Hortscience | 2019
Metalized-striped Plastic Mulch Reduces Root-zone Temperatures during Establishment and Increases Early-season Yields of Annual Winter Strawberry
Abstract
Black plastic mulch is used predominantly for winter strawberry (Fragaria 3ananassa Duch.) production in Florida because of its warming effects. However, black plastic mulch can increase heat stress during establishment, especially when growers advance planting dates (e.g., late September) to improve earliness. Consequently, we designed a new plastic mulch film that has a metalized center stripe with black shoulders. We hypothesized that metalized-striped mulch can minimize heat stress during establishment, while maintaining the warming effects of black mulch during winter. We conducted field trials over two seasons to evaluate black mulch, fully metalized mulch, and metalized-striped mulch using two cultivars differing in heat stress tolerance and fruit production patterns: ‘Florida Radiance’ and ‘Florida Beauty’. The effect of plastic mulch type on plant growth and yield was generally consistent across both seasons. Compared with black mulch, metalized-striped mulch reduced afternoon rootzone temperature (RZT) by up to 3.1 8C and reduced the duration of heat stress conditions (RZT > 30 8C) by 119 hours across October and November, but exhibited equivalent soil warming during winter. Yield increases by metalized-striped mulch compared with black mulch ranged from 19% to 34% in the early season (November– January), 6% to 20% in the late season (February–March), and 12% to 26% over the entire season. Statistical significance was detected for the 2016–17 early-season yield and when yield data were expressed on a weekly basis. Compared with black mulch, metalized-striped mulch improved fruit number significantly without affecting fruit weight or canopy area, suggesting that heat stress on black mulch negatively affects flower and fruit development more than plant growth. Weekly fruit yield data indicate that metalized-striped mulch can produce greater yields than fully metalized mulch. Metalized-striped mulch is an easily implementable strategy for reducing establishment heat stress and improving fruit earliness in subtropical winter strawberry production regions. Florida is the primary producer of winter strawberries in the United States because of its mild subtropical climate. The price received by Florida strawberry growers is highly variable throughout the season, with the greatest prices typically seen during the early-season months ofNovember, December, and January. According to U.S. strawberry market data from 2012–17, the average grower price in November was $22.80 per 3.6-kg flat, followed by $18.94, $14.38, $11.40, and $8.88 for the months of December through March, respectively (U.S. Department of Agriculture, 2018). Because the Florida strawberry industry is threatened by new challenges such as international competition, rising production costs, and growing labor shortages, Florida strawberry growers require further improvements in early yield to remain profitable (Wu et al., 2015). Historically, Florida strawberry growers have planted in early to mid October. Bareroot transplants are typically established in raised beds covered with black plastic mulch, which has long been considered important to achieve adequate wintertime soil warming (Brooks, 1959). After planting, the fragile transplants can be exposed to high daily air temperatures exceeding 30 C for up to 6 weeks. During this time, flower buds initiate and develop in the crowns while the plants establish a leaf canopy capable of supporting fruit production, which usually begins in mid November. Yields increase slowly throughout the winter, with peak production occurring in March. To speed up establishment and increase early yields, Florida strawberry growers have recently begun to advance transplanting dates from mid October to late September. As a result, plants are exposed to even greater heat stress conditions than they normally would when planted in October. For example, maximum daily air temperatures were 34 C in the third week of Sept. 2016, but only 28 C in the second week of Oct. 2016. Combining advanced planting dates with black plastic mulch can likely cause excessive heat stress conditions, especially because establishment period soil temperatures often exceed 35 C under black plastic mulch. A number of studies indicate that temperatures above 30 C can induce physiological complications in strawberry, including slowed, abnormal growth (Geater et al., 1997; Hellman and Travis, 1988; Zhang et al., 1997); reduced protein content (Gulen and Eris, 2015); and low rootoxygen consumption (Sakamoto et al., 2016). Plants could experience excess heat stress, which leads to inhibited growth and fruit development when planted on black plastic mulch in late September. To avoid this, growers need an alternative to black plastic mulch. Metalized mulch films have the potential to improve early-season fruit development by alleviating, at least in part, heat stress