Comparison of Freeze Protection Methods for Strawberry Production in Florida

Abstract

During strawberry (Fragaria × ananassa) freeze protection, the standard practice is applying sprinkler irrigation on plant canopies to prevent and reduce freezing damage. This method is highly inefficient because it uses large volumes of water. The overall goal of this study was to compare the effects of freeze protection methods on water savings, and growth and fruit weight of strawberry. Treatments consisted of the following: sprinkler heads delivering 17 L∙min^–1, sprinkler heads delivering 13 L∙min^–1, light-weight row covers (21 g∙m^–2) on the crop canopy, light-weight row covers on 60-cm-high mini-tunnel hoops, heavy-weight row covers (31 g∙m^–2) on the crop canopy, heavy-weight row covers on 60-cm-high mini-tunnel hoops, and foliar application of a polymer (Desikote). There were eight freezing and near freezing nights (≤1.1 °C) at the experimental site during both seasons with a minimum air temperature of –2.8 °C. Minimum temperature inside the row covers ranged between 0.6 to 4.4 °C at the canopy level. There were significant treatment effects on early and total marketable fruit weights. The highest early marketable fruit weight was found in plots protected with light-weight row covers with hoops, heavy-weight row covers on canopy, and foliar polymer, ranging between 5.0 to 5.5 t∙ha^–1. For total marketable fruit weight, using non-irrigation methods resulted in the highest fruit weight with 23.0 t∙ha^–1. In conclusion, data showed that using non-irrigation techniques provided satisfactory freeze protection under the evaluated conditions and increased total marketable fruit weight, possibly due to reduced injury of fruits and flowers when using alternative techniques.

KEYWORDS:

• Di-1-p-menthene polymer
• Fragaria × ananassa
• row covers
• reduced-volume sprinkler irrigation
• water usage

Introduction

Florida is the major producer of strawberries during winter in the U.S. with about $360 million in sales during the 2011–12 season (USDA, 2012). Production fields are concentrated in the west-central area of the state with more than 4000 ha (Boriss et al., 2012; USDA, 2012). Water use in this area is a concern due to competition between agricultural and urban uses (Hochmuth et al., 2006; Santos et al., 2011a, 2012). During the season, strawberries are subjected from four to six freezing nights.

Sprinkler irrigation is commonly used to protect the crop during the cold weather. This practice consumes about 559 m³∙ha^–1 per freezing night (Hochmuth et al., 1993, 2006; Santos et al., 2011a, 2012). During the unusual winter of 2010, around 11 freezing nights occurred and large volumes of water were pumped to protect the crop. As a result of these freezing nights, the aquifer level dropped 18 m, allegedly causing 750 residential dried wells and more than 140 sinkholes in the area (Aurit et al., 2013; Southwest Florida Management District, 2011). This phenomenon has occurred three times over the last 10 years. After the last period of freezing nights in 2010 and 2011, the Southwest Florida Water Management District began working on trying to implement a water management plan to protect the aquifers. In consequence, there is a need to investigate alternative freeze protection methods that could reduce volumes of water used during strawberry production.

The quantity of water used to protect the strawberry crop from freezing damage represents about one-third of the total amount of water utilized during the whole season (Hochmuth et al., 2006; Santos et al., 2011a). The physical phenomenon behind this method of freeze protection is based on how water absorbs and releases energy when it changes from one physical state to another. A calorie is described as the quantity of heat needed to raise by 1 °C, 1 g of water at 1 atm (Perry, 1998). Under freezing conditions, the applied water turns from liquid to ice, releasing energy. For 1 g of water turning to ice, approximately 80 calories are released, supplying heat needed to keep plant tissues near or above the freezing point; this process is called latent heat of fusion (Perry, 1998). At the same time, evaporative cooling is occurring. In this process, water absorbs approximately 540 cal when it turns from liquid to vapor. This is nearly seven times what is being released by latent heat of fusion. Consequently, it becomes necessary to apply seven times the volume of water needed in order to keep a balance between the energy released by latent heat of fusion and the energy absorbed by evaporative cooling. Otherwise, the effect will be the opposite, damaging the crop (Perry, 1998).

Reduced-volume sprinklers, foliar polymers, and row covers could be alternatives to sprinkler irrigation in strawberry. However, more research is needed to assess the effect of these techniques on growth and yield. In general, sprinklers are set for the worst scenarios and deliver more water than is needed for crop protection and growth (Fisher and Shortt, 2009; Perry, 1998). Environmental factors, such as wind speed, dew point, and relative humidity, affect appropriate application and distribution of water. Reduced-volume sprinklers might be a suitable option to decrease volumes of water used during freeze protection. However, during windy conditions, when sprinkler nozzle size is reduced and distance between sprinklers is larger, freeze protection might be compromised (Fisher and Shortt, 2009; Locascio et al., 1967).

