In contrast to the majority of the world’s major food crops in which the primary climatological production constraint is the amount and timing of precipitation during the growing season, the primary weather-related constraint for most temperate tree fruit crops is the frequency and severity of spring freeze events. In the Great Lakes region, the frequency of freeze events following initial phenological development during the spring season has increased during the past few decades, with profound impacts on regional fruit production. For example, Michigan’s tart cherry and apple production in 2012 was reduced by about 90% and 88%, respectively, compared to the previous year’s production due to a series of spring freeze events that followed the warmest March on record. The timing of the onset of phenological development in the spring is a key factor in determining potential cold damage risk, as the vulnerability of vegetation to freeze injury increases rapidly with the stage of development. One technique that shows some potential for reducing overall cold damage risk is the application of water to cool vegetative tissue by latent evaporation and delay phenological development. Evaporative cooling with water has been used in the past (mid 70’s and early 80’s) prior to bud break to delay early development of flowers, with one to three weeks of observed delay. However, related problems (greater disease risk, poor fruit set, and high water demand) reduced its potential for commercial use. This study re-examines the approach with the use mist-cooling to delay bloom through a solid set canopy delivery system (SSCD), and evaporative cooling application based on changes in temperature and humidity. In our study, bloom was delayed by 6-11days in apple with the mist application of 14-15 cm/ha and bloom was delayed by 9 days in sweet cherry with the mist application of 10.8cm/ha.
Abbreviations- Solid set canopy delivery (SSCD), Growing degree days (GDD), Relative Humidity (RH), South west Michigan research and extension centre (SWMREC), Horticulture Teaching and Research Centre (HTRC).
The production of the majority of the world’s major food crops is constrained by climatic variables such as the amount and timing of precipitation during the growing season. However, the primary weather-related constraint for most temperate tree fruit crops such as apple and cherry is the frequency and severity of spring freeze events (Flore, 1994). Unfortunately, for tree fruit producers in the Great Lakes region, the frequency of spring freeze events following initial phenological development has increased during the past few decades, resulting in relatively greater risk of production losses with time (Andresen et al., 2012). These trends have had significant impacts on regional fruit production in recent years. An unprecedented heat wave in March, 2012 brought fruit crops out of their dormant state more than 1 month earlier than normal. A subsequent series of 15-20 freeze events during April and May resulted in catastrophic freeze damage, with tart cherry and apple yields reduced by 90% and 88% respectively relative to the previous year’s production (USDA, 2013). A similar early warm up and freeze event in Michigan reduced the cherry yields by more than half during the 2002 season compared to that in 2001(Michigan Agriculture Statistics, 2003).
Given the increasing trend in spring freeze events, new technology or strategy is needed to reduce climate-related risks for fruit producers. Development of new, more freeze-resistant varieties may be a long term solution, but they will not likely be available in the foreseeable future. Conventional frost protection methodologies such as wind machines and overhead sprinklers offers some defense, although they may not be effective in all freeze situations (e.g. wind machines in an advective freeze situation) or may require large amounts of water which result in flooding, leaching of fertilizer or pesticides, or collateral tree damage from ice formation (e.g. overhead sprinklers).
One potential approach to reduce the vulnerability of tree fruit to freeze events is the application of water during the late stages of dormancy and early vegetative stages to cool the plant tissue and delay the rate of growth and development. Plant vulnerability to cold damage decreases rapidly from the end of dormancy through bloom. So, any delay in phenological development potentially increases chance of the bud tissue survival in early spring freeze events. This method has been shown to effectively delay the onset of vegetative growth by one to as many as three weeks, although the conventional overhead sprinkler system used in the research consumed very large amounts of water (Andersen et al., 1975; Hewitt and Young, 1980). A promising new variant of this approach is the application of water mist through solid set canopy delivery system (SSCD), which is increasingly being used in high density orchards for application of fertilizer and other spray applications (Agnello and Landers, 2006). The SSCD can theoretically provide the water necessary for cooling at a tiny fraction of rates consumed by a conventional sprinkler. This system comprises of micro-emitters attached to the main or lateral pipe lines and dropped partially into the canopy using drop-tubing. This research examines the use of water mist as an adaptive strategy to delay the bloom and reduce the risk of frost damage to tree fruit buds. The successful application of such technology could sustainably increase production efficiency and grower profitability.
