The Pacific Northwest (PNW, composed of Washington and Oregon in the United States and British Columbia in Canada) is an important region for floricane-fruiting red raspberry (Rubus idaeus L.) production (DeVetter et al., 2020; USDA NASS, 2019). Typically, growers plant bareroot canes or root cuttings in the spring, allow plants to establish the year of planting, and begin harvesting fruit from floricanes the next summer. Subsequent floricane production over future years allows the planting to produce fruit annually for the life of the planting.
Raspberry growers in the PNW have recently started to transition from using bareroot canes and root cuttings to TC transplants (P. Moore, personal communication). TC is a technique that allows nurseries to rapidly produce clean plants that are free from diseases, nematodes, insects, and viruses (Theiler-Hedtrich and Baumann, 1989). In addition, some desirable cultivars are available only as TC transplants, and many other raspberry cultivars are increasingly being offered as TC transplants (Peerbolt, 2020). However, TC transplants are more expensive than traditional forms of planting material. They also can be difficult to establish because they compete poorly with weeds, and the green foliage on TC transplants limits the use of post-plant herbicides. Poor planting establishment may ensue, resulting in a field that produces low yields and possibly with shorter planting longevity, which reduces on-farm profitability.
To promote the establishment and growth of TC transplants, some growers have started planting in late summer (August) because this planting time allows plants a longer period of time for establishment. In this system, TC transplants overwinter and grow primocanes during the first and second years, with first fruit production occurring ≈22 months after planting. Late summer plantings also eliminate the challenges associated with spring planting, such as rain, saturated soils, and poor weather conditions that can delay planting. Although late summer planting systems are promising, weed management is still a challenge.
Plastic mulches, such as PE, have been used globally in agriculture since the 1960s, primarily in annual production systems, and provide the benefits of weed control, improved plant growth, modified soil temperature and moisture, and increased yields (Kasirajan and Ngouajio, 2012; Lamont, 2017; Miles et al., 2012). PE mulch use in perennial systems is limited, but has demonstrated benefits such as improved weed management and establishment in spring-planted TC raspberry (Trinka and Pritts, 1992; Zhang et al., 2019). However, PE mulch removal and disposal can be expensive and is estimated to cost up to $534/ha in horticultural systems with a 2.4-m spacing between bed centers (Velandia et al., 2019). PE mulch also has waste management challenges (Kasirajan and Ngouajio, 2012). Mulch is seldom recycled because of contamination with residual soil and plant debris, high cost of transport, and limited availability of recycling facilities that can handle PE mulch waste (Grossman, 2019; Levitan and Barros, 2019). It is estimated that less than 10% of agricultural PE mulch in the United States is recycled, and most is landfilled, stockpiled, or burned (Goldberger et al., 2015; Kasirajan and Ngouajio, 2012; Levitan and Barros, 2019; Miles et al., 2017).
One potential alternative to PE mulch is BDM. BDMs are designed to biodegrade at least 90% into CO2, water, and microbial biomass through microbial activities within 2 years under ambient soil conditions or standardized laboratory testing conditions [American Society for Testing and Materials (ASTM) 5988-18, 2018; Brodhagen et al., 2015; European Norms (EN) 17033, 2018]. In the past decade, many studies have demonstrated that BDMs can provide benefits comparable to PE mulch in annual production systems in terms of weed management, soil temperature and moisture modification, and yield enhancement (Anzalone et al., 2010; Costa et al., 2014; DeVetter et al., 2017; Ghimire et al., 2018; Miles et al., 2012). Few studies have explored the use of BDMs in perennial production systems. Mulch requirements for perennial systems are different from those for annual systems because perennial crops typically require a longer mulching period. Furthermore, mulch incorporation via tillage cannot be done annually in perennial systems. Therefore, it is unknown whether BDMs designed for relatively short periods of use are suitable in perennial production systems like floricane-fruiting raspberry.
Trinka and Pritts (1992) found PE mulch suppressed weeds and provided improved soil moisture and temperature for primocane-fruiting red raspberry in New York. In addition, Król-Dyrek and Siwek (2015) and Tecco et al. (2016) demonstrated that BDMs suppressed weeds and improved primocane-fruiting raspberry yield. However, those studies were predominately focused on primocane-fruiting raspberry, which differs physiologically and in its cultivation relative to floricane-fruiting raspberry (DeVetter et al., 2020). Although Zhang et al. (2019) found both PE mulch and BDMs provided weed control and increased yield of floricane-fruiting raspberry compared with the standard grower practice of BG cultivation (herbicides plus handweeding), the mulches were removed after 10 months. In addition, all of the previously mentioned studies were conducted in a spring-planted system in which growers expect mulch to stay in place for ≈6 to 10 months. In contrast, growers expect the mulch to stay in place for 18 to 24 months during the establishment phase of summer-planted floricane-fruiting raspberry.
The objectives of the current study were to evaluate the deterioration of PE mulch and BDMs, mulch mechanical properties after agricultural weathering, and the impacts of PE mulch and BDMs on soil temperature and moisture, plant growth, yield, fruit quality, and weed suppression in a late summer planting of floricane-fruiting raspberry established as TC transplants. This study contributes knowledge regarding the feasibility and implications of using PE mulch and BDMs in late summer floricane-fruiting raspberry plantings established using TC transplants in the PNW, as well as general information about the use of PE mulch and BDMs in perennial fruit crops.
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