Many chemical products for freeze protection prevent or reduce freezing damage on plant tissues. The mode of action of these products varies depending on the composition. Some of them are antitranspirants that create a physical barrier, reducing heat loss from plant tissues (Burns, 1970). Desikote (40% di-1-p-menthene) is a pinolene-based polymeric material derived from conifer resins. Many studies have been conducted using similar products with inconclusive results. Research conducted with young citrus (Citrus spp.) plants using antitranspirants resulted in no protection with minimum temperatures of –5 and –3.9 °C (Burns, 1970). Other studies conducted in tomato (Solanum lycopersicum), pepper (Capsicum annuum), and peach (Prunus persica) plants using a cryoprotectant and an antitranspirant did not provide freeze protection when temperature decreased to –3.3 and –1.1 °C (Aoun et al., 1993; Perry et al., 1992). However, a study conducted with tall fescue (Festuca arundinacea) resulted in 22% higher seed weights compared with the unprotected seeds when temperature went down to –1.1 and –2.2 °C (Hare, 1995). Gardea et al. (1993) reported positive results when using an antitranspirant to protect grape (Vitis vinifera) plants at –2.2 °C. However, at lower temperatures limited protection was found.

Row covers are flexible, transparent, or semi-transparent blankets made up of polyethylene, polypropylene, polyester, and other materials. They have been used extensively to prolong growing seasons. During winter, these blankets are placed over the crop between 2 and 4 weeks to provide a warmer environment that enhances crop growth (Dickerson, 2009; Wells and Loy, 1993). In the 1980s, row covers started to be used on commercial farms as an alternative for Florida growers to protect their crops during freezing nights (Hochmuth et al., 1986). Row covers used for freeze protection commonly range from 20 to 50 g∙m^–2, and they are placed only during freezing nights (Hochmuth et al., 1986; Perry, 1998; Wells and Loys, 1993). The mechanism of action for using row covers is to enclose the mass of air around plant canopies, thus trapping the radiational heat already absorbed by the plant and soil during the day. Using galvanized or fiberglass hoops is a common practice to anchor row covers and avoid plant canopy damage due to direct contact with the material (Perry, 1998; Wells, 1996). Diverse degrees of protection are accomplished with row covers, ranging from 2 to 7 °C, depending on material and thickness (Dickerson, 2009; Hochmuth et al., 1993; Poling et al., 1991; Santos et al., 2011a). The overall goal of this study was to determine the effectiveness of water-saving strategies during strawberry freeze protection and their effect in growth and yield.

Materials and methods

The study was conducted during two seasons (2011–12 and 2012–13) at the Gulf Coast Research and Education Center (GCREC) of the University of Florida in Balm, Florida. The soil at the experimental site is a Myakka sandy, siliceous hyperthermic Oxyaquic Alorthod with <1.5% organic matter and a pH of 6.6. Prior to the experiment the soil was tilled twice at approximately 20 cm deep to ensure proper soil structure. In late August of each season, planting beds were formed using a standard bedder measuring 69 cm wide at the base, 61 cm wide at the top, 20 cm high, and 33 cm apart between bed centers. Simultaneously with bedding, the soil was fumigated with 1,3-dichloropropene plus chloropicrin (40:60 v/v) at a rate of 336 kg∙ha^–1. One drip tape line (28 ml∙m^–1 per min, T-Tape Systems International, San Diego, CA, USA) was buried 5 cm below the surface. Beds were covered with high-density black polyethylene mulch (0.025 mm thick; Intergro Co., Clearwater, FL, USA). Transplants were set in double rows with 30 cm separation between rows and they were spaced 38 cm from each other.

