HORTSCIENCE 54(1):110–116. 2019. https://doi.org/10.21273/HORTSCI13583-18

Metalized-striped Plastic MulchReduces Root-zone Temperaturesduring Establishment andIncreases Early-season Yields ofAnnual Winter StrawberryStephen S. Deschamps and Shinsuke Agehara1

Department of Horticulture, University of Florida, Gulf Coast Research andEducation Center, 14625 County Road 672, Wimauma, FL 33598

Additional index words. aluminized, earliness, Fragaria 3ananassa Duch., heat stress,microclimate, plasticulture, subtropical

Abstract. Black plastic mulch is used predominantly for winter strawberry (Fragaria3ananassa Duch.) production in Florida because of its warming effects. However, blackplastic mulch can increase heat stress during establishment, especially when growersadvance planting dates (e.g., late September) to improve earliness. Consequently, wedesigned a new plastic mulch film that has a metalized center stripe with black shoulders.We hypothesized that metalized-striped mulch can minimize heat stress duringestablishment, while maintaining the warming effects of black mulch during winter.We conducted field trials over two seasons to evaluate black mulch, fully metalizedmulch, and metalized-striped mulch using two cultivars differing in heat stress toleranceand fruit production patterns: ‘Florida Radiance’ and ‘Florida Beauty’. The effect ofplastic mulch type on plant growth and yield was generally consistent across bothseasons. Compared with black mulch, metalized-striped mulch reduced afternoon root-zone temperature (RZT) by up to 3.1 8C and reduced the duration of heat stressconditions (RZT > 30 8C) by 119 hours across October and November, but exhibitedequivalent soil warming during winter. Yield increases by metalized-striped mulchcompared 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 theentire season. Statistical significance was detected for the 2016–17 early-season yield andwhen yield data were expressed on a weekly basis. Compared with black mulch,metalized-striped mulch improved fruit number significantly without affecting fruitweight or canopy area, suggesting that heat stress on black mulch negatively affectsflower and fruit development more than plant growth. Weekly fruit yield data indicatethat metalized-striped mulch can produce greater yields than fully metalized mulch.Metalized-striped mulch is an easily implementable strategy for reducing establishmentheat stress and improving fruit earliness in subtropical winter strawberry productionregions.

Florida is the primary producer of winterstrawberries in the United States because of itsmild subtropical climate. The price receivedby Florida strawberry growers is highly vari-able throughout the season, with the greatest

prices typically seen during the early-seasonmonths ofNovember, December, and January.According to U.S. strawberry market datafrom 2012–17, the average grower price inNovember was $22.80 per 3.6-kg flat, fol-lowed by $18.94, $14.38, $11.40, and $8.88for the months of December through March,respectively (U.S. Department of Agriculture,2018). Because the Florida strawberry industryis threatened by new challenges such as inter-national competition, rising production costs,and growing labor shortages, Florida strawberrygrowers require further improvements in earlyyield to remain profitable (Wu et al., 2015).

Historically, Florida strawberry growershave planted in early to mid October. Bare-root transplants are typically established inraised beds covered with black plastic mulch,which has long been considered important toachieve adequate wintertime soil warming(Brooks, 1959). After planting, the fragiletransplants can be exposed to high daily air

temperatures exceeding 30 �C for up to 6weeks. During this time, flower buds initiateand develop in the crowns while the plantsestablish a leaf canopy capable of supportingfruit production, which usually begins in midNovember. Yields increase slowly through-out the winter, with peak production occur-ring in March.

To speed up establishment and increaseearly yields, Florida strawberry growers haverecently begun to advance transplanting datesfrom mid October to late September. As aresult, plants are exposed to even greater heatstress conditions than they normally wouldwhen planted in October. For example, max-imum daily air temperatures were �34 �C inthe third week of Sept. 2016, but only 28 �Cin the second week of Oct. 2016. Combiningadvanced planting dates with black plasticmulch can likely cause excessive heat stressconditions, especially because establishmentperiod soil temperatures often exceed 35 �Cunder black plastic mulch. A number of studiesindicate that temperatures above 30 �C caninduce physiological complications in straw-berry, including slowed, abnormal growth(Geater et al., 1997; Hellman and Travis,1988; Zhang et al., 1997); reduced proteincontent (Gulen and Eris, 2015); and low root-oxygen consumption (Sakamoto et al., 2016).Plants could experience excess heat stress,which leads to inhibited growth and fruit de-velopment when planted on black plastic mulchin late September. To avoid this, growers needan alternative to black plastic mulch.

