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1. Proc. Fla. State Hort. Soc. 99: 307-311. 1986.
FREEZE PROTECTION OF STRAWBERRIES WITH FLOATING ROW COVERS
G. J. HOCHMUTH, S. R. KOSTEWICZ, AND S. J. LOCASCIO
Vegetable Crops Department
University of Florida, IFAS
Gainesville, Fl 32611
E. E. Albreghts and C. M Howard
Agriculture Research and Education Center
University of Florida, IFAS
Dover, FL 33527-9664
C. D. Stanley
Gulf Coast Research and Education Center
University of Florida, IFAS
Bradenton, FL 34203
Additional index words, drip irrigation, trickle irrigation,
sprinkler irrigation, early yield, Fragaria x ananassa.
Abstract. Three types of nonwoven and 3 types of polyethylene
row covers were evaluated alone, and with drip or sprinkler
irrigation for freeze protection of strawberries (Fragaria x
ananassa, Duch.) at Gainesville and Dover, Fla. Treatments
were applied only during freeze events. Thirteen freeze events
occurred at Gainesville during the 15 Dec, 1985 to 31 Jan.,
1986 period while only 4 freeze events occurred during the
same period at Dover. Air and flower temperatures were
higher under row covers compared to that in the uncovered
treatment. However, during the 25/26 Dec, 1985 freeze, only
the 1.5-oz polypropylene (Dover) and the 2.0-oz poly
propylene and the polyethylene blanket row covers (Gaines
ville) maintained temperatures above 30°F. Early (Dec. and
Jan.) fruit yield produced with some covers used alone was
equal to that obtained with sprinkler irrigation alone. At
Gainesville, the use of certain row covers in conjunction with
sprinkler irrigation, but not with drip, resulted in increased
early yield over use of row cover or sprinkler irrigation alone.
Florida produces about 5,000 acres of strawberries
yearly using the annual-hill system on polyethylene-
mulched beds (2). Most of the crop is grown in the Plant
City area and harvested from Dec. through Apr. During
this period, the plants and fruits are generally exposed to
several periods of freezing temperatures. Sprinkler irriga
tion is presently used to protect the plants, flowers, and
fruit during these freezes (4). During particularly severe
freezes, 8 to 10 inches of water might be applied to achieve
protection during a 2-day freeze period. Prolonged and
intense irrigation for freeze protection can result in field
erosion, fertilizer leaching, electrical shortages, and a low
ering of the water table. The problems are intensified dur
ing freezes accompanied by wind because more water is
required than during calm freezes (3). Uniform applica
tion of water is difficult under these conditions, and dam
age to plants and fruits from ice and water may occur.
Because the problems associated with freeze protection
by sprinkler irrigation, alternative freeze protection
methods are of interest. Row covers are used elsewhere in
the country for growth enhancement and certain of these
row covers have been reported to provide a minimal
amount of freeze protection of warm-season crops (5).
These row covers are light-weight (0.6 oz/yd), non-woven
materials that are laid over the plants in the field. Heavy
weight materials (1.0 to 2.0 oz/yd) are available but have
not been investigated for freeze protection.
The objective of our studies was to investigate the
freeze-protection capabilities for strawberries of various
row covers used alone or in combination with drip or
sprinkler irrigation. This paper reports the effects of row
covers and irrigation methods on air and flower tempera
tures during the severe freeze event of December 25/26,
1985. In addition, the effects on early strawberry yields
are presented.
Materials and Methods
Studies were conducted at Gainesville and Dover, Fla.
during the winter of 1985-86 using split-plot experiments
in randomized complete-block designs. Main plots were
irrigation treatments and sub-plots were row cover mater
ials. Irrigation treatments, at both locations were none,
drip, and overhead sprinkler (Gainesville) or overhead
spray-jets (Dover). Row cover treatments used at Gaines
ville and Dover are described in Table 1. Irrigation and
row cover treatments were applied only during a particular
freeze event.,
The experimental units at Gainesville consisted of 3
raised beds each about 6 inches high with 4 ft between
centers. The plots were 20 ft in length. During bed prepa
ration, the soil (pH = 7.2) was fertilized by incorporation of
N, P, K and a micronutrient fertilizer mix at the rate of
Table 1. Row cover materials used in strawberry freeze protection studies.