Our long term goal is to develop an effective, environmentally friendly method to protect fruit tree buds and flowers from spring frost damage through the delay of phenological development by cooling the buds once dormancy has been broken.
Specific short term goals include:
1) Determining the potential delay in early vegetative development of cherry and apple buds by evaporative cooling using the solid set canopy delivery system.
Further ahead, intermediate term goals include:
2) Identification of optimal water application rates and timing based on ambient temperature and humidity relative to the effective rate of cooling and associated delay in phenological development.
3) Development of improved methods to estimate tree growth and development based on internal tissue temperatures.
In 2014, field research was carried out at three apple orchards in southern Lower Michigan located St. Joseph, Charlotte, and Hillsdale, and a sweet cherry orchard at the MSU Southwest Michigan Research and Extension Centre (SWMREC) near Benton Harbor, MI. Similar experiments with dormant potted trees and cut tree branches were conducted in a growth chamber and in an outdoor environment at the MSU Horticulture Teaching and Research Center (HTRC) near Holt, MI to examine the mist interval necessary at different combinations of temperature and relative humidity.
Instrumentation to accurately monitor environmental conditions and physical impacts of the misting system was a major part of this research. Air temperature and relative humidity (RH) was monitored with a research grade hygrothermometer (HMP60, Campbell Scientific Inc.). The probe was sheltered in a radiation shield in order to eliminate the radiation errors. Table 1 lists the monitoring instruments used in the different study locations. Type E 24 gage-chromium constantan thermocouples (Campbell Scientific Inc) and infrared temperature sensors (OS 212, OS 315, Omega Scientific) were used to measure the internal adn external skin temperatures of buds, respectively. A prygeometer (CN3, Kipp and Zoenen) and cup anemometer (03001, R.M. Young) were used at the HTRC site to provide supplemental observations. The data from the sensors were collected and processed by CR1000 dataloggers (Campbell Scientific Inc.) The monitoring equipment was powered by battery with continuous charging via solar panels. Water mist was applied to the buds via the SSCD system. The water flow mist application was regulated by a two way solenoid valve (Asco redhart 3/8″-3/4″) connected to a datalogger and battery power supply.
Solid Set Canopy Delivery System (SSCD)
SSCD systems were installed in three different apple orchard located in South west Michigan, Charlotte and Hillsdale and a cherry orchard located in SWMREC . Mist was also applied through a scaled down version of the SSCD system in the potted plant experiment at HTRC. The SSCD experimental plots in orchards had three rows with wooden posts to support high density polythene pipes. The main water pipe was divided into three different laterals. The laterals ran above the height of the canopy and openings were made at required locations to attach emitters. The setup was such that each tree had one and half emitters (Figure 1). The full emitter was above the canopy and we had one sub-lateral lines dropping down from laterals in between the trees which had two half emitters. The pressure gage was used before the valve in the main line to maintain the required pressure. Also, we used a water filter before the pressure gage to filter unnecessary materials such as sand and other particulate matter. The pressure was maintained between 30 to 40 psi. The nozzle size of the emitter was 0.032″ with the discharge rate of 0.153 gpm (0.579 lpm) at 40 psi (Trickl-eez company, nozzles (NET-3036)).
Misting, evaporative cooling and data collection
The rate of growth and development of plants is largely driven by thermal time and internal tissue temperature. In our experiment, water mist was applied to reduce bud temperature. Ambient air temperature and RH was measured at all research sites. The valve operation was programmed based on ambient air temperature and RH. The internal bud temperature of both treatment and control fruit buds was measured at four different sites (two apple orchards and one sweet cherry orchard, HTRC) to understand the differences between control and misted buds. The bud temperature was measured every 60s to observe the cooling effect and changes in temperature due to misting and to understand the heat transfer exchange within the bud.
At all experimental orchards misting was started after the end of endo-dormancy. At St. Joseph misting was started on April 2, 2014 and turned off on May 13 (after king bloom of control) and May16, 2014 (after full bloom of control). At Charlotte, MI mist was applied from April 12, 2014 to May 1, 2014 (first bloom of control). The mist was turned off after full bloom of control buds in Hillsdale (April 9 -May 19, 2014), MI. At SWMREC sweet cherry buds were misted between April 9, 2014 (after endo-dormancy) to May 8, 2014 (full bloom of control). At HTRC, we utilized potted plants during the fall of 2014. Three set of experiments were carried out with misting and complete measurement of weather parameters. Potted dormant trees were removed from a cooler to a warmer outside environment and mist was applied until full bloom of control. The results were used to develop a biofix of mist application and estimate of necessary misting interval for different combinations of temperature and RH.
