The Effects of Colored Plastic Mulches andRow Covers on the Growth and Yield of Okra

Garry G. Gordon", Wheeler G. Foshee 1112, Stewart T. Reed',

James E. Brown', and Edgar L. Vinson 1112

ADDITIONAL INDEX wORDS. Abelmoschus esculentus, soil temperature, air,stem diameter

SUMMARY. Okra (Abelmoschus esculentus 'Clemson Spineless') was grown on anOrangeburg sandy loam soil in Shorter, AL. Okra was direct-seeded in single rows.Treatments consisted of five mulch colors: black, white, red, silver, and blueinstalled either with or without spun-bonded row cover. Soil temperatures were 4to 7 °C lower than air temperatures in all treatments. The use of darker (black, blue,red) -colored plastic mulches increased early and total yield of okra compared withbare soil with and without row cover. Increased soil and air temperatures did notalways correlate to an increase in yield. It can be concluded that the use of darkplastic mulch is advantageous to growers of okra in climates that do not have coolsprings, but the added use of row covers to plastic mulch has no effect on growthand yield. The profit of marketable okra produced using a row cover was $1.37versus $1.35 per pound without a cover in 2003 and $1.28 versus $1.29 per poundin 2004. Blue plastic mulch is c$0.08 per foot more expensive than black plastic.Our data do not show an economic advantage for blue over black mulch for okra,but the positive effect cited by other authors may be more pronounced with leafyvegetables.

p

lastic (polyethylene) mulcheshave been used in vegetableproduction in the United States

since the 1950s. Black polyethyleneplastic mulch is the standard plasticmulch used in vegetable production.Black plastic alters the plant's growingenvironment by generating warmersoil temperatures (Dodds et al., 2003;Hanna et at., 2003) and holding moresoil moisture (Ham et al., 1991;Lamont, 1993) than bare soil. Re-searchers using black plastic instead of

'USDA-ARS, Subtropical Horticultural Research Sta-tion, 13601 Old Cutler Road, Miami, FL 33158

2 Departinent of Horticulture, 101 Funchess Hall,Auburn Universit,', AL 36849

'Corresponding author. E-mail: Garv.Gordon@ARS.USDA.GOV .

bare soil have recorded higher yields(Brown et al., 1995; Leib et al., 2002;Summers and Stapleton, 2002), ear-lier harvests (Bonanno and Lamont,1987; Ibarra et al., 2001; Lamont,

1993), and cleaner fruit (Brown andChannell-Butcher, 2001; Loughrinand Kasperbauer, 2002), all attributedto soil temperature and moisture dif-ferences under plastic mulch. Some-times black plastic mulches can createsoil temperatures that are too high andthis will cause deleterious effects onplant growth (Diaz-Péréz et al., 2000;Orzolek and Murphy, 1993). Othermulch colors besides black have beenused by growers and researchers invegetable production. White plasticmulch has been shown to generatecooler soil temperatures than blackplastic (Diaz-Péréz and Batal, 2002;Lamont, 1993). White plastic is pre-ferred during the summer growingseason in warmer regions of theworld compared with black plasticbecause it has the ability to maintainsoil moisture while having coolertemperatures than black plastic. Theuse of silver plastic mulch has re-sulted in less insect-transmitted dis-ease (Csizinszky et al., 1995; Lamontet al., 1990) in certain vegetablecrops. Red plastic mulch has shownincreased yield in tomato (Solanumlvcopersicnm) (Decoteau et al., 1989)and other crops (Decoteau et al.,1990; Kasperbauer, 1992).

Row covers are used to insulatea plant's growing environment topromote early yield. Floating spun-bonded polyester row covers areused with various vegetable crops(Shadholt et al., 1962; Wells andLoy, 1985). Floating row covers havebeen shown to alter the plant's nii-croenvironment by increasing tem-peratures for the crop during the dayand into the night (Arancibia andMotsenhocker, 2002). The use ofrow covers with plastic mulch hasshown to sometimes have a greatereffect (Powell, 2000) and sometimesless effect (Lamont et al., 2000) onplant yield than the use of plastic

UnitsTo convertU.S. to SI, To convert SI to U.S.,multiply by U.S. unit SI unit multiply by

0.3048 ft in 3.28089.3540 gal/acre lJia 0.10692.54 inch(es) cm 0.39370.4536 lb kg 2.20461.1209 lb/acre kg-ha 0.89220.0254 mil mm 39.3701

70.0532 oz/acre gha 0.01431.1692 pt/acre 1.-ha 0.85532.3385 qt/acre Lha 0.4276°F- 32) - 1.8 IF oC 1.8 x °C) + 32

224 February 2010 20(1)

mulch alone. The response of a plantto row covers is greatly dependent onthe temperature in the region dur-ing the time the row covers are in-stalled (Bonanno and Lamont, 1987;Contreras Magana and Snchez delCastillo, 1990).