Material and wt.
per yd
or thickness
Extruded fabric (0.5 oz)
Polypropylene #1
Polypropylene #2
Polypropylene #3
(2.0 oz)
Polyester #1 (0.6 oz)
Polyester #2 (1.0 oz)
Polyester #3 (1.75 oz)
Polyethylene tunnel
(perforated)
Clear polyethylene sheet
(.0011 inch thick)
Polyethylene blanket
(.125 inch thick)
Abbrevia
tion
EF
PP-1
PP-2
PP-3
PE-1
PE-2
PE-3
PT
PS
PB
Loca
tion7
G
G
G,D
G
G,D
D
D
G
D
G,D
Source
CDK Internation Corp.
3191 Wicks Creek Trail
Marietta, GA 30062
Kimberly-Clark Corp.
1400 Holcomb Bridge Rd.
Roswell, GA 30076
E. I. Dupont, Inc.
Textile Fibers Dept.
Center Road Bldg.
Wilmington, DE 19898
Agplast, Inc.
PO Box 318
Ellenton, FL 33532
Asgrow—Florida
Plant City, FL 34289
Industrial Pkg.
Materials
PO Box 1702
Eaton Park, FL 33801
Florida Agricultural Experiment Station Journal Series No. 7833.
Proc. Fla. State Hort. Soc. 99: 1986.
'Locations were Gainesville (G) and Dover (D).
307

2. 80, 43, 66, and 30 lb. per acre respectively. This fertilizer
represented 40, 100, 40, and 100% of N, P, K, and micro-
nutrients, respectively. The remaining N and K fertilizer
was applied in weekly applications by drip irrigation. Fol
lowing fertilization, the beds were fumigated with methyl
bromide-chloropicrin (67-33% mixture) at the rate of 400
lb./acre. The beds were covered with black polyethylene
mulch.
Drip irrigation tubes were placed under the mulch for
supplying fertilizer to all plots, and for supplying water to
specific main plots during freeze events. Drip emitters
were spaced 12 inches apart and applied 0.5 gal per emit
ter per hr. Revolving, sprinkler irrigation nozzles were
positioned (4 nozzles per main plot) to apply 0.25 inch/hr
of water uniformly over plots during freeze events.
Plots at Dover were similar except that the experimen
tal unit consisted of single beds, 10 ft in length. All fer
tilizer (200, 22, and 166 lb. per acre of N, P, K respectively)
was applied before mulching. One-fourth of the fertilizer
was incorporated in the bed and 3/4 was banded in the
center of the bed. Routine irrigation at Dover was supplied
by overhead sprinkler irrigation. Drip irrigation was used
only on specific treatments during a freeze event.
'Douglas' strawberries were placed in twin rows on beds
at each location. There were 12 inches between rows on
beds and between plants in rows. Sprinkler irrigation was
used during the first 2 weeks to aid establishment of trans
plants.
Row covers were placed in the plots by covering one
edge with soil and furling the cover in the alley between
beds to await use during a freeze. Thermocouples were
placed in the plant crown area at Gainesville and attached
to open flowers at Dover for measuring temperatures dur
ing freeze events.
Row covers were deployed in the afternoon prior to an
impending freeze by covering plants and anchoring edges
with weights or soil. Irrigation treatments were applied
during the period when air temperature at 2-inch height
above the bed of an uncovered, unirrigated plot was below
32°F. Irrigation was discontinued when the air tempera
ture rose above 32°F and ice began to melt. Row covers
were removed after ice had melted.
Table 2. Description of freeze events from Dec. 15, 1985 through Jan.
31, 1986 at Gainesville and Dover, FL
Period below 32°F (hr) Lowest air temperaturey (°F)
1 1 ~~£.V-
event
Dec,1985
15/16
16/17
17/18
19/20
21/22
25/26
26/27
27/28
30/31
Jan., 1986
5/6
13/14
27/28
28/29
yMeasured 2
308
Gainesville
12
8
8
8
11
14
16
6
8
8
10
16
12
inches above bed
Dover
8
12.5
9.5
8.5
surface.
Gainesville
24
31
31
30
26
15
20
31
28
26
26
14
27
Dover
24
25
22
29
Mature fruits were harvested thrice weekly and graded
to determine marketable,cull, and freeze-damaged fruits.
Data on early (Dec-Jan.) marketable yields are reported in
this paper.