The misting interval needed to cool plant tissue is dependent on ambient air temperature and relative humidity levels. Growth chamber experiments over a range of environmental conditions (e.g. sunny and cloudy days) were used to determine an appropriate misting on/off interval. Observations of bud phenology for control and misted buds were taken twice a week following the methods described by Zavalloni et al. (2006) and Edson (1985). Based on known base temperatures of the tree species and observations from the growth chambers, we defined our misting application decision rule to turn on misting whenever air temperature was more than 4.4 0C and the RH less than 90%. The misting interval was based by the growth chamber experiment and modified according to the environmental condition of orchards. The mist off interval was different for different range of temperature and relative humidity, but was on for 60 seconds at St. Joseph, and 105 seconds at SWMREC, Charlotte, Hillsdale. At each site, 5 trees per variety were selected for phenological observations, carried out twice per week. During reproductive stages after bloom, spurs and fruits per flowering spur were also counted, along with the numbers of fruit per spur after last thinning spray, and the fruit per flowering spur.
Fruit diameters were measured on a weekly basis to determine crop maturity. Fruit quality observations were also made in a laboratory from samples taken from the plots, including brix, shape, weight, firmness, and redness (apple only).
- Mist application
The rate of mist application at each site was dependent on the ambient air temperature and relative humidity of the orchard. Higher temperature and lower relative humidity resulted in higher water application rates. On average during the entire period of application, mist was applied approximately 100 times per day. The mist application was directly proportional to the air temperature of the site, with higher application rates for relatively higher air temperatures and vice versa. Thus with the seasonal warm up of the spring season, application rates generally increased with time. These changes were tempered by ambient RH, since lower RH values resulted in a relatively higher rates of evaporation and cooling. Also, the evaporation and heat loss from the bud is influenced by wind and net radiation (Landsberg et al., 1974) which is not included in the misting control program.
Total observed mist application rates at the sites ranged from 10.8 ha-cm at SWMREC to 26.3 ha-cm at Hillsdale (Table 2). The maximum amount of mist (26 ha-cm) applied at the Hillsdale, MI site was associated with relatively windy and warmer conditions at that site which enhanced evaporation rates. We also observed relatively greater drift of water mist at that site. Since we did not consider the wind speed in the mist control program, the misting duration period was modified to 105 seconds at the Hillsdale while compared to other sites which operated with a 60 second spray duration. Despite the higher volume of mist application at Hillsdale, the observed phenological delay was not greater than that at the other sites. Compared with a calendar- or similar timing based system, the use of the weather monitoring sensors and spray control program with the SSCD system likely reduced the volume of water needed.
- Bud temperatures
The influence of evaporative cooling is readily apparent in the differences of misted and control (non- misted) bud temperatures (see example in Figure 2). The bud temperature in the misted buds was lower than the control buds throughout the season. The temperature in the misted buds dropped on average 1-2?C immediately after misting and continued to drop for a few minutes due to evaporation and convective heat losses. After several minutes of mist application and the evaporation of the applied spray mist, buds warmed again due to convective heat exchange. This illustrates the temporal changes in the energy exchange between the buds and their environment. The cooling associated with the misting slowed down the development of bud at all sites, with delays in all phenological stages of misted buds versus control buds. Phenological differences between control and treated buds at St. Joseph, MI are illustrated in Figure 3. The higher difference (>7 deg C) between the control and misted bud temperature was noticed when the air temperature ranged from 20-25 deg C and RH between 45- 50%. Difference between the control and misted bud temperature was lower and nominal when the relative humidity was higher (85 % or above).