The use of row covers with plasticmulch has been shown to generateearlier and increased yields than rowcovers and plastic mulch used sepa-rately (Brown and Chan nell-Butcher,1999a; FarIas-Larios et aL, 1998;Purser, 1993). Brown et al. (1998)observed that row covers with plasticmulch increased yield in sweetpotato(Ipomoea batatris). Gerber and Brown(1982) found superior and earlieryields of muskmelon (Cucumis melo)with the combination of row coversand plastic mulch. Cucumber (Cucu-mis sativus) exhibited a positive in-crease in early and total yield with theaddition of row covers to plastic mulch(Wolfe et al., 1989).

Phytochrome is the photorecep-tor responsible for light-regulatedgrowth responses. The phvtochromemolecule is a dissolvable chromopro-tein with subunits that are made up ofa linear tetraprvrrole chromophorecovalently linked to a polypeptide of120 to 127,000 molecular weightdepending on the plant species (Pratt,1982; Vierstra et al., 1984). Ph yto-chrome receptors within plant cellshave the ability to detect wavelengthsfrom 300 to 800 run. Red (R) light isabsorbed from 660 to 680 nm andfar-red (FR) light is absorbed from730 to 740 nm (Decoteau et al.,1993; Kasperhauer, 1999; Rajapakseand Kell y, 1994). Tomato plantsgrown with red plastic mulch pro-duced higher marketable yields thanthose grown with black plastic mulch(Decoteau et al., 1989). The re-searchers who conducted this studybelieved that their findings were re-lated to the effects that light reflectedfrom red plastic mulch had on theplant's phvtochrome regulatory sys-tem. Orzolek et al. (2000) found sil-ver and red plastic mulches reflectedhigher FR:R ratios than other mulchcolors used in their study. This studyalso revealed an increase in market-able fruit yield in tomato using silveror red mulch as compared with stan-dard black plastic. Bell pepper (Cap-sicum annuum) plants grew tallerand were heavier when grown onred plastic, which exposed them to

a greater FR:R ratio compared withother plastic mulches used in theexperiment (Decoteau et al., 1990).

Plants have shown a multitude ofdifferent responses when treated withblue (wavelength 400 to 500 nm)light. Blue light treatments haveshown to affect morphological, met-abolic, and directional reactions inplants (Senger and Schmidt, 1994).Some of the documented findingson plant response to blue light in-clude: phototropism (Lipson, 1980;Shropshire, 1980) enz yme synthe-sis (Hart, 1988; Ruvters, 1982),chloroplast development in leaves(Akoyunoglou et al., 1980), and sto-matal opening (Hart, 1988). Hattet al. (1993) and Kasperbauer andLoughrin (2004) have found whiteplastic mulch to reflect more bluelight than the other colored mulchesused in their experiments. Antoniouset al. (1996) reported that turnip(Brassica rapa) roots grown on whiteplastic mulch, which reflected thelargest amounts of blue light, hadthe least distinct or mildest flavor.Decoteau et al. (1988) reported thatshorter stems and more auxiliarygrowth could result from the bluelight reflected from the use of whiteplastic mulch with tomato. Field-grown cotton (Gossvpium spp.) pro-duced more boll fibers and seeds perplant when grown on ground coveredwith colored plastic mulches thatreflected less blue light resulting inhigher FR:R ratios (Kasperbauer andThibodeaux, 1997).

Research on growing vegetablecrops with various colored plasticmulches along with spun-bondedrow covers is limited. The purpose ofthis study was to evaluate the effectsof colored plastic mulches with andwithout row covers on growth andearliness of fruit production on okra.

