Frost Protection for High Density Orchards

Joe Hannan
Commercial Horticultural Field Specialist
Iowa State University Extension and Outreach
jmhannan@iastate.edu

Fruit crops in Iowa are highly susceptible to spring freezes during bloom.  The primary methods to protect fruit crops from injury are heat, air movement, row covers, and water.  

Heat

Small heaters can be used to warm the area around the trees and are effective under both advection and radiation freeze.  However, heaters provide radiant heat.  The surface to be warmed (or protected) must be in a direct line of site with the heaters.  Furthermore, the energy dissipates the further from the source meaning a lot a small heaters are required to be effective.

Some orchards will substitute bonfires for heaters.  Generally, this is not a recommended practice due to difficulty of regulating temperatures.  It is very easy to get too much heat and break through the upper inversion layer drawing in cold air or burn the actual trees themselves.  Another challenge with bonfires is that they require significant effort to manage.  Each fire must be started and tended to in order to keep them burning with enough intensity to be effective.  If it is has recently rained or been damp, this is challenging.

Air Movement

Wind machines that mix warm air from above an inversion layer with cooler surface air during radiation freezes can prevent freeze damage to flower blossoms.  Wind machines draw down warm air and are capable of providing protection for up to 10 acres.  However, they are not effective under advective freezes and are expensive at roughly $30,000 per unit.

Covers

Spunbon polyester row covers, such as Agribon™ or Reemay®, act as blankets which slows the rate of heat loss from the soil.  Row covers protect plants by trapping heat released from soil and are available in several weights and dimensions.  The heavier the row cover, the more protection it provides but the less light is transmitted.  While very common in vegetable production, they are very cumbersome to place over high-density orchards due to wind and unsuited for larger sized trees.  A 0.5 oz/yd2 cover will provide 2°F to 4°F of protection outside.  They must be closely monitored so that temperatures do not get too warm during the day.  If it is a sunny day, expect to have to remove the cover.

Water

Typical water based frost protection systems use temporary overhead irrigation systems with high output to supply the water.  These systems work well on strawberries that are low growing and dense.  However, they require a large pump and a large volume of water supplied through a surface water source such as a pond or river or a large well.  Water based frost protection systems have not been suitable for taller crops such as apples or grapes until recently.  A new system developed by Michigan State University uses micro sprinklers permanently mounted to a trellis system to supply water for frost protection.  This system, known as solid set canopy irrigation, uses less water than traditional overhead sprinklers found in strawberry fields, which reduces well and pump size requirements, while protecting a diverse array of crops.  

Solid set canopy irrigation may be suitable for use by Iowa farmers but there are challenges of this system that must be addressed.  There is limited information on appropriate design and parts needed to build the system.  This article provides specific details to help farmers build their own solid set canopy irrigation system.  Like traditional irrigation based frost protection systems, solid set canopy irrigation uses a significant amount of water, which can lead to very wet or flooded soil conditions under long operational times or frequent daily use. This article addresses best practices to minimize standing water or saturated orchard soils.  Lastly, many Iowa farms are operating from standard farm wells or rural water supply rates of 10 gallons per minute (gpm) up to 20 gpm and cannot provide frost protection with their current well over a large area.  This article provides suggestions for operating with limited water capacity.

Types of Frost

During full bloom, a damaging freeze occurs when temperatures reach below freezing.  When temperatures reach 28 degrees Fahrenheit, approximately 10 percent of blooms are killed and are no longer capable of producing fruit.  As temperatures decrease to 25 degrees Fahrenheit, approximately 90 percent of blooms are killed.  These temperatures where injury occurs are known as the critical temperatures.  The critical temperatures prior to bloom are lower and vary by bud growth stage as shown in Table 1.

Table 1. Critical temperatures of apple buds at each stage of development

https://extension.psu.edu/orchard-frost-critical-temperatures-for-various-fruits

There are two types of freeze events, advection and radiation.  An advection freeze (Figure 1) occurs when a dry, cold air mass moves through.  Conditions are windy throughout the night and early morning.  Plant tissue is warmer than the air.  Damage occurs when warm air radiating from the soil is carried away by an early morning wind.

Figure 1.  Advection Freeze Characteristics

A radiation freeze (Figure 2) occurs when a layer of cold, dry air forms below a layer of warm air.  This warm air can be 3 to 10 degrees Fahrenheit warmer than surface temperatures.  The sky is clear and there is no air movement at all during a radiation freeze.  Often there will be a slight breeze all night that will stop just prior to sunrise.  Plant tissue is colder than the air temperature.  All methods of frost protection such as covers, wind machines, heaters, and water application are effective at preventing damage to flower buds.

Figure 2.  Radiation Freeze Characteristics

System Design

Water flow rates to provide adequate protection to blossoms are 0.1 to 0.2 acre inches per hour or roughly 2,700 to 5,400 gallons per acre per hour.  This is significantly more than the output of most standard 20 gpm wells in Iowa that output a mere 1,200 gallons per hour (gph).  It is believed that adequate protection can be achieved with flow rates less than 0.1 to 0.2 acre inches per hour but the technology to do so has only recently become widely available.

Current industry standard uses a Nelson R5 or R10 rotator nozzle and riser mounted to the top of the trellis.  The Nelson R5 rotator nozzles have the advantage of having very low flow rates, industry leading, at 9.5 gph.  However, they are significantly more expensive at $10 to $15 than alternative options as shown in Table 2.  Less expensive alternatives such as frame jets, rotor mini sprinklers, and fan spray jets are available from $0.25 to $1.25 each at slightly higher flow rates (Table 2).  Additional fittings are required to connect to the supply lines on the trellis.