- Bloom delay
In general, the mist applications through SSCD system delayed the bloom phenological stage by a few days. In 2014, observed delays ranged from a maximum 11 days in Honey Crisp variety, 10 days in Red delicious and 9 days in Gala (Table 5) at St. Joseph, MI. Also, at Hillsdale, 9 days of bloom delay was observed between control and misted buds with 81 hours of mist operation. Number of days delayed depended on the volume of mist apply, variety of apple and location of orchards. In 2014, 7-8 days of bloom delay was obtained when the mist was turned OFF after king bloom and 9-11 days of bloom delay was obtained when mist was turned OFF after full bloom of control buds. These bloom delays in apple were less than 18 days delays reported by Andersen et al. (1975) in Utah, USA and Hewett and young (1980) in 1976 in Otago, New Zealand. However, we achieved the effective days of bloom delay with lower water application rate (less than 1.158 L/min per tree) and lower number of misting hours than in the previous studies; the rate of water application per tree was10 L/min per tree (Andersen et al., 1975) to achieve 18 days of bloom delay in apple. Chesness et al. (1977) sprinkled 46 cm of water in 209.1 hours in area of 0.0021 ha to protect peach from frost. Chesness et al. (1977) sprinkled the bud when air temperature was above 7.22 0C with 1.25 minutes on and off cycle to delay peach bloom by 14 days.
For the sweet cherry site, bloom was delayed by 9 days, which was longer than that observed by Tsipouridis et al.( 2006), who obtained 5-6 days of delay in the cherry bloom using sprinkler irrigation and relatively higher water application rates.
- Fruit set and fruit quality
Fruit per flowering spur for the misted buds were generally similar to the non-misted buds at both St. Joseph and Charlotte locations, with a average differences between control/non-misted and misted of +9.2, -43.4, and +5.2 fruits per flowering spurs for Gala, Red Delicious, and Honey Crisp varieties respectively at St. Joseph, MI and +5 fruits per spur for Honey Crisp at Charlotte (Table 3). Just as importantly, there was no any evidence of increased disease and/or pathogen problem or reduction in fruit size. The size of Honey Crisps were actually observed to be larger in misted trees than in non-misted one. Very little difference in size was observed between misted and control Gala and Red delicious apples. Apples from the misted trees were observed to have good sugar content, redness and weight. Similarly, the starch content and firmness did not show any statistical difference between misted and non-misted fruits (harvested on September 14, 2014). For cherries, misted fruit had good average size (28 mm), weight, firmness and brix (15.66%). The subsequent maturity of sweet cherry was also delayed by a week in 2014, but apple maturity was not delayed significantly. These results differ from those of Andersen et al., (1975), who observed a delay of 18 days in a Utah experiment.
Agnello, A. and A. J. Landers. 2006. Current progress in the development of a fixed spray pesticide application system for high-density apple plantings. NY Fruit Quarterly 14(4):22-26.
Andersen, J. L., G. L. Ashcroft, E. A. Richardson, J. F. Alfaro, R. E. Griffin, G. R. Hanson, and J. Keller.1975. Effects of evaporative cooling on temperature and development of apple buds. Journal of American Society of Horticultural Science, 100: 229-231.
Andresen, J. A., S. Hilberg, K. Kunkel, and Midwest Regional Climate Center. 2012. Historical Climate and Climate Trends in the Midwestern USA. Great Lakes Integrated Sciences and Assessments Center Occasional Paper #12002 (Prepared for the Midwest Technical Input Report, U.S. National Climate Assessment). online: http://glisa.msu.edu/docs/NCA/MTIT_Historical.pdf.
Chesness, J. L., C. H. Hendershott, and G. A. Couvillon.1977. Evaporative cooling of peach trees to delay bloom. Transactions of the ASAE, 20:266-468.
Flore J.A. 1994. Stone fruit. In: Schaffer B, Andersen PC (eds) Handbook of environmental physiology of fruit crops, vol I, Temperate crops. CRC Press, Boca Raton, pp 233-270.
Hewitt, E. W. and K Young.1980. Water sprinkling to delay bloom in fruit trees. New Zealand Journal of Agricultural Research, 23:37-41.
Landsberg, J. J., Butler, D. R., & Thorpe, M. R. 1974. Apple bud and blossom temperatures. Journal of Horticultural Science 49:227-229.
Michigan Agriculture Statistics, accessed at 2002http://www.nass.usda.gov/Statistics_by_State/Michigan/Publications/Annual_Statistical_Bulletin/stats04/agstat04.pdf . Accssed on January, 2015.
Tsipouridis, C., T. Thomidis, and I. Xatzicharisis. 2006. Effect of sprinkler irrigation system on air temperatures and use of chemicals to protect cherry and peach trees from early spring frost. Animal Production Science 46(5): 697-700
United States Department of Agriculture, National Agricultural Statistics Service, Michigan Field Office. accessed at http://www.nass.usda.gov/Statistics_by_State/Michigan/Publications/Current_News_Release/nr1308.pdf. Accssed on March, 2013.