Materials and methodsThe research was conducted at

the EX. Smith Research ExperimentStation in Shorter, AL. The soil type isan Orangeburg sandy loam (fine-loamy siliceous thermic Typic Kan-diudult). Field plots to evaluate theeffects of colored plastic mulches androw covers on the growth and pro-duction of 'Clemson Spineless' okrawere established in May 2003 andApr. 2004. The plots were 10 ft longand 5 ft wide. The colored plasticmulches and plastic drip irrigation

lines were applied simultaneousl y onraised beds (6 inches in height) pre-pared with a plastic la yering machineattached to a medium-sized tractor.The experimental plots were arrangedin a randomized complete block de-sign with four replications of eachtreatment. Each of the four replica-tions represented a different block.Okra was direct-seeded into the fieldon 8 May 2003 and 28 Apr. 2004. Asoil pH of 7.0 was recorded in Year2003 and 6.2 in 2004. Treatmentsconsisted of five mulch colors: black(BPM), white (WPM), red (RPM),silver(SPM) (all from Ken-Bar, Read -ing, MA), and blue [BLUPM (Pliant,Schamburg, IL)], installed either withor without spun-bonded row coverERG (Ken-Bar)]. Drip irrigation tapeapplied together with the BPM wasleft in place for all treatments. Thesilver, red, white, and black plasticmulch were all 1.5 mil thick and 36inches wide. The blue plastic mulchwas 1.25 mil thick and 48 inches wide.The row cover was 1.7 x 0.77 in.The drip irrigation tape was 1 cm indiameter with 12-inch emitter

Table 1. Fertilizer and insecticideapplied preplant and fertilizer,insecticide, and herbicide applied inseason to okra plots with coloredplastic mulch with and without rowcover or bare soil in 2003 and 2004at Shorter, AL.

Year

2003 2004Amendment Preplant (oz/acre)

Phosphorus 0.14 0.29Potassium 0.57 0.57Magnesium 0.57 0.14Calcium 3.14 2.14Nitrogen 955 1012Chloropicrin 4790 4790

Preplant (lb/acre)

Methyl bromide 300 300

In season (lb/acre)

20N-8.73P-16.6K 47.7 104.8Calcium nitrate 54.9Carharvl 4.0 1.2

(gal/acre)

Trifluralin 0.13 0.13Spinosad 0.007Malathion 0.13 0.13Sethoxvdim 0.13 0.13Glvphosatc 0.25 0.50'i oz/acre 70.0532 gha 1 lb/acre = 1.1209kgha 1 I gal/acre = 93540 Uha

HrIdnoIogy February 2010 20(1) 225

TECHNOLOGY AND PRODUCT REPORTS

Table 2. Analysis of variance results for the effects of mulch color by row cover, year, block, and their interactions on okraplant height, fresh weight, stem diameter, and marketable and unmarketable yields (early and total yields) at Shorter, AL.

Early Early Total TotalPlant Fresh Stem marketable cull marketable cull Total No. Soil Air

Treatment ht wt diam yield yield yield yield yield branches temp temp

Mulch color' *** *** *** *** *** *** *** *** ***

Row coverv *** *** *** *** *** NS **

Year' NS *** *** *** *** *** * *** ***

Block" NS NS NS NS NS NS NS NS NS NS NS

Mulch x row cover * NS NS NS NS ES NS

Mulch x row cover x year * ** ** NS ES ES

'Bare soil, black plastic, blue plastic, red plastic, silver plastic, and white plastic mulch.'Plots with and without row cover.'2003 and 2004.'Each replication was a block.

nonsignificant at the 0.05 Plevel; , significant at the 0.05, 001, and 0.001 Plevels, respectively.

spacing. Bare soil (BS) plots with andwithout RC were used as controls. Six364-ft strips of black plastic were in-stalled over beds. Several sections inthe black plastic (10 x 5 ft) were cutout, removed from the soil, andreplaced with an equal size of white,blue, red, or silver plastic mulch. Un-cut sections remaining in place servedas the BPM treatment and cut sec-tions left uncovered served as con-trols. The same day plastic mulcheswere installed in the field, 8 May 2003and 28 Apr. 2004, okra was direct-seeded with three seeds for each of the10 holes placed in the plastic mulch orbare soil treatments. The okra wasthinned to one plant per hole after14 d of growth and then 2-rn-widerow cover strips were spread overdesignated treatments on 22 May2003 and 12 Apr. 2004. Row coverstrips were tucked into the soilaround the edges of each plot and leftto float loosely in an effort not tohinder plant growth as well as toprevent the loss of captured heat.The row covers were removed whenokra plants developed flowers.

Soil temperature to a depth of 4inches was measured with a soil probethermometer (Taylor® SwitchableDigital Thermometer; Taylor Preci-sion Products, Oak Brook, IL). Airtemperature was measured withTaylor® Indoor/Outdoor Thermom-eters (Taylor Precision Products)with sensors attached to heat-con-ducting wires and placed in the centerof each plot at 5 inches above groundlevel. Temperatures were recordedfor 14 d from 30 May to 15 June2003 for air and soil temperatures.In 2004, air temperature was recordedfor 12 d from 20 May to 7 June andfor soil temperature 15 d from 14

May to 7 June from 1200 to 1300 HR

on each day. The air temperatureswere recorded during the okra vege-tative stage.