Metalized mulch films have the potentialto improve early-season fruit developmentby alleviating, at least in part, heat stressconditions during the establishment period(Andino and Motsenbocker, 2004; Vos et al.,1995). In general, reflective metalized mulchfilms can reduce soil temperatures comparedwith black mulch by reflecting a greaterproportion of incoming solar radiation(Ham et al., 1993). Reflective metalizedmulch films have proved widely effective atincreasing marketable fruit yields comparedwith black mulch for a number of horti-cultural crops, including tomato (Solanumlycopersicum) and bell pepper (Capsicumannuum), which are both major crops inFlorida (Andersen et al., 2012; Díaz-P�erez,2010; Greer and Dole, 2003; Hutton andHandley, 2007). Relatively few studies havecompared reflective mulch films to the stan-dard black plastic mulch for subtropicalstrawberry production. Perhaps most notably,Albregts and Chandler (1993) found entirelywhite- and yellow-painted mulch films toimprove early-season yields compared withblack plastic mulch for Florida strawberryproduction. However, black plastic mulchoutperformed yellow mulch and white mulchin the late season, possibly resulting frominsufficient wintertime soil warming by thesetwo more reflective mulch films. Conse-quently, we believe that subtropical winterstrawberry growers would benefit from amulticolored mulch design that is reflectivein the center and black on the shoulders. Wehypothesized that by having the dual benefits

Received for publication 18 Sept. 2018. Acceptedfor publication 10 Dec. 2018.Thisworkwas funded by Imaflex, Inc., and the FloridaStrawberry Research and Education Foundation.This paper is based on a presentation given duringthe annual meeting of the Florida State Horticul-tural Society, which was held 10 to 12 June 2018 inFt. Lauderdale FL.We thank the Horticulture Lab at the University ofFlorida’s Gulf Coast Research and EducationCenter for carrying out the field experiment. Wealso thank Dr. Vance Whitaker, Dr. Kevin Folta,Dr. Seonghee Lee, and Dr. Ralf Dujardin for theirscientific guidance and constructive discussion.1Corresponding author. E-mail: sagehara@ufl.edu.This is an open access article distributed under theCC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

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of reflective mulch and black mulch,metalized-striped mulch should maintain suf-ficiently cool RZTs during establishment,and sufficiently warm RZTs during the coolwinter months.

Several previous studies have examinedthe effect of combining two different colorsin one plastic mulch type, mostly by adding ablack center stripe to reflective silver mulch.The positive effects of this multicoloredmulch type compared with entirely black orsilver mulch include increased canopy-levellight capture, soil warming during springestablishment, improved yield of bell pepper,and reduced incidence of virus symptoms intomato (Csizinszky et al., 1999; Díaz-P�erez,2010; Hutton and Handley, 2007). Metalizedmulch with a black center stripe is meant towarm transplants during spring establishmentand reduce soil temperatures during hotsummer months, which is the opposite soilwarming pattern that we expected to observeusing black mulch with a metalized centerstripe. To our knowledge, the strategy ofusing black plastic mulch with a metalizedcenter stripe has never been evaluated forstrawberry production, or any other cropproduction system.

The objective of our study was to examinethe effects of metalized-striped plastic mulchon soil temperature, plant growth, fruit yield,and earliness using two of Florida’s mostcurrent early-yielding strawberry cultivars.‘Florida Radiance’, which currently accountsfor about 60% of Florida’s strawberry mar-ket, is a short-day cultivar not recommendedfor September planting because it has arelatively weak plant habit and fruit canbecome elongated and unmarketable underhigh temperatures (Whitaker et al., 2008).‘Florida Beauty’ is an early-yielding, weakday-neutral that possesses a compact canopy,making it well-suited for advanced plantingdates (Whitaker et al., 2017). ‘Florida Radi-ance’ is a seasonal flowering genotype, so itsflowering and fruiting are tightly controlledby photoperiod and temperature, whereas

‘Florida Beauty’ is an everbearing genotypethat can basically produce flowers and fruitthroughout the entire growing season (Heideet al., 2013). To understand the potentialimportance of using a multicolored mulchfilm, we compared metalized-striped mulchagainst fully metalized mulch as well as theindustry standard black plastic mulch. Tofollow recent Florida strawberry-growingtrends of advancing planting from midOctober to late September, we examinedthe effect of metalized-striped mulch whenplanting was advanced to late September intwo seasons.

