Weather is a troublesome thing. Rain is welcome in the winter, and usually the temperature rises to meet the precipitation. But when a cold snap is on, it’s time for a farmer to act. During the time of year when trees start to come out of dormancy and put out their buds, they are particularly vulnerable to frost damage.
Over-canopy sprinklers for frost protection on apples.
Extent of Damage to Fruit
What effect does frost have on blooming fruit? It depends on the crop and what stage of bloom. Buds not exposed yet are safer than those in full blossom. At its worst, a freeze of 28°F will kill 10% in full bloom, and 25°F will kill 90% in full bloom. The earlier in the bloom cycle, the better, since a chill down to 18°F will kill 10% of buds that have not yet emerged. To put that in perspective, the coldest recorded temperature in Fresno was 18°F on December 24, 1990.
Methods for Frost Protection
Certain ways to mitigate frost damage have shown good results.
Over-canopy sprinklers coat the whole plant with water. These are especially useful on berry crops. Overhead frost water adds a few degrees from water at ground temperature, and coats the whole plant, encasing it in ice. Because plants are encased in ice, and due to the continual addition of water coating the outside, they are kept from freezing below 32°F. This works for vines and trellised trees, as well. For apples, overhead sprinklers also work as cooling during summer heat. Keep in mind that the higher off the ground you raise a sprinkler, the more pressure is needed to lift water up. Overhead frost is probably not a good option for unsupported trees, as it might add too much weight to the limbs. When a berry bush is covered in ice, the branches will bend to the ground, and if you keep coating it with water until it thaws, you should see it spring back up.
Under-canopy sprinklers for frost protection on peaches.
Under-canopy sprinklers for frost protection work by covering the ground to prevent heat escaping. Water on the ground limits heat loss because dry ground loses heat faster than wet ground. Coverage is a big factor to consider. We advise full coverage, if possible, because dry spots between the wetted patterns still lose heat. Your coverage will depend on the flow rate available. Fan-Jets® or microsprinklers are great for tree fruit and nuts because they are part of the irrigation system. Farmers typically turn their riser valves from drip to frost in winter, then back to drip when the risk of damage is over. Frost protection is important for newly planted citrus because the trees are evergreen. On stone fruit and almonds, it is usually added around year three when the trees go into production.
For frost sprinklers, availability of water is the main thing to consider. It’s important to think not only how many gallons per minute, but how many acre inches for a night of running sprinklers. This is a big reason why reservoirs are becoming more prevalent—they allow a ranch manager to charge them with water during the day and dispense it at night. An appropriately sized reservoir acts as a battery to run frost water when it’s needed.
Wind machines are another method, and are common in connection with citrus trees. On a freezing night, the propellers turn to mix and move the air around. They work because of an inversion layer: when it freezes, the air is stratified and warmer layers sit above the cold air below. Wind machines can raise an orchard’s temperature by a few much-needed degrees by bringing air down to field level and keeping it moving.
Flood or furrow irrigation can be somewhat effective, mainly because of wetting the ground and trapping the daytime heat, as mentioned with sprinklers. Because flood irrigation sinks into the ground fastest where it enters the field, it’s not as uniform or as quick across the entire field as sprinklers.
Whatever your preferred method, it’s worth considering a combination of tactics to build an effective frost protection plan.
“Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.”
– Robert Frost, Fire and Ice