The stem diameter was takenwith a digital caliper (Model 500-196; Mitutoyo Digimatic, Kanagawa,Japan). Caliper measurements weretaken 2 inches above ground levelaround the base of plants. Canopyheights were taken with a meter stickby measuring from the base of theplant to the tip of the highest leaf.Plant heights were taken from alltreatments in both years on the daythat the row covers were removed.The amount of branching per okraplant was determined by hand count-ing all branches over 3 inches inlength. The okra yield was separatedbased on appearance (size, shape,color, and presence of insect or dis-ease damage) into marketable and cullfruit and the weight of each grouprecorded. Yields for each treatment

Row coverNone 353 a' 161Yes 186b 127

Mulch colorBare soil 225 38Black 311 134Blue 295 229Red 272 187Silver 307 181White 208 97

were determined by weighing mar-ketable and cull fruit at each harvestdate.

Preplant fertilizer and pesticidesapplied to okra plots in the years 2003and 2004 were in accordance with soiltesting recommendations from Au-burn University Soil Testing Labora-tory (Auburn, AL) and are given inTable 1. No limestone was added tookra plots either year. Before layingthe plastic mulch, methyl bromide(67% methyl bromide + 33% chloro-picrin; Soil Chemicals, Hollister, CA)was applied to the soil as a fumigant ata rate of 300 lb/acre on 17 Apr. 2003and 27 Mar. 2004, respectively. Liq-uid calcium nitrate [Ca(NO 3 ) 2 ] fer-tilizer was injected into the soilthrough plastic drip fertigation tubesfrom 30 May to 28 Aug. 2003 andnitrogen (N), phosphorus (P), andpotassium (K) as 20N-8.73P-16.6Kfrom 30 May to 28 Aug. 2003. Nitro-gen, P, and K as 20N-8.73P-16.6K

9,865 b 5,435

15,300 b

12,630 a 6,396

19,026 a

7,762 c 3,740 c 11,502 c

13,738 a 7,662 a 21,401 a

12,084 ab 6,875 ab

18,959 ab

11,275ab 5,593b

16,868 b

11,197b 5,977ab

17,174 b

11,429ab 5,644b

17,074 b

Table 3. Effect of row cover and plastic mulch color on early and total okra yieldfor marketable, cull, and the combined total of marketable plus cull fruit in 2003at Shorter, AL.

Early yield (lb/acre)' Total yield (lb/acre)Treatment Marketable Cull Marketable Cull Combined total

'First 2 weeks of harvest; ilb/acre = 1.1209 kg ha-'.'Mean separation within columns was determined by Fisher's least significant difference test. Within a column andfactor, treatment means followed by the same letter are not significantly different at Pc 0.05. Means followed byno letter were not significantl y dithrent at Pc 0.05.

226 Hainoiogy February 2010 20(1)

964 a'689 b

329 d1,020 ab1,081 a1,101 a

754 be678 e

500 a 7,579 b 3,009 10,587 a199h 8,897a 3,212 12,108a

114d 3,027d 1,128c 4,155c502 a 10,803a 4,423a 15,226a648a 10,019ab 4,115ab 14,133 a377 be 7,668 c 2,958 h 10,626 b258 cd 7,932 be 2,937 h 10,868 h199 cd 9,978 ab 3,103 h 13,081 ab

were applied to plots in 2004. Tn-fluralin at 1 pt/acre a.i. (Treflan;Bayer CropScience, Research Trian-gle Park, NC) was applied in May2003 and 2004. In May 2003, 1 qt/acre a.i. of glyphosate (Round-Up;Monsanto, St. Louis, MO) was ap-plied between the rows of the okraplots. In May 2004, 2 qt/acre a.i. ofglyphosate was applied. Carbaryl(Sevin 80S; Bayer CropScience) wasapplied at a rate of 4 lb/acre a.i. inJune 2003 and at 2 pt/acre a.i. inJune 2004. Sethoxydirn (Poast; BayerCropScience) was applied at 1.2Lha a.i. on 25 July 2003 and 23June 2004. Malathion (Pestanal®;Superleco, Superleco Park, PA) andspinosad (SpinTor; Dow AgroScien-ces, Indianapolis, IN) were applied on18 July and 8 Aug. 2003, respectively.