Materials and Methods

Bed preparation and plant establishment.Strawberry field trials were conducted at theUniversity of Florida’s Institute of Food andAgricultural Science Gulf Coast Researchand Education Center (GCREC) in Balm,FL, during the 2016–17 and 2017–18 winterand spring growing seasons. The field site’ssoil (Myakka fine sand siliceous hypothermicOxyaquic Alorthod) had a pH and organicmatter content of 6.8 and 1.5%, respectively.On 10 Sept. 2016 and 14 Aug. 2017, com-mercial equipment was used to prepare theGCREC’s strawberry fields such that all bedswere covered initially with entirely blackplastic mulch. The beds used in our experi-ment measured 91 m long, 81 cm wide at thebase, 71 cm wide at the shoulders, and 25 cmhigh at the bed top. During the initial prep-aration, beds were fumigated with PicClor 60(1,3-dichloropropene + chloropicrin; 303 kg/ha) (TriEst Ag Group, Inc., Greenville, NC)to reduce incidence of soil pathogens andweeds. One line of drip irrigation tubing(0.95 L/h/emitter) with a 30.5-cm emitterspacing was laid 2.5 cm deep at each bed’scenter. To reduce the effect of experimentalmulch types on fumigation efficacy, 0.02-mm-thick black plastic mulch was stretchedover all beds. The experimental mulch typeswere not applied during the initial bed prep-

aration days of 10 Sept. 2016 and 14 Aug.2017. A map showing the experimentallayout and bed dimensions is provided inFig. 1.

On 27 Sept. 2016 and 14 Sept. 2017, fourfull-length beds underwent final preparation,which consisted of replacing the previouslylaid black plastic mulch with the experimen-tal mulch films used in our trials. Each ofthe four beds were divided randomly intoequal-length sections of black mulch, fullymetalized mulch, and metalized-stripedmulch (Can-Block XSB v-TIF silver/black;Imaflex, Inc., Thomasville, NC). Metalizedcoatings on the fully metalized mulch andmetalized-striped mulch are a 25-nm-thicklayer of aluminum. The center stripe on themetalized-striped mulch is 51 cmwide. Over-head photos of each plastic mulch film areprovided in Fig. 1C. The experiment used asplit-plot design, with mulch type as thewhole-plot factor and cultivar as the subplotfactor. During each season, 24 plots wereused to test two cultivars on three mulchtypes across four beds, with each bed servingas a replicate. On 28 Sept. 2016 and 25 Sept.2017, bare-root transplants with three to fourleaves were received from Crown Nursery(Red Bluff, CA). On 29 Sept. 2016, trans-plants of ‘Florida Radiance’ were planted intheir respective plots at a density of 32 plantsper plot; transplants of ‘Florida Beauty’ wereplanted at a density of 16 plants per plot as aresult of plant material constraints from thenursery. On 26 Sept. 2017, transplants of‘Florida Radiance’ and ‘Florida Beauty’were both planted in their respective plotsat a density of 20 plants per plot. Plants werespaced in double rows with 38 cm betweenplanting holes and 30 cm between rows.Following commercial practices, the plantsreceived 9 h of overhead irrigation duringdaylight hours for 10 d after transplanting. Acommercial standard strawberry fertilizer(6N–0.9P–3.3K) was administered throughthe drip tape three times per week at a rate of1.12 kg N/ha/d. Plants were watered daily via

Fig. 1. (A) Experimental layout in the 2016–17 and 2017–18 seasons. (B) Overall bed width, bed shoulder width, and metalized stripe width are provided alongwith the location of planting holes and sensors for bed center temperature (BCT) and bed shoulder temperature (BST). (C) Overhead photos showing ‘FloridaRadiance’ on black (left), fully metalized (middle), and metalized-striped (right) plastic mulches at 70 d after planting in the 2017–18 season.

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drip irrigation. Pest and disease control wereperformed following current recommenda-tions from the University of Florida (Peres,2015).

Soil temperature. Soil temperature wasmonitored from November through Februaryof the 2016–17 season and October throughFebruary of the 2017–18 season. Soil tem-perature was monitored using two DecagonRT-1 temperature sensors in two plots ofeach of the three mulch types; thus, 12 RT-1sensors were used in total. Soil temperaturewas logged every 30 min by EM50G dataloggers (Decagon Devices, Pullman, WA).Soil temperatures were monitored at both thebed center and the bed shoulder. Bed centertemperature (BCT) was recorded at a 10-cmdepth and 20 cm from the bed shouldertoward the bed center. Bed shoulder temper-ature (BST) was recorded at a 10-cm depthand 5 cm from the bed shoulder toward thebed center.

Plant growth. Canopy area measurementswere taken three times during the 2016–17season and six times during the 2017–18season. Overhead photos of each experimen-tal plot were captured using a camera ele-vated on a monopod to a height of 2.1 m overthe bed surface. Canopy area was determinedby analyzing the photos using the imageprocessing software Image J (National In-stitute of Health, Bethesda, MD). The imagescale was set according to the known bedwidth in each photo. Image color thresholdwas determined by adjusting hue, saturation,and brightness values to distinguish greencanopy pixels from all background pixels.Threshold values were determined manuallyfor each day on which photos were taken toaccount for differences in ambient light,cloud cover, and shadow distribution. Afterthresholding, the images were converted to abinary format in which canopy pixels becameblack and background pixels became white.The total area of the black canopy pixels wasmeasured and recorded. Individual plantcanopy areas were determined by dividingthe whole-plot canopy area by the number ofliving plants per plot.