Results and Discussion
Freeze events for Dec. and Jan. are summarized in
Table 2. Two severe freezes occurred at both locations dur
ing 25-27 Dec, 1985 and 27-29 Jan., 1986. At Gainesville,
several additional freezes also occurred during Dec. and
Jan. At Gainesville, 35 inches of water were applied by
sprinkler irrigation during Dec and Jan. for freeze protec
tion.
Row covers used alone during the 25-26 Dec. freeze at
Dover (min. temp. 25°F) maintained 2 to 6°F higher flower
temperatures than in uncovered plots (Fig. 1). However,
only the PP-2 treatment maintained the flower tempera
ture above the critical point of 30°F (1). Flower tempera
tures were below 30°F under other covers for periods rang
ing from 1 to 3 hours. In most cases, drip irrigation did
not increase flower temperatures over those in plots co
vered with row covers alone (data not shown). Flower tem
peratures in all plots were above 32°F when sprinkler irri
gation was used.
Minimum air temperature at Gainesville during 25-26
Dec was 15°C. With no irrigation, only the PP-3 and PB
row covers maintained air temperature at, or above 32°F
(Fig. 2). The use of drip irrigation did not increase air
temperatures over those obtained with row covers alone
(Figs. 2 and 3). The use of sprinkler irrigation, however
maintained air temperature with all row covers at, or above
32°F, for the duration of the freeze (Fig. 4). Row covers,
in combination with sprinkler irrigation maintained air
temperature in all plots above 35°F.
Irrigation and row covers interacted in their effects on
early fruit yield at Dover (Table 3). Strawberries protected
with row covers alone produced similar early yields to
those obtained with row covers combined with drip or
sprinkler irrigation. Sprinkler-irrigated plots with no row
covers were not included at Dover but a non-factorial un
irrigated, uncovered plot yielded only 80 flats per acre.
Irrigation and row covers interacted in their effects on
early fruit yield at Gainesville (Table 4). In most cases,
" 25
A No cover
35
30
25
0200 0300 0400 0500 0600 0700 0800
TIME (hrs) Dec. 25-26, 1985
Fig. 1. Effect of row covers on strawberry flower temperature, Dover,
25-26 Dec, 1985
Proc. Fla. State Hort. Soc. 99: 1986.

3. B
50
45
40
L. 35
UJ
S 30
<
jjj 25
2E
UJ
H- 20
flC
<
15
10
• No cover
O PT
▲ PP-1
- D PE-1
■ EF
-
>•*
-
-
-
hi i i
NO IRRIGATION
GAINESVILLE
i
//
//// //
JJL-a
tN*ci*l fr/I
m •**?-« M*//
T
>
1 I 1 1 1 1
50
45
40
35
30
25
20
15
10
LL
o
UJ
or
OC
UJ
Q.
s
UJ
<
50
45
40
35
30
25
20
15
10
>
_ • No cover
A PP-2
# pp.o
_O PB3
-
-
-
NO IRRIGATION
GAINESVILLE
/
u
A A A ^^«*-^
//
/
/
V
i i i i
/"
//
r
-
-
-
—
50
45
40
35
30
25
20
15
10
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
Fig. 2. Effect of light-weight row covers (A) and heavy-weight row covers (B) on air temperature in strawberry plant crown area for unirrigated
plots, Gainesville, 25-26 Dec, 1985.
B
50
45
DRIP-IRRIGATED
GAINESVILLE
50
45
40
15
10 10
• No cover
A PP-2
# PP-3
O PB
DRIP-IRRIGATED
GAINESVILLE
L>
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
>L
50
45
40
10
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
Fig. 3. Effect of light-weight row covers (A) and heavy-weight row covers (B) on air temperature in strawberry plant crown area for drip-irrigated
plots, Gainesville, 25-26 Dec, 1985.
Proc. Fla. State Hort. Soc. 99: 1986.
309

4. B
50
r 45
• No cover
- 0 PT
▲ PP-1
0 PE-1
■ EF
SPRINKLER
IRRIGATION 50
45
40
35
oc
D
OC
LU
Q.