Okra was harvested from 3 Julyto 8 Sept. 2003 and from 21 June to27 Aug. 2004. Fruit were picked onMonday, Wednesday, and Friday dur-ing the harvest period. Early yieldswere defined as okra fruit collectedduring the first 2 weeks of harvest,which were from 3 to 16 July 2003and 21 June to 4 July 2004. All datawere analyzed using SAS (Version9.1; SAS Institute, Cary, NC). Theeffects of mulch color, row cover,replication (block), year, and theirinteractions on selected plant physicalcharacteristics and yield componentswere tested by the analysis of variance(ANOVA) procedure. Where maineffects were insignificant, those vari-ables were dropped from the modeland the ANOVA rerun. Because therewere no significant block effects, thedata were run in a factorial modelwith interaction. Temperature datawere analyzed using the general linearmodel, repeated measures procedureof SAS (SAS Institute). Both air andsoil temperatures were reported as themean for values measured over therecording period during each year.Unless otherwise indicated, meanseparation for all variables was accom-plished with the Fisher's least signif-icant difference test at P< 0.05.

Results and discussionSignificant differences between

treatments were found for mulchcolor (Table 2). The row cover effectwas significant for all treatments ex-cept total cull and year had a signifi-cant effect on all but fresh weight.There were significant mulch-by -

cover- by-year treatment interactionsfor early yield components; however,by the end of harvest, these effectshad vanished. All yield data are pre-sented by year for early versus totalyield comparisons. Table 3 lists yielddata for 2003. Early yields, both mar-ketable and cull, tended to be higherin plots without row covers, but thistrend reversed in time. The combinedtotal yields (marketable yield plus cull)were greater with row covers thanwithout. Significantly lower yields wereproduced with bare soil than with anycolored mulches used. Blue plastic

Row coverNoneYes

Mulch colorBare soilBlackBlueRedSilverWhite

mulch resulted in the highest earlyyield and maintained its productivitythrough harvest yielding the secondhighest combined total yield. At har-vest, black plastic mulch resulted in thehighest combined total yield.

There were no significant differ-ences in early marketable and cullokra yields among the mulch colortreatments in 2003. The bare soilproduced the lowest marketable andcombined total yields and black plas-tic produced the highest. On average,marketable yields were 66% of thetotal fruit produced with row cover

Table 4. Effect of row cover and plastic mulch color on early and total okra yieldfor marketable, cull, and the combined total of marketable plus cull fruit in 2004at Shorter, AL.

Early yield (lb/acre)' Total yield (lb/acre)Source Marketable Cull Marketable Cull Combined total

'First 2 weeks of harvest; lib/acre = 1 1209 kg-ha-'.'Mean separation within columns was determined by Fisher's least significant difference test. Within a column andfactor, treatment means followed b y the same letter are not significantl y diflcrent at P< O.O.S. Means followed b y noletter svere not significantl y different at Pc 0.05.

Table 5. Effect of colored plastic mulch and row cover on air and soiltemperatures for okra production in 2003 and 2004 at Shorter, AL.'

With row cover Without row coverAir Soil Air Soil

Mulch color 2003 temp (°G7

Blue 38.9 ab' 30.3 a 34.2 ab 31.0 aBlack 38.4 ab 30.1 a 33.7 ab 30.1 bBare soil 36.5 c 27.9 e 33.0 c 28.8 eRed 38.0 b 30.2 a 33.5 be 30.5 bSilver 39.7 a 27.5 e 33.5 be 27.5 dWhite 38.4 ab 28.9 h 34.3 a 28.5 e

2004 temp (°C)

Blue 34.1bc 31.6a 31.7 32.9aBlack 37.5 a 31.2 a 32.2 32.2 abBare soil 31.1c 27.1c 33.2 28.9eRed 34.6 ab 30.8 a 32.3 31.7 bSilver 36.8 ab 28.6 h 33.0 28.2 cWhite 36.1 ab 29.2 b 32.1 28.3 e'Soil temperature to a depth of4 inches(10.2 cm) and air temperature 5 inches (12.7cm) above ground level wererecorded daily over the time during which the row cover was present./1.8x°C)+32=°F.'Mean separation within columns was determined b y Fisher's least significant difference test. Means with differentlower case letters are significantly different at P< 0.05. Means followed by no letter were not significantl y differentat P< 0.05.

florlkhnology February 2010 20(1) 227

20

a a a a a a a a a a a a0 -s N m or U, SQ N SQ Or rt U,

Date

2003A50

45

40

35

a

30a

Ea

25

••.. black plastics row cover airtemp

—U-- black plastic air temp

— bare soil air temp

2003B40

35

U

30

CL

4-.(5

Ew25

0lin

TECHNOLOGY AND PRODUCT REPORTS

treatments producing a slightly higherpercentage of marketable fruit thanthose without covers.