During the 2016–17 season, runners wereremoved, counted, and weighed at 10 and 12weeks after planting. During the 2017–18season, runners were sampled from all plotsat 9 weeks after planting. At the end of the2016–17 season, three plants from each plotwere sampled randomly; separated into roots,leaves, and stems; and then dried at 65 �C for48 h to determine tissue dry weight. At theend of the 2017–18 season, two plants fromeach plot were sampled randomly to deter-mine fresh weight, crown number, petiolenumber, and leaf area.

Fruit yield and quality. Strawberries wereharvested twice weekly from mid Novemberto earlyMarch of both seasons. The harvestedfruits were graded according to U.S. De-partment of Agriculture (2006) grading stan-dards. Fruits weighing more than 10 g andfree from disease, pest, or mechanical dam-age were considered marketable. The numberand weight of fruit from each plot were

recorded. To determine soluble solids content(SSC) for the 2016–17 season, four fruit fromeach ‘Florida Radiance’ plot were sampledon 25 Jan. and 15 Feb. To determine SSC forthe 2017–18 season, four fruit from each‘Florida Radiance’ plot were sampled on 25Jan. Strawberry juice from the aggregatesamples was analyzed with a digital refrac-tometer to measure SSC for each plot.

Statistical analysis. During both seasons,there were six treatments, which were facto-rial combinations of three mulch types andtwo cultivars. There were four replicationsper treatment in both seasons, for a total of 24plots (Fig. 1A). The experiments used a split-plot design, with mulch type as the whole-plot factor and cultivar as the subplot factor.All statistical analyses were performed usingSAS Enterprise Guide 7.1 (SAS Institute,Cary, NC). Soil temperatures were comparedbetween mulch types for each hourly averagewithin a month using standard one-way anal-ysis of variance (ANOVA). Yield, quality,and growth data were analyzed separately foreach planting date by two-way ANOVA. TheGLIMMIX procedure was used to analyzeelongated fruit proportion data for both sea-sons. Mean separation was performed usingthe Tukey-Kramer method, and significancewas established at P < 0.05 unless otherwisenoted.

Results and Discussion

Because early-season strawberries garnerhigher market prices, improving early-seasonfruit yield is important to Florida growers.Metalized mulch films have the potential toimprove early-season fruit development, atleast in part, by alleviating heat stress condi-tions during the establishment period (Andinoand Motsenbocker, 2004; Vos et al., 1995). Incombination with the continued developmentof early-yielding cultivars, which can producefruit at higher temperatures and longer photo-periods than the current major cultivars, met-alized mulch films could prove critical to thelong-term viability of Florida’s strawberryindustry by reducing heat stress and furtherimproving early-season yields.

Root-zone cooling by metalized mulchtypes. High temperatures capable of inducingplant heat stress are a major concern forstrawberry growers, especially during estab-lishment, when plants are developing roots,crowns, and canopies capable of supportingfruit production. Black plastic mulch has highshortwave radiation absorption, causing it toincrease RZTs drastically by conductingthermal radiation toward the bed center(Ham et al., 1993). Conversely, metalizedmulch films have low shortwave radiationabsorption, so they conduct less radiation intothe soil and maintain lower RZTs than blackmulch. To evaluate the RZT cooling effectsof fully metalized mulch and metalized-striped mulch, we monitored BCT and BSTat a 10-cm depth throughout the growingseason.

Because soil temperature data showed thesame trend in both seasons, only the more

comprehensive data from the 2017–18 sea-son, which included October soil temperaturemonitoring, are presented in Fig. 2A. Plantson black mulch experienced significantlyhigher BCTs than plants on fully metalizedmulch or metalized-striped mulch, especiallythroughout the establishment period (Octoberand November). In October, the averageafternoon BCT was reduced significantly by3.7 �C under fully metalized mulch and3.1 �C under metalized-striped mulch com-pared with black mulch (Fig. 2B). In Novem-ber, both fully metalized mulch andmetalized-striped mulch reduced the averageafternoon BCT significantly by 3.2 �C com-pared with black mulch. In addition to re-ducing the magnitude of establishmentperiod heat stress conditions, the metalizedmulch films also reduced significantly theduration of heat stress conditions. Becausemany reports suggest RZTs above 30 �C canresult in poor growth and development ofFragaria species (Geater et al., 1997; Sakamotoet al., 2016; Zhang et al., 1997), we used 30 �Cas a heat stress temperature threshold in ourstudy. Throughout October, BCTs exceeded30 �C for an average of 164 h under blackmulch (Table 1). This duration was reducedsignificantly to 136 h under fully metalizedmulch and reduced nonsignificantly to 153 hunder metalized-striped mulch. ThroughoutNovember, BCTs exceeded 30 �C for anaverage of 108 h, but the duration was reducedsignificantly to only 40 and 46 h under fullymetalized and metalized-striped mulch, respec-tively. Many previous studies have found met-alized mulch films to maintain lower daytimeRZTs than black mulch by reflecting moreincoming shortwave radiation (Díaz-P�erez,2010; Díaz-P�erez and Batal, 2002; Díaz-P�erezet al., 2005; Lamont, 2005; Tarara, 2000). Ourresults of reduced RZTs under more reflectivemulch types are consistent with these previousobservations.

