end of the season. Mechanical strength of BioAgri and BioTelo did not change over the course of the season in either the open field or high tunnel, even though the ability of these mulches to elongate or stretch declined 89% in the open field and 82% in the high tunnel. SBPLA and PEmulches did not show a change in PVD ormechanical properties in either the high tunnel or the open field. Overall, PVD was three to six times greater by midseason in the open field than in the high tunnels. Although there were significant relationships between visual assessments and various mechanical properties for each mulch except SBPLA, the relationships differed for eachmulch when evaluated separately and had coefficients of determination (R) below 30%. Furthermore, PVD overestimated mechanical deterioration of BioAgri and BioTelo. Results of this study indicate that mulch visual assessments may reflect general trends in changes in certain mechanical properties of the mulch; however, visual assessment and mechanical properties provide different information on deterioration. Each should be used as needed, but not as a substitute for each other. PE plastic mulch is used widely in specialty crop production to control weeds, conserve soil moisture, increase crop yields, modify soil temperature, and shorten the time to harvest (Hill et al., 1982; Schonbeck, 1998; Schonbeck and Evanylo, 1998; Shogren, 2000). These benefits provide farmers worldwide with significant horticultural and economic advantages (Takakura and Fang, 2001). However, the widespread use of PE mulch creates removal and disposal costs to growers as well as costs to the environment due to waste plastics being buried in landfills and on farm, or burned. As an alternative, plastic mulches that are biodegradable have been developed. These products first appeared on the market in the 1980s, and may bemade from renewable, natural, or sustainable feedstocks (Hayes et al., 2012; Miles et al., 2009). They provide the benefits of PE mulch, such as weed suppression and improved crop yields, and are purported to completely degrade between crop seasons (Cowan et al., 2014; Miles et al., 2012; Minuto et al., 2008; Moreno and Moreno, 2008). Because farmers must bear the annual cost of mulch removal and disposal, estimated at $250 per hectare (Shogren and Hochmuth, 2004), the ability to till mulch into the soil following harvest with confidence that biodegradation will occur is an economic incentive (Olsen and Gounder, 2001). Additionally, the environmental benefit of a mulch that degraded completely in soil without producing toxic byproducts would be highly desirable. Biodegradable plastics typically undergo a two-phase degradation process: disintegration or weathering during use in the field, and biodegradation, which occurs after being incorporated into the soil (Krzan et al., 2006; Kyrikou and Briassoulis, 2007). Moisture, temperature, and light are key factors that impact disintegration, and interactions among these factors may further enhance degradation (Hakkarainen, 2002; Ho et al., 1999; Krzan et al., 2006). Disintegration can lead to decreased molecular weight and increased water solubility of the plastic, which subsequently facilitates biodegradation (Lucas et al., 2008). During biodegradation, biotic processes mineralize the polymer fragments to carbon dioxide, water, and microbial biomass (Lucas et al., 2008). In this study, the word ‘‘deterioration’’ is used to reflect a reduction in the function or visual intactness of the mulch without respect to any particular mode of action or biological process. Parameters typically measured in the field and laboratory to evaluate deterioration of biodegradable mulch films include reduction in soil coverage, weight loss, and changes in mechanical properties. Measuring the decrease in soil coverage provided by mulch is relatively inexpensive and can be performed in the field without the use of special equipment. Measuring changes in mulch weight requires either large samples, or sensitive laboratoryquality balances, and cannot be reliably performed in the field. Althoughmeasuring mulch weight is less subjective than a visual assessment, weightmeasurements are subject to error because of soiled samples.Measuring changes in mulchmechanical properties, includingbreaking HORTSCIENCE VOL. 51(3) MARCH 2016 245 force and elongation to break, among others, are objective and accurate, but requires specialized laboratory equipment and climate-controlled facilities. If visual assessment of soil coverage reliably predicts degradation of biodegradable plastic mulches, the need for more expensive and less accessible methods may be obviated, despite the potential for greater accuracy. To quantify biodegradable mulch deterioration and compare mulch performance in the field,Miles et al. (2012)measured the number of rips, tears, and holes (RTH), as well as the PVD in three distinct climates of the United States. However, the authors found RTH to be an unreliable measure of deterioration due to the coalescence of RTH over time, leading to underestimation of deterioration. Other studies have used similar observations to visually rate or evaluate loss of integrity of biodegradablemulch on the soil surface (Minuto et al., 2008; Moreno and Moreno, 2008; Moreno et al., 2009; Ngouajio and Ernest, 2005; Ngouajio et al., 2008), but none have been adopted as a standard of measurement. In addition, other investigators have evaluated changes in mulch mechanical properties over time to evaluate deterioration (Briassoulis, 2006, 2007; Candido et al., 2006; Cascone et al., 2008; Kijchavengkul et al., 2008; Scarascia-Mugnozza et al., 2006; Tocchetto et al., 2001). Martı́n-Closas et al. (2007 and 2008) implied that visual observations of biodegradable mulch deterioration were related to changes in mechanical properties, though no statistical comparisons were made between the measures. Studies that statistically analyze the relationships between visual assessments of mulch deterioration in the field and mechanical properties in the laboratory have not yet been reported. However, an increased awareness and understanding of such relationships could contribute to more accurate interpretations about visual measures of deterioration. The research reported here builds on the work of Miles et al. (2012), and seeks to determine whether visual assessments of biodegradable mulch intactness during the growing season (independent variable) predict statistically significant changes in mechanical properties (dependent variables). Additionally, the study was designed to contrast mulch deterioration in open-field and high tunnel production environments to expand on potential agricultural settings. Tomato (Solanum lycopersicum cv. Celebrity) was selected as the model crop for this study because it is an important vegetable crop throughout the United States and plastic mulch is a common and important component of tomato production systems (Lamont, 1993). Materials and Methods The field portion of this study was carried out at Washington State University (WSU) NorthwesternWashington Research and Extension Center at Mount Vernon, WA. Soil type is a Skagit silt loam and a high water table persists during the growing season (76–91 cmbelow the surface) (Klungland and McArthur, 1989; National Resources Conservation Service, 2010). Throughout the study, as well as the 2 years prior, the field site was managed according to the standards for certified organic production set by U.S. Department of Agriculture National Organic Program. However, the field site was not certified for organic production. The laboratory portion of this study was carried out at the Textiles Laboratory in the Department of Apparel, Merchandizing, Design and Textiles, WSU, Pullman, WA. Environmental data were previously presented in Cowan et al. (2014). Experimental design. The experimental design was a split-plot design with four replicate blocks; main plots were arranged as a randomized complete block design. The two main plot treatments were high tunnel and open field, and were assigned randomly within each replication and remained fixed throughout this 3-year study (2010–12). The subplots were fivemulch treatments that included two plastic films marketed as biodegradable [BioAgri (Bi

Visual Assessments of Biodegradable Mulch Deterioration Are Not Indicative of Changes in Mechanical Properties