Plant nutrients, such as nitrogen, potassium, magnesium, iron, zinc, boron, and manganese, were applied following the current recommendations for the crop in the state (Santos et al., 2011b). Other essential elements were at sufficient levels according to soil tests conducted one month before planting. Daily fertilizer applications started at 2 weeks after transplanting (WAT) through the drip lines using a hydraulic injector (Dosatron, Clearwater, FL, USA). Irrigation volumes were based on the average reference evapotranspiration for the area from October to March and the crop growing stages (Simonne and Dukes, 2009). Irrigation volume was split equally into two daily cycles each starting at 8 am and 1 pm. Recommendations for insect and disease control were followed depending on pest pressure (Santos et al., 2011b). ‘Strawberry Festival’ bare-root transplants with three to five leaves from nurseries in Canada (Lareault Nursery, Lavaltrie, Quebec, Canada) were planted in the field in mid-October during both seasons. Treatments were set in a randomized complete block design with three replications. Each plot consisted of four adjacent 9.5-m-long rows with 50 plants per row. The outside rows on each plot were considered as borders. The two central beds were used for harvests. A 7.6-m-long non-planted buffer zone at the end of each plot was set to avoid water overlapping across treatments. Immediately after transplanting, sprinkler irrigation with 17 L∙min^–1 sprinkler heads was turned on at 8 am each morning for 8 h∙day^–1 during the first 10 days to ensure transplant establishment.

Treatments consisted of: (a) sprinkler heads delivering 17 L∙min^–1 (4.3-mm-diameter nozzle; Rain Bird, Azusa, CA, USA); (b) sprinkler heads delivering 13 L∙min^–1 (3.6-mm-diameter nozzle; Rain Bird, Azusa, CA, USA); (c) light-weight row covers on the crop canopy (21 g∙m^–2; Agribon row cover, Gardener’s Supply Company, Burlington, VT, USA); (d) light-weight row covers on 60-cm-high mini-tunnel hoops; (e) heavy-weight row covers on the crop canopy (31 g∙m^–2; Agribon row cover, Environmental Green Products, Phoenix, OR, USA); (f) heavy-weight row covers on 60-cm-high mini-tunnel hoops; and (g) foliar application of the polymer (Desikote, 40% di-1-p-menthene; Engage Agro, Prescott, AZ, USA). Sprinklers were set 14.6 m apart and they were turned on when air temperature at 60 cm above the surface was 1.1 °C. Irrigation was turned off when ice completely melted. Row covers were placed between 2 and 5 pm on the afternoon of the forecast freezing event and 2-kg sand bags were used to hold the row covers. They were removed once the freezing event ended. The foliar polymer was applied with a 7.6-L hand-held foliar sprayer at the rate of 5.1 L∙ha^–1 at the same time when row covers were placed. This procedure allowed the formation of a protective film on leaves, fruit, and flowers.

To assess the effect of the treatments on strawberry growth and development, five randomly selected plants were chosen to measure canopy plant diameter at 4, 8, and 12 WAT. Border plants were not used for evaluation. The same plants were used for all three observations. Canopy plant diameter was measured by using a ruler with a slide; the ruler was placed on the top of the plant canopy, perpendicular to the direction of the rows to ensure measure consistency. Early and total marketable fruit weight and number were collected for the first 10 and 24 harvests, respectively. Moreover, the first six harvests after the freezing nights of 5 Jan. and 13 Feb. 2012 were analyzed to identify immediate treatment effects after the freezing nights. Plots were harvested twice a week on Mondays and Thursdays, starting in early December of each season until early March for a total of 24 harvests per season. Marketable fruit was defined as a fruit over 10 g in weight, physiologically mature with more than 80% dark red skin, free of defects or disease injury. Climate data from both seasons were collected from Florida Automated Weather Network (FAWN). Minimum temperatures were measured at 15 cm above canopy level using temperature loggers with an accuracy of ±0.7 °C at 21 °C and ±1.3 °C at 70 °C (HOBO data loggers, model H08-002-02, Onset Corp., Bourne, MA, USA). Data were analyzed using the general linear model (P ≤ 0.05) and treatment values were separated using Fisher’s protected least significant difference test (Statistix Analytical Software, version 9, Tallahassee, FL, USA).