Because they have different shouldercolors, fully metalized mulch and metalized-striped mulch had contrasting effects onBSTs throughout the growing season. Com-pared with black mulch, the average hourlyBST in October was reduced significantlyby as much as 4.1 �C under fully metalizedmulch compared with only 1.1 �C undermetalized-striped mulch (Fig. 2B). The sametrend can be seen throughout November andDecember. By January, however, there wereno significant differences in daytime BSTsbetween black mulch and metalized-stripedmulch. In contrast, fully metalized mulchreduced the average afternoon BSTs in Jan-uary by as much as 2.9 �C compared withblack mulch. The reason that black mulchand metalized-striped mulch caused similarwintertime soil warming is their black shoul-ders. Plant canopies mostly covered the bedcenter by January (Fig. 2C). As a result, onlythe bed shoulders remained exposed to in-teract with incoming solar radiation. Thiscaused greater daytime soil warming underblack mulch and metalized-striped mulch,which both have black shoulders that absorbsolar radiation effectively and conduct it as

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thermal radiation along a temperature gradi-ent toward the bed center. Consequently,metalized-striped mulch did not reduce BCTscompared with black mulch in January andFebruary. Our data indicate that metalized-striped mulch is capable of warming wintersoils just as well as black mulch, which hassoil-warming properties that have long beenconsidered necessary to attain profitable win-ter strawberry yields in Florida (Albregts andChandler, 1993; Brooks, 1959).

Beyond reducing daytime soil tempera-tures compared with black mulch, we alsofound that fully metalizedmulch andmetalized-striped mulch resulted in higher nighttime andpredawn RZTs than black mulch. In October,fully metalized mulch and metalized-stripedmulch increased significantly the average BCTcompared with black mulch for 9 h out of thenight (data not shown), with a maximum in-crease of 1.15 �C occurring just before dawn(Fig. 2B). The ability of fully metalized mulchand metalized-striped mulch to act as insulatorsand trap heat in the root zone at night is a result

of the optical properties of their aluminumcoating—specifically, aluminum’s low emissiv-ity. Emissivity (e) is a measure of the relativeamount of longwave radiation emitted from amaterial’s surface compared with the longwaveradiation emitted by a perfectly emitting black-body (e = 1.0) at the same temperature andwavelength (Tarara, 2000). Ham et al. (1993)studied the optical properties of different plasticmulch surfaces and reported that mulch surfaceswith high longwave transmittance also had highlongwave emissivity. In their study, blackmulch(e = 0.87) resulted in the highest daytimetemperatures and coolest nighttime tempera-tures, whereas aluminum-painted mulch (e =0.28) resulted in the coolest daytime tempera-tures but highest nighttime temperatures of alleight mulches examined.

Metalized (aluminum) mulch surfaces arecharacterized by low shortwave absorption,high shortwave reflection, and low longwavetransmittance (Ham et al., 1993). Because oftheir low shortwave absorption, metalizedmulch films are slow to gather energy through-

out the day. However, the low emissivity andlongwave transmittance of metalized mulchfilms produce an insulating effect wherebythermal radiation is not emitted rapidly fromthe bed surface at night. By comparison, blackmulch gathers heat rapidly throughout thedaytime and also emits heat rapidly to thecooler surrounding nighttime air. In this way,plants on black mulch experience greater di-urnal RZT fluctuation than plants on fullymetalized mulch or metalized-striped mulch.Despite this, there are several studies thatsuggest that diurnal RZT fluctuations havelittle overall effect on strawberry growth andyield when temperature extremes are avoided(Gonzalez-Fuentes et al., 2016; Kumakuraand Shishido, 1994). We suspect that smallchanges in diurnal RZT fluctuation by themetalized mulch films were unimportant toaffecting early yield in our study. Instead, webelieve earliness improvement resulted from areduction in the magnitude and duration ofheat stress conditions throughout the hot early-season afternoons.