UJ
-; 30
I J_
50
45
40
35
30^-
- # No cover
APP-2
#PP-3
OPB
SPRINKLER
IRRIGATION
50
45
40
35
- 30
I j_
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
1900 2300 0300 0700
TIME (hrs) Dec. 25-26, 1985
Fig. 4. Effect of light-weight row covers (A) and heavy-weight row covers (B) on air temperature in strawberry plant crown area for sprinkler-ir
rigated plots, Gainesville, 25-26 Dec, 1985.
there was no benefit from operation of drip irrigation dur
ing the freeze. Only with the thickest polypropylene mater
ial (PP-3) was there benefit from drip irrigation compared
to none. Generally, not enough heat was produced with
drip irrigation to influence early fruit yield. Sprinkler irri
gated strawberries yielded more early fruits than drip or
unirrigated strawberries. With some row covers, there was
an additive effect of cover and sprinkler irrigation for in
creasing early yield. For example strawberries with EF, PP-
3, PT, or PB in combination with overhead irrigation pro
duced more fruits than plants with row cover or sprinkler
irrigation alone (Table 4).
Where no irrigation was used, highest yields were ob
tained with PB, PP-3, and PP-1 treatments. Lowest yields
were from the uncovered plants or from plants covered
with the light-weight materials such as EF, PE-1, and PT.
With some row covers, early yields were equal to the uncov
ered, sprinkler-irrigated check (LSD .05 = 82). With some
row covers (EF, PP-3, PT, and PB), used in combination
Table 3. Effects of irrigation and row covers
strawberries, Dover, Fl.
Row cover
PE-1
PE-2
PE-3
PS
Early
None(l)
227az
230a
264a
162b
on early (Dec-Jan.) yield of
marketable yield (flats/acre)
Irrigation method
Drip(2)
88b
253a
253a
163b
Sprinkler(3)
266a
287a
278a
241a
Irrigation
significancey
(1,3)(2)
NS
NS
NS
with sprinkler irrigation, yields of early fruit were greater
than with sprinkler irrigation alone. Possible reasons for
this depend on the type of cover. The EF material is very
porous to air and water. The benefit from its use in combi
nation with sprinkler irrigation might have resulted from
enhanced water coverage of the plants with the cover in
place. This cover might have broken water droplets and
facilitated water coverage of plants. The basis for the addi
tive effect of sprinkler irrigation with the other covers
might result from their non-porous nature. Ice was ob
served to accumulate on these covers during severe
freezes. The added insulating capacity of ice might be re
sponsible for the increased yields.
These data indicate that certain row covers, when used
alone can protect strawberries and that early yields were
equal to those produced with sprinkler irrigation alone.
Table 4. Effects of row covers and irrigation on freeze protection of early
(Dec-Jan.) strawberry yield, Gainesville, 1985-86.
Early marketable yield (flats/acre)
Row cover
None
EF
PP-1
PP-2
PP-3
PE-1
PT
PB
None(l)
4cz
20c
HOab
63bc
lOlab
43bc
52bc
161a
Irrigation method
Drip(2)
Oc
40bc
81bc
127ab
179a
100b
67bc
107b
Sprinkler(3)
128d
276a
146cd
I76bcd
231ab
161bcd
215abc
240ab
Irrigation
significancey
(3)(1,2)
(3)(2,1)
NS
<3f2)(2,l)
(3,2)(1)
(3,2)(2,1)
(3)(2,1)
(3,1)(1,2)
yNumbers in parentheses not significantly different by Duncan's multiple
range test, 5% level or effects were not significant (NS).
zMean separation in columns by Duncan's multiple range test, 5% level.
310
yNumbers in parentheses not significantly different by Duncan's multiple
range test, 5% level or effects were not significant (NS).
zMean separation in columns by Duncan's multiple range test, 5% level.
Proc. Fla. State Hort. Soc. 99: 1986.

5. Operation of drip irrigation during a freeze did not pro
vide a beneficial amount of heat to be trapped by a row
cover and did not protect strawberries from freezing.
Combinations of some row covers with sprinkler irrigation
gave an additive effect on early yield protection over either
alone. In our research, the amount of sprinkler-applied
water was the same with all row covers. Future research
will address the question of whether the amount of sprink
ler-applied water can be reduced with a row cover in place.
Literature Cited
Albreghts, E. E. and C. M. Howard. 1984. Strawberry production in
Florida. Univ. Fla. Agr. Expt. Sta. Bui. 841.
Anonymous. 1984. Florida Agricultural Statistics—Vegetable Sum
mary. Fla. Crop and Livestock Reporting Serv. Orlando.
Harrison, D. S., J. F. Gerber, and R. E. Choate. 1974. Sprinkler irriga
tion for cold protection. Univ. Fla. Coop. Ext. Serv. Cir. 348.