Table 2.  

Antelco 360 degree frame jet 11.3 gph @20 psi* $0.38 each Spray radius = 16 ft.	Antelco rotor spray mini sprinkler 11.7 gph @ 20 psi $0.38 each Spray diameter = 16 ft.
Antelco 360 degree fan spray jet 11.0 gph @ 20 psi $0.12 each Spray diameter = 16 ft.	Dig micro sprinkler 10 gph @ 20 psi $1.21 each Spray radius = 12 ft.
Barbed Riser	Plastic Cap

 *Flow rates and coverage diameter vary depending on actual model selected and pressure pressure.

Once a nozzle is selected, the remainder of the design is straightforward.

1. Determine nozzle spacing. Follow the manufacturers recommended nozzle spacing.  When a range is provided, error conservatively to ensure adequate coverage.
2. Determine mainline and supply line pipe sizes.  To estimate supply line (the line hanging on the trellis) requirements, divide the average row length by the nozzle spacing to determine how many nozzles are required per row.  Next, multiply the number of nozzles by the nozzle flow rate provided by the manufacturer to estimate flow rate per row.  Reference a polyethylene pipe sizing chart to select a pipe size capable of meeting the flow rate requirement with minimal pipe loss (listed per 100 ft).  In most cases, a ½ inch pipe for a 300 ft row should be sufficient.  For rows longer than 300 ft, a ¾ inch line may be needed.  Reference a polyethylene pipe (or appropriate chart if other pipe type is used) size chart to determine size of mainline needed that can handle total flow of the system.  A 1” to 1.5” mainline is likely suitable in most instances.
3. Layout the system. Lay the system out so that the least amount of pipe is used as possible.  The more pipe used and the further from the well the orchard is, the smaller the zone of protection is.  Complete protection of an orchard is unlikely.  Prioritize coverage on high value cultivars such as Honeycrisp.  Offset nozzles every other row for best coverage by starting some nozzles ½ radius in from the end of the row to create a triangular coverage pattern.
4. Include important add-ons. Some features, while not essential, help reduce maintenance and ease operation of the system.  Install a filter at the start of the system to remove any small particles or sediment that can clog the nozzles.  In addition, add a valve to each supply line going down the row.  The valve allows the rest of the system to run while a repair is being made to the supply line.  This is critically important when an issue arises during a frost protection event.  You absolutely do not want to turn off the system while temperatures are below freezing as you can do significant damage to the buds.  In addition, the valves allow you to turn on just one line at a time while blowing the excess water out of the system at the end of the season.  Another nice feature is a dedicated connection port for connecting an air compressor used to blow out the line at the end of the season.  

Construction Details

1. Begin construction by attaching the supply line to the top trellis wire.  Supply lines can be attached via zip ties, tie tubes, or other trellising materials.  
2. Splice supply line at each nozzle location and connect fitting, riser, cap and nozzle.  This process is simpler if fitting, riser, cap, and nozzle are preassembled as shown in Figure 3.  The clamp, barbed fitting, and plastic pipe nipple are all available at local hardware stores.
3. Layout mainline from source hydrant to each supply line.  Connect mainline to each supply line.  Be sure to include a valve for each supply line.  
4. Flush the system.  Cap each end.  Pressure test and seal any leaks.  

Figure 3: Fitting, riser, cap, and nozzle installed into supply line.

System Operation

The frost protection system is used when temperatures are expected to dip below the critical temperature for the current growth stage.  The system should be turned on before temperatures reach the critical temperature and must stay on until temperatures rise above freezing and all ice has melted from the flower buds.  Do not turn the frost protection system off while ice remains on the buds or damage will occur.  Thawing ice will pull energy away from the buds causing injury.  In addition, if the dew point is 5 degrees Fahrenheit below the predicted temperature, evaporative cooling will occur causing damage instead of preventing damage.  Invest in a quality digital thermometer and dew point meter. 

Tips for Success

One common challenge with all overhead frost protection systems is soil saturation or flooding when these systems are operated for several hours per day over the course of several days.  While it is always a best practice to plant on well-drained soils, even the best sites can flood after several days of heavy use of a frost protection system.  When water is used for frost protection, additional tile drainage or surface drainage ditches may be needed to help move excess water out of the field; even on well-drained soil types that do not normally need added drainage.

The technology around flow rates is still limited and therefore limits the acreage coverable, about 400 trees, with standard farm wells.  A nozzle with a square pattern that can be centered over the trees and avoid wasting water between the rows would greatly help in reducing the overall flow rate per acre and increase the number of trees covered.  However, nozzles with that type of coverage pattern and low flow are few and far between.  The cheapest and simplest solution to increasing coverage area without drilling a new well or pumping from a surface water source is to incorporate storage tanks and an auxiliary pump.  A 5,000 gallon tank refilled by the well or rainwater catchment could provide enough water to protect 1 acre of high density trees for about 2.5 hours if the supply well was started simultaneously to refill the tank.  A tank and a pump is approximately a $1,500 investment.

Summary

Providing protection to flower buds in bloom during spring freeze events is an ongoing challenge that orchards contend with every year.  Low flow, low pressure spray systems are an option for providing protection but protection capacity is still very much limited to water supply.  The technology isn’t yet capable of providing significant protection without a large water source available; either well, surface water, or storage tanks.  If a water source is available, protection can be provided at a reasonable price.  

This project was funded through a Specialty Crop Block grant.