Fig. 2. (A) Average hourly root-zone temperatures at a 10-cm depth for the bed center and bed shoulder. (B) Tables showing the number of hours of statisticallysignificant differences as well as the maximum change in temperature ([ = increased temperature;Y = decreased temperature) for each comparison shown onthe left (B = black mulch; M = fully metalized mulch; MS = metalized-striped mulch). Comparisons are given as the second mulch type relative to the firstmulch type. (C) Representative photos of the growth stage of ‘Florida Radiance’ are given for each month.

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Metalized mulch types improve earlyyield. There was a trend of improved early-season fruit yield by fully metalized mulchand metalized-striped mulch in both seasons,although the effect was significant onlyduring the 2016–17 season (Table 2). Con-versely, the plastic mulch · cultivar interac-tion did not affect fruit yield significantly ineither season. As a result, data presented inthis section are pooled by the main effect ofplastic mulch type.

During the 2016–17 season, fully metal-ized mulch and metalized-striped mulch in-creased early yields significantly by 11% and19%, respectively, compared with blackmulch. During the 2017–18 season, fullymetalized mulch and metalized-stripedmulch led to nonsignificant early-yield in-creases of 22% and 34%, respectively. Bycontrast, neither late-season nor total-seasonyields were affected significantly by plasticmulch type during the 2016–17 or 2017–18season. Regardless, there was still a prom-inent trend of total-season yield improvementby fully metalized mulch and metalized-striped mulch. For example, metalized-striped mulch led to a nonsignificanttotal-season yield increase of 26% comparedwith black mulch during the 2017–18 season.Metalized-striped mulch resulted in numeri-cally higher early and total yields than fullymetalized mulch during both seasons.

As expected, early-season yields weremore affected by plastic mulch type thanlate-season yields. Plastic mulches influencethe daytime energy balance around the cropboth above and below the soil, mostly byeither reflecting or absorbing incoming solar

radiation per their specific optical properties(Ham et al., 1993; Tarara, 2000). Becauseexposure of the mulch surface to incomingsolar radiation is highest during the earlyseason, when plant canopies are small, themulches should impart their greatest impact onthe plant energy balance during establishment.

Two previous studies have examined theeffect of reflective mulch for annual raised-bed strawberry production under subtropicalconditions. In Brazil, Yuri et al. (2012) foundno difference in winter strawberry yieldsbetween black and silver reflective mulchtreatments. In Florida, Albregts and Chandler(1993) evaluated eight colors of plasticmulch for winter strawberry production. Fortwo consecutive seasons, yellow mulch andwhite mulch increased early-season yields(then considered November through Febru-ary) by 26% to 33% compared with blackmulch, but they had no significant effect ontotal-season yields because of their negativeimpact on late-season yields. Compared withour study, Albregts and Chandler (1993) usedcultivars and planting dates that were lesslikely to promote high early yields. Becauseof different market conditions, they plantedroughly 3 weeks later and continued toharvest for almost 2 months longer than us.Their longer harvest period may account fordifferences in the effect of reflective mulchon late-season yields between our study andtheirs. Using modern early-yielding cultivarsand late-September planting dates, we ob-served significant increases in early yieldand nonsignificant increases in late yield byfully metalized mulch and metalized-stripedmulch. To our knowledge, our study is thefirst to report the successful use of reflectivemetalized mulch for improving early-seasonyields and producing numerically greatertotal-season yields than black plastic mulchunder subtropical winter strawberry produc-tion conditions. The effects of root-zonecooling by the fully metalized mulch andmetalized-striped plastic mulch on improvedstrawberry yields are discussed further in thefollowing sections.

Metalized mulch types improve fruitdevelopment without affecting plant growth.

In our study, yield gains by fully metalizedmulch and metalized-striped mulch resultedfrom an increase in the number of fruitproduced by each plant rather than an im-provement in fruit size. In fact, the averagefruit size during the 2016–17 early seasonwas the same across all three mulch treat-ments (Table 3). During this same period,plants on fully metalized mulch andmetalized-striped mulch produced 13%and 20% more fruit than plants on blackmulch, respectively. This implies that sig-nificant early marketable yield increasesduring the 2016–17 season were almostentirely a result of improved fruit develop-ment and fruit set.