Locascio, S. J., D. S. Harrison, and V. F. Nettles. 1967. Sprinkler irri
gation of strawberries for freeze protection. Proc. Fla. State Hort. Soc.
80:208-211.
Wells, O. S. and J. B. Loy. 1985. Intensive vegetable production with
row covers. HortScience 20:822-826.
Proc. Fla. State Hort. Soc. 99: 311-314. 1986.
USE OF HYBRIDS TO DEVELOP HEAT TOLERANT TOMATO CULTIVARS
J. W. Scott
IFAS, University of Florida
Gulf Coast Research £ff Education Center
5007 60th Street East
Bradenton, FL 34203
R. B. Volin
Northrup King Company
10290 Greenway Road
Naples, FL 33962
H. H. Bryan
IFAS, University of Florida
Tropical Research £s? Education Center
18905 S.W. 280th Street
Homestead, FL 33031
S. M. Olson
IFAS, University of Florida
North Florida Research £sf Education Center
Route 3, Box 4370
Quincy, FL 32351
Additional index words, heterosis, yield, fruit-size, Lycopersi-
con esculentum.
Abstract. Hybrids were made between tomato (Lycopersicon
esculentum Mill.) genotypes with various levels of fruit set
ting ability under high temperature, high humidity condi
tions. Parents and hybrids were tested under such conditions
in 1982, 1983, and 1986. When both parents were heat toler
ant, 2 of 2 hybrids had heterosis for yield (fruit wt.) at Braden
ton and 1 of 2 had heterosis for yield at Homestead. When
one parent was heat sensitive and the other was heat toler
ant, 1 of 2 hybrids was heterotic at Homestead but neither
was heterotic at Bradenton. When small fruited, heat tolerant
inbreds were crossed with larger fruited heat sensitive in-
breds, the hybrids had intermediate fruit size, often greater
than that of the heat tolerant parent. Fruit numbers for such
hybrids were comparable or less than that of the heat tolerant
parent and yields were comparable or greater than the heat
tolerant parent. When larger fruited heat tolerant and heat
sensitive parents were crossed, fruit numbers were generally
Florida Agricultural Experiment Station Journal Series No. 7832.
Proc. Fla. State Hort. Soc. 99: 1986.
comparable for the hybrids and their respective heat tolerant
parents and often the hybrid fruit number was greater than
that of the respective heat sensitive parent. Fruit size of these
hybrids generally was not significantly different than that of
the heat tolerant parent but hybrid fruit size was significantly
larger than the heat tolerant parent in a few cases. The data
support the use of hybrids to facilitate commercialization of
heat tolerant cultivars.
Florida's tomato production season extends from Oc
tober through June with virtually no production during
the summer. Although tomato production in the northern
U.S. and Canada precludes exports in the summer,
Florida's high temperatures and frequent rains prevent
local market production as well. These conditions inhibit
fruit set and are conducive to several diseases, with bacte
rial spot incited by Xanthomonas campestris pv. vesicatoria
being the most severe. Heat tolerant cultivars would make
it possible to produce tomatoes either during the summer,
for local markets, or for earlier, more productive fall crops
for shipping.
There are virtually no heat tolerant cultivars adapted
to Florida's hot, humid conditions. Most heat tolerant ac
cessions, such as those from the Asian Vegetable Research
and Developmept Center (AVRDC) are too small fruited
for the Florida market. Another recently released breeding
line Treshmarket 9' (6) has a small vine which is very sus
ceptible to bacterial spot and thus not of commercial poten
tial (Scott, unpublished data). Development of heat toler
ant cultivars is a difficult task in part because the trait is
complex (2, 5, 7, 8) and subject to low (9) or moderate (3)
heritabilities which make selection difficult. In breeding
larger fruited cultivars for hot, rainy conditions, additional
problems such as fruit cracking, black shoulder, and rough
blossom-ends become major obstacles. Thus, it is difficult
to fix all desired traits in a single line.
In 1981, the senior author observed a high degree of
fruit set in hybrids between heat tolerant and heat sensitive
breeding lines. It appeared that utilizing such hybrids,
combining desirable traits from both parents, might be
useful in developing commercially acceptable heat tolerant
cultivars. Several experiments were conducted to test the
feasibility of this approach. The objective of this paper is
to report the findings from these experiments.
311