Zavalloni, C., J. A. Andresen, and J. A. Flore. 2006. Phenological models of flower bud stages and fruit growth of ‘Montmorency’ sour cherry based on growing degree day accumulation. J. American Society of Horticultural Science 131(5): 601-607.
[Companies name are for introduction purpose]
- Air and bud temperatures during misting cycles at a) St. Joseph, MI (May 5, 2014 in apple) and b) SWMREC (April 22, 2014 in cherry).
- Table 3: Bloom date of apples and accumulated growing degree days (GDD) from green tip on control based on ambient temperature, 2014 season at St. Joseph, and Charlotte, MI.
- Visual comparison of phenological development of Control (upper left), Mist off on May 13(upper right), and Mist off on May 16 (lower left) on Red Delicious apples, May 16, 2014 at SWMREC site.
- Table 2: Total volume of mist applied in the area of 1 ha
Educational & Outreach Activities
Conference and other presentation: The results were presented at the conference of American Society of Horticultural Science (ASHS), July 28-31, 2014, Orlando, FL. The results were presented at Great Lakes fruit, vegetable and farm market EXPO, Dec 9-11, 2014, Grand Rapids, MI. This EXPO was targeted to growers of the Great Lakes region. Also, the techniques of mist cooling and results were shared with growers at a field day, July 10, 2014 at Michigan State University (MSU) Clarksville Research Center. Part of the results from this research will be presented at the Climate Change Symposium – Adaptation and Mitigation, May 3-5, 2015 in Chicago, IL.
Progress report: The methods and results were shared with the Michigan Cherry and Apple community via a progress report.
Scientific Publication: A major deliverable product of the of project will be a series of scientific publications describing the impact of the solid set canopy delivery system (SSCD) on the rate of bud and flower development of tree fruit and its value as a frost protection methodology.
PhD Thesis: Part of the results from this research will be used to complete the requirements for a PhD degree.
Water was used to cool the plant and protect it from frost damage. This study used solid set canopy delivery (SSCD) system integrated with advance weather monitoring equipments, which helped to optimize the water use. This system is potentially cost effective because growers could use the SSCD system for fertilizer and chemical application. Mist applied before bud break (and after the end of endo-dormancy) resulted in bloom delay for apples and sweet cherries with no increases in disease risk. Mist application through the SSCD system delayed the bloom by 5 to 11 days on average depending on the length of mist application and variety. Keeping the buds in the dormant stage for a longer time reduces their overall vulnerability to spring freeze damage. Also, delaying the development of each phenological stages is an effective way of protecting it from frost damage; later stages are relatively more susceptible to freezing temperatures than early stages.
Comparing with the earlier method of frost protection this method is environment friendly, no noise pollution and had greater efficiency in application. However, this system could be expensive if only used for mist cooling to delay bloom. The system requires mini weather station, Solid set canopy delivery system, which could be expensive in the initial phase of installation. However, growers could use same system for different purposes and be cost effective: mist cooling to delay bloom or to improve crop performance if applied during the hot summer days, and fertilizer, pesticide, growth regulator application. Also, the application of growth regulator sprays when environment condition are favorable (optimum temperature and no wind) might result in better performance and less follow up manual work.
It is difficult to talk yet about farmer adoption of this practice. However, looking at the positive results of 2013 and 2014, many fruit growers are interested in adopting the method in their orchards for this coming spring. Apples, sweet and tart cherry and apricot growers are attracted by the method of mist cooling to delay bloom and prevent buds from frost damage.
Areas needing additional study
There are several potential modifications in the proposed system that would result in more effective and efficient cooling. Perhaps the most important of these is the addition of net radiation and wind speed environmental variables into the mist application control program, which could improve the accuracy of estimated evaporation and potential spray drift rates. On the application side, filtering the water to separate coarse material would reduce the frequency of clogged emitters (a common problem). On a related note, the water source used for misting should be relatively free of salt and iron to avoid phytotoxicity problems and deposition on plants which might repel pollinators.
This system could be extended to cool the fruits in summer to delay the harvest and get better fruit performance. Working on using the same SSCD system for mist cooling and application of growth regulator, fertilizer, pesticides and so on and optimizing the application rate based on environmental parameter could be a good research focus.