Okra marketable and cull yieldsfor 2004 are shown in Table 4. Yieldtrends in 2004 were roughly similarto those in 2003. Early yields weregreater without a row cover than withone. There was no significant differ-ence between row cover treatmentsfor combined total yield. This was theresult of red and blue plastic mulch

treatments producing greater yieldswithout a row cover. Despite thesetwo exceptions, by the end of harvest,plots with a row cover produced sig-nificantly more marketable yield thanthose without a row cover. Earl y mar-ketable plus cull yields were similar forblue and black mulches and lowestwith bare soil. This trend continuedthrough to harvest. Plastic mulchhas been identified for its ability toproduce earlier okra yields than bare

soil (Incaleaterra and Vetrano, 2000;Simone et al., 2002). In our study theresults were inconsistent on this point.Bare soil always produced the lowestearly yield; however, the differencewas only significant in 2004.

At harvest, marketable yield washigher with row cover than without;however, the total combined yieldwith row cover was not significantlyhigher. Blue and black plastic mulchesgave the highest early marketable plus

••.. black plastic * row ca per soil temp

20—e— black plastic soil temp

a a a a a a a a a a a aw co — —baresoilsoiltemp

Date

Fig. 1. Temperature reading for 30 May through 15 June 2003, the time period when row covers were present, for okragrown under black plastic mulch with and without row covers and bare soil. (A) Air temperature recorded 5 inches (12.7 cm)above each plot. (B) Soil temperature recorded 4 inches (10.2 cm) below soil surface in each plot; (1.8 x °C) + 32 = °F.

228 Hii.o4ogy February 2010 20(1)

2004A50

45

40

35

U

30

CLw

EU

. 25

cull yield and gave the two highest yields were 26% and 28% of the com- BLUPM + RC ^! SPM + RC RPM +total combined yields, significantly bined total for treatments with and RC> BS + RC and BLUPM BPM >higher than all but white plastic without a row cover, respectively. RPM = WPM = SPM > BS for treat-mulch. Bare soil had the lowest total Combined total yield for mulch treat- ments with and without row covers,marketable yield of all treatments; this ments for each row cover followed the respectively (data not shown). Brownwas true for BS and BS + RC. Cull order: BPM + RC ^! WPM + RC ^! and Lewis (1986) and Khan et al.

20 I +• black plastic + row cover air temp

—*— black plastic air temp

a a a a a a a a a a a aor or cv — —baresoilairtemp

Date

B

200440

IR

U

30

UQ.EU•4

25LA

20 Ici ci ci •..*.. black plastic * row cover soil temp

--*— black plastic soil temp

Date --A — bare soil temp

Fig. 2. Temperature reading for 20 May through 7 June 2004, the time period when row covers were present, for okragrown under black plastic mulch with and without row cover and bare soil. (A) Air temperature recorded 5 inches (12.7 cm)above each plot. (B) Soil temperature recorded 4 inches (10.2 cm) below the soil surface in each plot; (1.8 x °C) + 32 = °F.

Wïfiigy February 2010 20(1) 229

(1990) also found black plastic mulchwith a row cover produced higher okrayield than bare soil.

The data for mulch color, rowcover, year, and mulch row coverinteraction show significant effectson both air and soil temperature atP < 0.01 (Table 2). In general, plotswith row cover had higher air tem-peratures than plots without rowcover (Table 5). In 2003, row coverhad no effect on soil temperature. In2004, 1 °C higher temperatures wererecorded in most plots without thanwith a row cover. The differencesin soil temperature between plotswith and without row cover weresignificant but not likely important.Temperature data for each year wereanalyzed separately.

Black plastic mulch is the stan-dard color of mulch used in horticul-ture. Figure 1A shows that in 2003,black plastic mulch with row coverhad a higher air temperature thanblack plastic mulch without row coveror bare soil. The black plastic mulchwith row cover averaged 4 °C higherair temperature compared with blackplastic mulch and 5 °C higher airtemperature compared with bare soil.In 2004 (Fig. 2A), air temperatureswere an average of 6 °C higher forblack plastic mulch with row covertreatment compared with black plas-tic mulch and 5 °C higher comparedwith bare soil. Okra mean air and soiltemperatures are shown in Table 5 for2003 and 2004. In 2003, most plotswith plastic mulch plus row cover hadstatistically similar air temperatures.However, BS + KG had a lower airtemperature than other plots witha row cover. During 2004, in plotswith a row cover, only BLUPM + RCfailed to generate higher air tempera-tures than BS + KG. These findingsare in agreement with other re-searchers who found higher air tem-peratures with the use of plastic mulch(Bonanno and Lamont, 1987; Brownand Channell-Butcher, 1999a; Ibarra-Jiménez et al., 2004; Khan et al.,1990). Without a row cover, WPM,BLUPM, and BPM had higher airtemperatures than BS treatments in2003. In 2004, there were no differ-ences in treatments without rowcovers.