conditions during the establishment period (Andino and Motsenbocker, 2004; Vos et al., 1995). In general, reflective metalized mulch films can reduce soil temperatures compared with black mulch by reflecting a greater proportion of incoming solar radiation (Ham et al., 1993). Reflective metalized mulch films have proved widely effective at increasing marketable fruit yields compared with black mulch for a number of horticultural crops, including tomato (Solanum lycopersicum) and bell pepper (Capsicum annuum), which are both major crops in Florida (Andersen et al., 2012; Díaz-P erez, 2010; Greer and Dole, 2003; Hutton and Handley, 2007). Relatively few studies have compared reflective mulch films to the standard black plastic mulch for subtropical strawberry production. Perhaps most notably, Albregts and Chandler (1993) found entirely whiteand yellow-painted mulch films to improve early-season yields compared with black plastic mulch for Florida strawberry production. However, black plastic mulch outperformed yellow mulch and white mulch in the late season, possibly resulting from insufficient wintertime soil warming by these two more reflective mulch films. Consequently, we believe that subtropical winter strawberry growers would benefit from a multicolored mulch design that is reflective in the center and black on the shoulders. We hypothesized that by having the dual benefits Received for publication 18 Sept. 2018. Accepted for publication 10 Dec. 2018. Thisworkwas funded by Imaflex, Inc., and the Florida Strawberry Research and Education Foundation. This paper is based on a presentation given during the annual meeting of the Florida State Horticultural Society, which was held 10 to 12 June 2018 in Ft. Lauderdale FL. We thank the Horticulture Lab at the University of Florida’s Gulf Coast Research and Education Center for carrying out the field experiment. We also thank Dr. Vance Whitaker, Dr. Kevin Folta, Dr. Seonghee Lee, and Dr. Ralf Dujardin for their scientific guidance and constructive discussion. Corresponding author. E-mail: [email protected]. This is an open access article distributed under the CC BY-NC-ND license (https://creativecommons. org/licenses/by-nc-nd/4.0/). 110 HORTSCIENCE VOL. 54(1) JANUARY 2019 of reflective mulch and black mulch, metalized-striped mulch should maintain sufficiently cool RZTs during establishment, and sufficiently warm RZTs during the cool winter months. Several previous studies have examined the effect of combining two different colors in one plastic mulch type, mostly by adding a black center stripe to reflective silver mulch. The positive effects of this multicolored mulch type compared with entirely black or silver mulch include increased canopy-level light capture, soil warming during spring establishment, improved yield of bell pepper, and reduced incidence of virus symptoms in tomato (Csizinszky et al., 1999; Díaz-P erez, 2010; Hutton and Handley, 2007). Metalized mulch with a black center stripe is meant to warm transplants during spring establishment and reduce soil temperatures during hot summer months, which is the opposite soil warming pattern that we expected to observe using black mulch with a metalized center stripe. To our knowledge, the strategy of using black plastic mulch with a metalized center stripe has never been evaluated for strawberry production, or any other crop production system. The objective of our study was to examine the effects of metalized-striped plastic mulch on soil temperature, plant growth, fruit yield, and earliness using two of Florida’s most current early-yielding strawberry cultivars. ‘Florida Radiance’, which currently accounts for about 60% of Florida’s strawberry market, is a short-day cultivar not recommended for September planting because it has a relatively weak plant habit and fruit can become elongated and unmarketable under high temperatures (Whitaker et al., 2008). ‘Florida Beauty’ is an early-yielding, weak day-neutral that possesses a compact canopy, making it well-suited for advanced planting dates (Whitaker et al., 2017). ‘Florida Radiance’ is a seasonal flowering genotype, so its flowering and fruiting are tightly controlled by photoperiod and temperature, whereas ‘Florida Beauty’ is an everbearing genotype that can basically produce flowers and fruit throughout the entire growing season (Heide et al., 2013). To understand the potential importance of using a multicolored mulch film, we compared metalized-striped mulch against fully metalized mulch as well as the industry standard black plastic mulch. To follow recent Florida strawberry-growing trends of advancing planting from mid October to late September, we examined the effect of metalized-striped mulch when planting was advanced to late September in two seasons. Materials and Methods Bed preparation and plant establishment. Strawberry field trials were conducted at the University of Florida’s Institute of Food and Agricultural Science Gulf Coast Research and Education Center (GCREC) in Balm, FL, during the 2016–17 and 2017–18 winter and spr