Interestingly, canopy image analysis didnot reveal any changes in canopy area byplastic mulch type for any sampling date ineither season (data not shown). This obser-vation suggests that fruit development ismore sensitive than vegetative growth to heatstress caused by black plastic mulch. It is welldocumented that temperatures above 30 �Ccan suppress floral development, pollen tubegrowth, pollen viability, and overall fruit setin strawberries (Kumakura and Shishido,1994; Ledesma and Sugiyama, 2005; Wangand Camp, 2000). In a growth chamberexperiment, Ledesma et al. (2008) found that,across two different short-day strawberrycultivars, increasing day/night temperaturesfrom 23/18 to 30/25 �C reduced the numberof inflorescences produced significantly. The30/25 �C day/night treatment subsequentlyhad a lower percentage of successful fruit setand number of fruits produced comparedwith the 23/18 �C treatment. Furthermore,Kadir et al. (2006) found that increasing day/night temperatures from 20/15 to 30/25 �Creduced fruit yields significantly withoutinhibiting shoot dry matter accumulation.They also found that exposure to 30 �C formore than 2 weeks inhibits strawberry flowerdevelopment regardless of cultivar heat sen-sitivity. In our study, as discussed earlier,fully metalized mulch and metalized-stripedmulch both reduced the magnitude and dura-tion of heat stress conditions significantlycompared with black mulch. Based on the

Table 2. Effect of cultivar, mulch type, and their interaction on early, late, and total yield of strawberries in the 2016–17 and 2017–18 seasons.

Cultivar Plastic mulch

2016–17 2017–18

Early Late Total Early Late Total

Florida Radiance Black 12.1 bcz 15.0 27.1 8.8 15.3 ab 24.2Fully metalized 13.2 ab 16.4 29.6 11.8 15.9 ab 27.7Metalized striped 14.2 a 15.9 30.1 13.4 19.0 a 32.4

Florida Beauty Black 11.2 c 13.9 25.1 10.5 12.1 b 22.6Fully metalized 12.9 ab 14.9 27.8 11.7 11.9 b 23.7Metalized-striped 13.7 a 14.8 28.4 12.6 13.9 b 26.5

Averaged dataFlorida Radiance 13.2 15.8 28.9 11.3 16.7 a 28.1 aFlorida Beauty 12.6 14.5 27.1 11.6 12.7 b 24.2 b

Black 11.7 b 14.4 26.1 9.7 13.7 23.4Fully metalized 13.0 a 15.7 28.7 11.8 13.9 25.7Metalized striped 13.9 a 15.3 29.3 13.0 16.5 29.4

Source of variation (P value)Cultivar 0.1633 0.2311 0.1450 0.7297 0.0002 0.0062Plastic mulch 0.0182 0.7326 0.1525 0.1796 0.0914 0.1272Cultivar · plastic mulch 0.8187 0.9825 0.9943 0.3790 0.5364 0.3063

zMeans within a column not followed by the same lowercase letter are significantly different at P < 0.05 according to Tukey’s test.

Table 1. Effect of mulch type on the averagenumber of hours in which bed centertemperature (BCT) exceeded 30 �C.

Mulch

BCT > 30 �C (h)

Oct. Nov. Dec. Jan. Feb

Black 164 az 108 a 21 a 1 a 57 aFully metalized 96 b 40 b 0 b 0 b 2 bMetalized striped 107 ab 46 b 4 b 0 b 42 azMeans within a column not followed by the samelowercase letter are significantly different at P <0.05.

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evidence from previous studies that highroot-zone and air temperatures can inhibitflower and fruit development, we believe heatstress mitigation by metalized mulch filmsin our study is responsible for alleviating, atleast in part, floral inhibition in the crown,and improving early fruit set and marketableyields.

Fully metalized vs. metalized-stripedmulch. The main objective of our new plasticmulch design is to create optimal growingconditions for winter strawberry productionunder subtropical conditions by using themetalized center stripe to cool the root zoneduring establishment, and by using the black

shoulders to warm soils during winter. Wehypothesized that metalized-striped mulch,by having the dual benefits of metalized andblack-mulch films, can outperform fully met-alized mulch. Several observations indicatethat metalized-striped mulch is more benefi-cial than fully metalized mulch, althoughthe direct comparisons between the twoplastic mulches showed no significant differ-ence. For example, metalized-striped mulchresulted in 7% more marketable fruit thanfully metalized mulch during the 2016–17early season (Table 2). During this sameperiod, yields of ‘Florida Radiance’ wereimproved significantly by metalized-striped

mulch, but not fully metalized mulch, com-pared with black mulch.

However, the benefits of adding a metal-ized center stripe to black plastic mulch werebest illustrated during the 2017–18 season.Despite an above-average number of hot daysduring establishment (Fig. 3), metalized-striped mulch produced 10% greater earlymarketable yields than fully metalized mulchfor both cultivars. Beginning in January,which experienced below-average daily tem-peratures, metalized-striped mulch began tooutperform fully metalized mulch noticeably(Fig. 4). Weekly yields of ‘Florida Radiance’were improved significantly compared withblack plastic mulch for 5 weeks usingmetalized-striped mulch, but for just 3 weeksusing fully metalized mulch. In addition,marketable late-season yields of ‘FloridaBeauty’ were reduced by fully metalizedmulch but improved by metalized-stripedmulch compared with black mulch (Table 2).Our findings indicate that metalized-stripedmulch is well suited to optimize soil microen-vironment conditions throughout the dynamicenvironmental conditions of Florida’s winterstrawberry production season.