Results and discussion

Treatment by season interactions were not significant. Eight freezing and near freezing nights (≤1.1 °C) occurred during both seasons (Table 1). The minimum air temperature registered was –2.8 °C according to data logger temperature sensors located in the fields. Calm conditions with wind speeds ≤8 km∙h^–1 prevailed during those nights (data not shown). The minimum temperatures directly above crop canopy in plots covered with light or heavy-weight row covers ranged between 4 and 7 °C higher than the outside air temperature, regardless of the use of hoops (Table 2). No water for freeze protection was needed in plots where row covers and the foliar polymer were applied. Treatments did not have a significant effect on plant diameter, averaging 41 cm at 6 WAT and remaining at 43 cm for the last two sampling dates (data not shown). Early marketable fruit weight and number were significantly different among treatments. Plots protected with light-weight row covers with hoops, heavy-weight row covers without hoops, and those treated with the foliar polymer obtained the highest early marketable fruit weight and number, averaging 5.2 t∙ha^–1 and 226,159 fruit∙ha^–1, respectively (Table 2). Sprinklers delivering 17 L∙min^–1 resulted in the lowest fruit weight and number with 3.7 t∙ha^–1 and 220,047 fruit∙ha^–1. Moreover, plots with non-irrigation methods for freeze protection had the highest marketable fruit weight and number at the end of the strawberry season with 23.0 t∙ha^–1 and 905,709 fruit∙ha^–1, respectively. The lowest fruit weight and number was registered in plots using sprinkler irrigation, regardless of the water volumes with 17.3 t∙ha^–1 and 889,645 fruit∙ha^–1, respectively.

Table 1. Freezing and near freezing nights occurred during 2011–12 and 2012–13 seasons, Balm, FL.

Date	Min. air T°^z	Period below 1.1 °C
	(°C)	(h)
2011–12 Season
4 Jan. 2012	–1.1	8.3
5 Jan. 2012	–2.8	5.8
15 Jan. 2012	–1.7	6.3
12 Feb. 2012	–0.6	1.8
13 Feb. 2012	–0.6	7.0
2012–13 Season
23 Dec. 2012	0.6	1.8
18 Feb. 2013	0.0	5.5
4 Mar. 2013	0.0	2.0

^zMinimum temperature measured at 60 cm above the ground in the open field with no irrigation.

Table 2. Effect of freeze protection methods on the minimum seasonal temperature for each treatment and the first six harvests after a freezing event on strawberry marketable fruit weight and number, Balm, FL, 2011–12.

	5 Jan. 2012			13 Feb. 2012
	Fruit no.	Fruit weight	Min. T°^z	Fruit no.	Fruit weight	Min. T°
Treatments	(fruit∙ha^–1)	(t∙ha^–1)	(°C)	(fruit∙ha^–1)	(t∙ha^–1)	(°C)
17 L∙min^–1 sprinklers (control)	126,930c^y	2.2c	–2.8	476,196bc	10.2bc	–0.6
13 L∙min^–1 sprinklers	143,262bc	2.6bc	–2.8	399,411c	8.3c	–0.6
Light-weight row cover on canopy	153,861bc	2.9ab	0.6	597,675a	13.3a	4.6
Light-weight row cover with hoops	177,928a	3.3a	0.6	557,570ab	12.2ab	5.2
Heavy-weight row cover on canopy	153,861bc	2.9ab	0.6	625,181a	13.4a	7.0
Heavy-weight row cover with hoops	142,402bc	2.7abc	0.6	546,104ab	12.2ab	7.0
Foliar polymer Desikote	160,737ab	3.0ab	–2.8	576,764a	12.5a	–0.4
Significance (P < 0.05)	*	*		*	*

^zMinimum air temperature in the row covered plots were taken 60 cm above plant canopies, whereas the temperature in the sprinkler-treated plots were the air temperatures without irrigation.

^yValues followed by the same letter do not differ according to Fisher’s protected least significant difference at the 5% level.

*Significant (P < 0.05).

Analysis of the first six harvests during the freezing event on 4 Jan. 2012, revealed that the highest fruit weight was recorded in plots using row covers and the foliar polymer, averaging 3.0 t∙ha^–1 and 157,758 fruit∙ha^–1. Freeze protection with 17 L∙min^–1 sprinklers resulted in a 25% decrease in fruit weight (Table 3). Similar results were found during the other freezing event on 13 Feb. 2012, with non-irrigation treatments having the highest fruit weight and number (12.7 t∙ha^–1 and 580,658 fruit∙ha^–1). Fruit weight reduction in plots where sprinkler irrigation was used was approximately 28%. This finding suggested that using sprinkler irrigation may have a detrimental effect on strawberry flower and fruit development after a freezing event.

Table 3. Effects of freeze protection methods on early and total marketable fruit weight and number, Balm, FL, 2011–12.