In 2003, soil temperatures werehotter under black plastic (Fig. 1B)than under black plastic with rowcover or bare soil. Like 2003, soil

temperatures for 2004 (Fig. 2B) weregreater under black plastic mulchthan black plastic mulch with rowcover or bare soil. The okra soil tem-peratures for 2003 and 2004 were

significantly higher in plots with blue,red, and black plastic mulches bothwith and without row covers. All ofthese treatments were darker coloredplastic mulches. Diaz-Pérez and Batal(2002) and Rangarajan and Ingall(2001) found cooler temperatureswith bare soil than under dark-colored plastic mulch. These findingsalso agree with Csizinszky et al.(1995) who found blue led to a highersoil temperature than orange, red,aluminum, white, or yellow-coloredmulches. Gough (2001) found thewarmest soil temperatures underred mulch. Soil temperature, in2003, for plots with row cover, fol-lowed the order of BLUPM + RC =RPM + KG = BPM + KG > WPM +RC> BS + KG = SPM + KG. A similartrend was followed in 2004. Soiltemperature in plots without a rowcover followed the order BLUPM >RPM = BPM > BS = WPM> SPM in2003 with a similar trend in 2004.

Air temperature was negativelycorrelated with early yield (Table 6).This is likely the result of abovenormal temperatures early in the sea-son delaying flowering. This wouldlead to low early yields and the neg-ative correlation with air temperature.Total yield and the number ofbranches produced by okra plantswere positively correlated to air tem-perature. Soil temperature was posi-tively correlated with early yield, butthis relationship diminished by theend of harvest.

Mulch color, row cover, and yearhad significant effects on plant height,stem diameter, and the number ofbranches produced by okra plants;mulch color and year had significanteffects on okra fresh weight (Table 7).

Pearson correlation coefficient

Early marketable yieldEarly unmarketable yieldTotal marketable yieldTotal unmarketable yieldTotal yieldNo. branches

Okra plants were taller, had greaterfresh weight, and produced morebranches with a row cover than with-out one. However, among the plasticmulch plus row cover treatments,BLUPM + KG produced significantlytaller plants than WPM + KG andSPM + KG in 2003 and 2004; SPM +RC resulted in the shortest okraplants. Without a row cover, BLUPMand BPM produced taller plants thanBS and WPM. Brown and Ghannell-Butcher (1999b) and Saikia et al.(1997) found that the use of plasticmulch with row cover generated tallerokra plants than the use of just plasticmulch or bare soil. The stem diame-ters produced under BPM was eitherthe largest or second largest (2004with row cover) of any treatment.

There were no differences in okrafresh weight between treatments in2003. In 2004, the fresh weights withblue, black, and red mulch colorswere always significantly greater thanwith bare soil regardless of the pres-ence of row cover.

The branch count produced byokra plants grown with a row coverfollowed the order BPM + RC =WPM + RC = BLUPM + KGRPM + KG SPM + KG BS + RCand BPM + RC ^! BLUPM + KG =RPM+RG>WPM+KG^:SPM +RC > BS + RC in 2003 and 2004,respectively. Without a row cover, thebranch count was only significant in2004 and followed the order WPM ^!BPM = BLUPM ^! RPM = SPM> BS.Khan et al. (1990, 1998) and Rahmanand Shadeque (1999) noted thatmore okra branches developed withplastic mulch with or without rowcovers than bare soil.

Results from our study show thata row cover can increase yield; how-ever, the presence of a row cover didnot reduce the percentage of cullfruit. Black and blue mulch consis-tently produced plants among the

Air tem p (°C) Soil

—0.43280 0.51026

—0.28249 0.58798

0.52478 0.22170

0.45840 0.18069

0.51701 0.21218

0.58546 0.20551

Table 6. Pearson correlation coefficient for air and soil temperatures versus okrayield components analyzed across mulch, row cover, and year combinations forexperiments conducted at Shorter, AL.