Practical applications of metalized-striped mulch. To stay cost-effective in anincreasingly competitive winter strawberrymarket, Florida strawberry growers need apractical, easily implementable solution forreducing establishment heat stress and im-proving early fruit yields (Guan et al., 2016).Two strawberry cultivars that differ in heatstress tolerance and early-season yield pat-terns were used in this study. ‘FloridaBeauty’, the earliest cultivar available inFlorida, is day neutral, so it has the abilityto initiate flowers at longer photoperiods andhigher temperatures than short-day cultivars(Whitaker et al., 2017). As a result, ‘FloridaBeauty’ is recommended for planting be-tween 20 Sept. and 1 Oct. in Florida to allowfor adequate vegetative growth before flow-ering begins. By contrast, ‘Florida Radiance’is a short-day cultivar with a relatively weakplant habit and low heat tolerance, so it is

Table 3. Effect of cultivar, mulch type, and their interaction on the number of marketable fruit per plant during the early, late, and total periods of the 2016–17 and2017–18 seasons.

Cultivar Plastic mulch

2016–17

No. fruit/plant Fruit size (g)

Early Late Total Early Late Total

Florida Radiance Black 14.4Bz 19.0 33.4 19.4ab 18.4 ab 19.0 abFully metalized 15.0B 20.3 35.3 20.0a 18.7 a 19.5 aMetalized striped 16.4A 20.4 36.8 19.8a 18.2 ab 19.2 a

Florida Beauty Black 14.0B 17.8 31.8 17.7bc 18.1 ab 17.9 bcFully metalized 17.0A 19.8 36.8 17.2c 17.8 ab 17.4 cMetalized striped 17.7A 20.8 38.4 17.4c 16.5 b 17.0 c

Averaged dataFlorida Radiance 15.3b 19.9 35.2 19.7 a 18.5 a 19.2 aFlorida Beauty 16.2a 19.4 35.7 17.4 b 17.5 b 17.4 b

Black 14.2b 18.4 32.6 18.6 18.2 18.4Fully metalized 16.0ab 20.1 36.0 18.6 18.3 18.5Metalized striped 17.0a 20.6 37.6 18.6 17.4 18.1

Source of variation (P value)Cultivar 0.0409 0.7245 0.7207 0.0000 0.0100 0.0000Plastic mulch 0.0079 0.5446 0.1038 0.9870 0.0964 0.2952Cultivar · plastic mulch 0.0997 0.8626 0.5869 0.2607 0.2881 0.1263

zMeans within a column not followed by the same lowercase letter are significantly different at P < 0.05, whereas means within a column not followed by the sameuppercase letter are significantly different at P < 0.10, according to Tukey’s test.

Fig. 3. Average weekly marketable fruit yield for ‘Florida Radiance’ (top) and ‘Florida Beauty’ (bottom)during the 2016–17 (left) and 2017–18 (right) seasons. Mean separation letters indicate significantdifference at P < 0.05 if lowercase and P < 0.10 if uppercase, with the top, middle, and bottom letterscorresponding to the means of black, metalized, and metalized-striped mulch, respectively.

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recommended for planting between 5 and 15Oct. (Whitaker et al., 2008). Despite suchcontrasting characteristics, no significant cul-tivar · plastic mulch interaction was found inour study. This observation suggests thatmetalized-striped mulch is a promising man-agement option that can improve early yieldsconsistently of a wide range of strawberrycultivars by mitigating heat stress associatedwith extremely early planting (e.g., lateSeptember in Florida).

In our trials, neither flavor (data notshown) nor fruit size were affected by plasticmulch type (Table 3). It is important thatgrowers do not sacrifice fruit quality for thesake of increased early-season yields, be-cause flavor and fruit size are two essentialquality parameters considered by intermedi-ary strawberry buyers and the end consumer(Gallardo et al., 2015). But perhaps mostimportant to growers, information providedby the manufacturer (Imaflex, Inc., Thomas-ville, NC) suggests that for equal thicknessand impermeability of fumigant gases,metalized-striped mulch costs only 2.7%more than entirely black plastic mulch. Over-all, our findings indicate that black plasticmulch with a metalized-center stripe canimprove early-season strawberry yields byup to 34% compared with black mulchwithout affecting fruit quality, late-seasonyields, or production costs negatively.

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