The popularity of high density orchards has led to some new research into the best manner of protecting those trees from spring frosts. Joe Hannan, commercial horticultural specialist with Iowa State Extension, conducted trials in two high density orchards using overhead irrigation sprinklers in a frost protection system.
Two types of freeze events can occur: advection and radiant freezing. In advection, cold, dry winds pull warmer air up and away from the ground. A radiant freeze occurs during calm conditions, when cold air settles down to the ground, with warm air remaining about 50 to 100 feet above. Radiant freezes often occur just prior to dawn, when the air stills and allows the colder air to settle.
Damage occurs if trees have already emerged from dormancy. Depending on the stage of bud development and the type of fruit, the temperatures at which damage occurs vary. For apples, a temperature of 28° F for 30 minutes when at the first pink to post-bloom stages will cause damage to 10% of the buds, while 30-minute exposure to 25° F will cause a 90% loss. Peaches and pears can also tolerate that 30-minute period at 28° in the late stages of bloom, as can sweet cherries and apricots.
Earlier bud development stages can tolerate lower temperatures, with silver tip in apple being tolerant of temperatures as low as 15° F with minimal loss. Penn State University Extension offers a chart for various tree fruits, showing their low temperature tolerances and the amount of damage which will occur at different growth stages (extension.psu.edu/orchard-frost-critical-temperatures-for-various-fruits). As bloom nears, the differences between temperatures which cause minimal damage and those which cause great loss narrows, as actively growing bud tissues have less ability to withstand cold temperatures.
Aside from reducing crop yield, freeze damage has longer-term effects on trees. Trees will go into biennial bearing mode, which is not healthy for the tree, and will cause excessive vegetative growth. That resulting canopy development inhibits the spread of light to next season’s crop.
Utilizing water to protect crops from freeze damage isn’t new – but it is risky and requires a large water supply. The water must be applied constantly until all of the ice has melted from the buds or blooms. If not, increased damage will occur.
In a standard orchard, one challenge with using water for frost protection is arranging a sprinkler system to reach all of the trees and to provide adequate, constant coverage to buds. In high density orchards, where trees are only eight – 12 feet tall, pruned with a narrow structure, spaced closely together in rows and grown on trellises, the potential for successfully using overhead irrigation systems to protect against freeze events exists.
Hannan’s goal was to design a system to fit the water availability of most orchards, resulting in a cost-effective and efficient permanent system of frost protection and irrigation for high-density orchard planting. Overhead drip irrigation, using micro sprinklers, fan spray jets and frame jets, presents a low-cost opportunity to protect these orchards.
“Our goal was to scale this so that it works with a common, standard well that has an output of 20 gallons per minute,” Hannan said, and to avoid costly specialty nozzles and fittings, which can be 10 times more expensive than standard sprinkler irrigation fittings.
Hannan’s research wasn’t without some mistakes. The sprinkler nozzles, which they fitted directly into half-inch drip irrigation supply lines using barbed fittings, ended up out of alignment and difficult to reposition well before season two. As the drip tubing expanded and contracted, the sprinkler nozzles become contorted, pointing everywhere but onto the trees. To correct this, they added extra adapters with pipe threads to make the nozzles easily adjustable, so any directional issues could be corrected as needed.
Spacing on the nozzles was originally set at approximately eight feet apart. Hannan said emitter spacing could be expanded to 14 to 16 feet, following manufacturers’ suggestions, saving time and money without compromising coverage.
Frame jets, which emit multiple jets of water in a 360° pattern and don’t rotate, provided good coverage. Rotating mini-sprinklers worked just as well. The fan jets also worked, providing good coverage, which was unexpected, Hannan said.
Overall costs for a drip irrigation overhead sprinkler system run several hundred dollars per acre, if $1.50 nozzles are installed every 15 feet on a half-inch drip line.
A square pattern was used when installing the system, with nozzles at tree number one, and then every so many feet down the row, repeating this pattern in every row. A triangular pattern actually provides much better coverage overall, and it better avoids any dry spots and decreases overlap. In the triangular system, the nozzles in the second row are placed halfway between the two nozzles in the first row, with the pattern alternating row to row.
Unfortunately, there wasn’t a freeze event during the trials. However, the water coverage was more than adequate had there been one.
“We know our system works. It puts out the appropriate water volumes out onto the canopy,” Hannan said. “It takes a lot of water, because so much water goes where we don’t want it. We learned a 20-gallon per minute well is only going to give you protection in a high-density orchard of about half an acre.”
As the trees are only about 4.5 feet in diameter from the nozzles, much of the water is deposited in the alleys between rows. Hannan is looking for lower flow nozzles, which would reduce the amount of wasted water yet keep the trees protected, adding the ability to protect more acreage – probably one to two acres – with the standard 20-gallon per minute well. He recommended growers place overhead sprinkler irrigation systems on their highest value cultivars.
Hannan is looking to trial systems using larger wells or large water storage tanks. A 5,000-gallon tank could be refilled as needed, providing longer duration freeze protection for more acreage. Both options would be more expensive to install than using existing wells, but provide enhanced coverage capabilities.
While using protective coverings, heat or air to prevent freeze damage in the high-density orchard is also viable, those methods come with challenges. Turbines are good in radiant freeze situations, where they mix warmer air above with cooler air settled below, generally covering about five – 10 acres each. Row covers don’t work well on trees, lasting only a season before tearing and requiring a lot of time and labor to install. Heaters require fuel and need to be distributed liberally, as they provide direct heat to the trees. These methods aren’t as cost-effective or as reliable as needed.
Hannan is looking to modify the automated controls for the overhead sprinkler irrigation system, making it more user friendly. Another possibility is adapting the system for pest management and fertigation, and to cool trees and reduce heat stress. By creating an overhead sprinkler system which is multi-faceted, and can be used year-round for a variety of standard orchard management tasks, the system becomes more affordable.
For further information on Hannan’s overhead sprinkler system, visit extension.iastate.edu/smallfarms/frost-protection-high-density-orchards.