230 I'gy. February 2010 20(1)

14.8 av14.8 a11.7 c13.4 b9.0 d13.2 b

0.51 a 0.62

0.50 a 0.42

0.40 b 0.20

0.44 b 0.35

0.33 c 0.37

0.48 a 0.49

7.1a 11.4ah 0.40a7.9a 11.9a 0.42a4.3 c 10.1 c 0.39 ab6.8ab 10.6 be 0.38ab5.3 he 10.5 be 0.41 a7.3a 8.8d 0.34b

0.33 4.2

0.35 3.7

0.24 2.3

0.22 3.4

0.42 3.1

0.22 3.8

Table 7. Effects of colored plastic mulch and row cover on okra plant height, stem diameter, fresh weight, and branch countfor each mulch by row cover combination in 2003 and 2004 at Shorter, AL.

Mulch color

BlueBlackBare soilRedSilverWhite

Plant ht Stem diam Fresh wt Branches Plant ht Stem diam Fresh wt Branches(inches)' (inches) (lb/plant)' (no.) (inches) (inches) (lb/plant) (no.)

With row over Without row cover2003

2004

Blue 17.9 ab 0.59 b 0.57 ab 6.0 ab 11.4 a 0.42 ab 0.31 abc 4.1 abBlack 15.6 cd 0.68 a 0.73 a 6.3 a 12.4 a 0.44 a 0.44 a 4.3 abBare soil 17.2 b 0.23 d 0.06 c 2.4 c 7.6 c 0.26 d 0.06 d 2.5 eRed 18.6 a 0.67 ab 0.51 ah 5.3 ab 12.3 a 0.43 ab 0.35 ab 3.9 bSilver 14.6 d 0.46 e 0.33 be 4.8 b 9.1 b 0.36 be 0.15 cd 3.6 bWhite 15.9 e 0.60 b 0.71 a 5.0 ab 7.8 be 0.30 cd 0.22 bed 5.0 a1 inch 2.54 cm; 1 lb 0.4536 kg.

Mean separation within columns was determined by Fisher's least significant difference test. Means with different lower case letters are significantl y different at P< 0.05. Meansfollowed by no letter were not significantly different at P < 0.05.

tallest, greatest fresh weight and mostprolific branching. Early and totalyields were also highest under blackand blue mulches.

ConclusionIn this study, plants grown in soil

under a row cover were taller andgenerally more robust than plantsgrown without row covers. Market-able yield was greater with the use ofa row cover. In contrast, early yieldswere reduced by the presence of rowcover. This is attributed to the highair temperature in late May associatedwith row covers. At times, air tem-perature surpassed 38 °F in plotscontaining a row cover. Soil temper-atures were 5 to 7 °C lower than airtemperatures. The dark colors, black,blue, and red, resulted in higher soiltemperatures than silver or white-colored mulches. Warmer air and soiltemperatures did not always corre-late to greater yield. Early yield wasgenerally greatest with dark mulchcolors and the combined total yieldgreatest with black and blue plasticmulch.

Fields et al. (2004) estimatedproduction cost of okra in Alabamawithout plastic mulch or row coverto be $2834.32/acre. The price ofblack plastic mulch is estimated at$252.00/acre and a row cover at$637.00/acre. Mossier and Dunn(2005) have given a price as high as$1.66/lb for okra. The profit of

marketable okra produced usinga row cover was $1 .37 versus $1.35per pound without a cover in 2003and $1.28 versus $1.29 per pound in2004. Combining row covers withplastic mulch did not give an advan-tage to the grower over plastic mulchalone. At locations where cool early-season temperatures do not occur,installing a row cover may increaseyield, but the additional yield may notbe sufficient to increase the return oninvestment. Plastic mulch did increaseyield over bare soil. Even the leastproductive plastic mulch color (silverin 2003 and red in 2004) increasedprofitability over bare soil by $0.10and $0.55 per pound in 2003 and2004, respectively. Neither plasticmulch nor row cover had a great in-fluence on the percentage of fruit lostas cull. There was no analysis of theeffects of different colored mulch oninsect and disease pressure.

Blue plastic mulch producedplant height, fresh weight, and earlyand total yield comparable to blackplastic. The FR:R and blue light re-flectance from the various plasticmulches was not measured duringthis experiment. It is possible thatthe FR:R and blue light reflectedfrom the plastic mulches could haveimproved okra growth and yield.Decoteau et al. (1989) and Hattet al. (1993) found the FR:R ratiohad a positive effect on vegetablecrops. Blue plastic mulch is $0.08

per foot more expensive than blackplastic. Our data do not show aneconomic advantage for blue overblack mulch for okra, but the positiveeffect cited by other authors may bemore pronounced with leafy vegeta-bles. More research needs to be doneto know the effect that row coverswith various colored plastic mulcheshave on the earliness and productionof vegetable crops.

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