	Early fruit weight^z		Total fruit weight^y
	Fruit no.	Fruit weight	Fruit no.	Fruit weight
Treatments	(fruit∙ha^–1)	(t∙ha^–1)	(fruit∙ha^–1)	(t∙ha^–1)
17 L∙min^–1 sprinklers (control)	220,047d^x	3.7c	889,645c	17.8b
13 L∙min^–1 sprinklers	259,587bcd	4.3bc	863,858c	16.7b
Light-weight row cover on canopy	248,397bcd	4.5bc	1,101,140ab	23.3a
Light-weight row cover with hoops	318,099a	5.5a	1,137,313ab	22.8a
Heavy-weight row cover on canopy	295,066abc	5.0ab	1,199,699a	23.9a
Heavy-weight row cover with hoops	243,150cd	4.2bc	1,049,291b	21.8a
Foliar polymer Desikote	306,218ab	5.1ab	1,170,255ab	23.0a
Significance (P < 0.05)	*	*	*	*

^zEarly fruit weight = first 10 harvests.

^yTotal fruit weight = 24 harvests.

^xValues followed by the same letter do not differ according to Fisher’s protected least significant difference at the 5% level.

*Significant (P < 0.05).

Results from this study were similar to others reported by several authors. Locascio et al. (1967) achieved protection in strawberries at a minimum temperature of –8.9 °C when using 13 and 17 L∙min^–1 sprinkler heads. However, when wind speeds were ≥8 km∙h^–1 and/or temperature was lower than –8.9 °C, 13 L∙min^–1 sprinklers provided limited protection. This result was probably associated to increased evaporative cooling due to high wind speeds. With regards to row covers, Poling et al. (1991) found that light- and heavy-weight row covers protected strawberries at minimum temperatures of –3.9 °C, with no differences on fruit weight compared with sprinkler heads delivering 17 L∙min^–1. Hochmuth et al. (1993) suggested that heavy-weight row covers (30 to 50 g∙m^–2) and 17 L∙min^–1 sprinklers protected strawberry plants at a minimum temperature of –4.4 °C, whereas sprinkler irrigation reduced fruit number by 15%, possibly due to water droplet damage. Santos et al. (2011a) indicated that row covers provided 7 °C protection when air temperature was –6 °C. The same study indicated that fruit weight declined sharply when sprinkler irrigation was used to protect the crop.

Research regarding the use of foliar polymers for freeze protection shows inconsistent results. Hare (1995) found a 22% decrease of freeze injury in tall fescue seedlings when using two types of foliar-applied polymers for freeze protection, when minimum air temperature was –2.2 °C. This effect was probably due to decreased freezing point on seedling tissue. Gardea et al. (1993) reported that two types of cryoprotectants reduced grape leaf disk injury by 25% at a minimum temperature of –2.2 °C; protection was linked to temperature and rate of application. In contrast to these results, research conducted in pepper and tomato transplants reported no protection at –3.3 and –1.1 °C when using two types of cryoprotectants and different application rates (Aoun et al., 1993; Perry et al., 1992). Burns (1970) found similar results in citrus, where 12 antitranspirants were tested and no protection was found when the minimum temperature was –5 °C. While few authors reported similar results to the ones found in this study, freeze protection with this type of polymers will vary depending on the crop, minimum temperatures, and product formulation.

In conclusion, when using freeze protection methods, data showed that non-irrigation methods provided satisfactory freeze protection at minimum temperatures of –2.8 °C. Reduction in yield was observed when using sprinkler irrigation to protect the crop. This might be related to damage on ripe fruit and flowers due to constant water droplet impact. During the strawberry season, about six to eight freezing nights may occur. Cost of these techniques varies: about 9.3 L∙h^–1 of diesel fuel is needed to pump water to irrigate 1 ha, which means approximately $70 per ha cost of application per night, which is translated to $632 per ha in eight freezing nights. When using the reduced-volume sprinklers, it is required to change the nozzle size and the same plumbing can be used; this will cost around $300 (approximately $5 per nozzle). Cost of row covers depends on thickness of the material. This ranges between $24,700 and $3200 per ha plus the labor cost of placing and removing covers, which would be around $370 per freezing event. Duration of the material is between 2 and 4 years depending on use and precaution when handling the covers. Approximate cost when using Desikote is around $160 per ha in one night, plus application cost, which is around $85 per ha including labor and tractor fuel. An advantage of using row covers and the foliar polymer is increase in yield because water damage in fruits and flowers is reduced.

Each of these techniques must be selected according to the specific conditions from each production area. They represent an alternative to reduce the amount of water used during freeze protection. For instance, if 30% of the production area adopted either of these non-irrigation techniques, it would mean 127 m³∙ha^–1 in water savings per freezing night, which will represent about 516,000 m³ in the Plant City area. Water volume saved might have direct implication in protecting surface water resources, reducing risk of sinkholes and dry wells due to water use during periods of prolonged water withdrawals in the strawberry production fields.