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Frost Protection for Apricots and Other Stone Fruits
Bob Purvis

Apricot production in the United States has been limited to only a few states because apricots bloom early and are therefore subject to crop losses due to frost. In this article, I’d like to outline the problem and to share with you my experiences in dealing with it in three states (Washington, Minnesota, and now Idaho), and to present some solutions that may be of help for your situation.

The Problem Outlined

The parts of the world where apricots originated, namely western China, central Asia, and Iran, are characterized by moderately cold winters, short, mild, relatively dry springs, and hot dry summers. Under these conditions, fungal and bacterial diseases are less of a problem than they are in wetter climates. Of all the states, California’s climate most closely approximates those in the birthplace of apricot production and not surprisingly, about 94% of the total US production comes from the Golden State, less than 6% from Washington State, and less than 1% from Idaho, Utah, Michigan, and New York.

There are several characteristics of apricots that make them difficult to grow. If a mid-winter thaw occurs, with temperatures getting up into the 50s F for more than a few days, the flower buds of apricots will de-harden. If temperatures drop below zero soon thereafter, the percent of killed flower buds will rise from none at 0F to 100% at -12F. This situation has occurred occasionally in central Washington State (1996) and also in Minnesota during my time living there.

Research done at the WSU Irrigated Agriculture Research & Extension Center in Prosser, Washington during the 1980s resulted in the publication of Extension bulletins delineating flower-bud mortality as a function of temperature at different bloom stages for commercial cultivars of apple, apricot, sweet cherry, peach, pear, and prune. Bulletin 1240, for apricots, gives the following critical temperatures for blossom buds: Click here for table showing critical bud stages

Flower buds were held at various temperatures for a half-hour and then the percent killed was tabulated.

Stage 1 is first swelling of flower buds; stage 2, tip separates on flower buds; stage 3, red calyx appears in flower buds; stage 4, first white (‘popcorn stage’); stage 5, first bloom; stage 6, full bloom; stage 7, fruitlets in the shuck (spent flower parts); stage 8, green fruitlets exposed. Anyone desiring an electronic copy of Bulletin 1240 (apricot), 0913 (apple), 1128 (sweet cherry), 0914 (peach), 0978 (pear), or 1186 (prune) may contact me through OFGA. The bulletins are out of print, but I have hard copies of them as well as the electronic version.

During the 26 years that I have grown apricots, I have seen different types of frost situations and have gotten a feel for how they arise. If a strong cold front passes through in March or April, there is usually enough wind mixing the atmosphere that frost is unlikely the first night. It is usually the second night, when the air is calm and the sky is clear, when the greatest danger of a damaging frost is present. In this situation, a temperature inversion can develop, with the air colder at ground level than 6 or 12 feet above the ground. Typically, temperatures will drop steadily on a clear evening until the dew point is reached and after that, much more slowly. The most hazardous situation is when dew points are in the teens, and it seems that cold, clear nights oftentimes happen at or close to Full Moon. Our Idaho orchard is 2,300 feet above sea level, a high-desert situation where we get only 10 inches of rain per year, and relative humidity is low most of the time. There is commercial production of apricots within 10 miles of our home; the western Treasure Valley is the only place in Idaho with commercial production of stone fruits.

My orchard currently has 64 bearing apricot trees representing 57 cultivars although I do not harvest the fruit off some of them. Dr. Joseph Goffreda, the Rutgers University apricot and peach breeder, told me that dry frosts generally pose less potential for damage to flowers than do wet frosts, when there is a lot of frost deposited. I have seen very few wet frosts at our location here.

Dealing with Frost

During the 1990s, when I lived in Washington State, commercial growers would use wind machines to break up temperature inversions on cold nights in March or April and bring down warmer air from aloft. At my location about five miles north of Yakima, I used under-tree micro-sprinklers to irrigate the orchard and also for frost protection. I would turn on the sprinklers when temperatures dropped to 40F because the black feeder lines for the sprinklers could freeze up at temperatures above 32F due to intense radiational cooling. As the well water hit the trees and froze, it would release latent heat of fusion and keep temperatures of the blossoms close to 32F. Of course, such protection would suffice only for one night. Sprinkling would need to be done until the temperatures were somewhat above freezing.

In Minnesota from 1999-2007, living in Cottage Grove, 13 miles SE of downtown St. Paul, I planted six apricot trees as part of a 49-tree backyard orchard. In March 2004, I attended my first Agro-K Corporation growers’ seminar in Minneapolis and learned about KDL (potassium bonded to dextrose and lactose, corn sugar and milk sugar), a foliar spray product that Agro-K had developed to supply foliar potassium to fruit trees and vegetables at critical times in their development but which also had shown promise as a means of frost protection, one notable example being a Michigan tart-cherry grower who had saved much of his crop when his neighbors lost all of theirs in a major frost event. I have grown apricots in Idaho from 2008 to the present (2019).

KDL imparts frost protection by raising the ratio of potassium to nitrogen in flowers, and this in turn lowers the freezing point of plant tissues. The product also contains small amounts of calcium and seaweed. That spring (2004) and in most succeeding years I used KDL on my apricots when frost threatened and kept records of what happened, in addition to seeing how much protection the sprays offered.

Because of the potassium being bonded to two common sugars, when KDL is sprayed onto a tree, the blossom and leaf tissues absorb it within a matter of seconds. KDL is alkaline, and the alkalinity can scorch tender blossom and leaf tissues. For that reason, I mix household vinegar 1:1 with KDL before spraying. Sysstem-Cal (calcium linked to phosphite), another Agro-K foliar product, can achieve the same effect if mixed 1:3 with KDL, to neutralize the alkalinity.

The frost-protection rate to use in mixing KDL is 3 oz. KDL + 3 oz. household vinegar (or 1 oz. Sysstem-Cal) per one gallon of water. The tank mix should be sprayed lightly (not drenched) on the apricot trees the evening before a frost event is forecast. The spray affords protection for 7-10 days if temperatures are warm by day (65-80F) or 10-14 days if temperatures are cool (45 to 65F). If cool weather prevails, blossoms are more tolerant to frost than if warm weather prevails before a frost event. The amount of protection afforded by the spray depends on the depth and duration of the frost event. Here in SW Idaho, our coldest temperatures usually occur just before dawn and generally begin rising soon after sunrise. Even the slightest amount of high cloudiness can mitigate a frost event.

My experience here, in Minnesota, and in Washington is that frosts and cold before bloom rarely will cause significant damage to developing apricot flower buds. The greatest danger here is typically around late full bloom. Our average full-bloom date for all the 57 apricot cultivars I grow is about March 30; in Cottage Grove, April 20. The latest I have seen frost here in our location is May 13. What follows is my experience spraying KDL during nine growing seasons, 2011-2019. Note that in what follows, temperatures given were at 1 meter above the ground in the very coldest spot in the orchard, about 4 feet lower than the highest spot in the apricot rows, which are 320 feet long.

My Experience Over the Years

2011-- Sprayed KDL April 6. Apricots at full bloom April 12. Temperatures of 28F April 19, 27F April 20 with apricots at average 80% of the way from full bloom to petal-fall (shuck stage). April 22 we had a 25F frost; April 23, 24F frost. (This was the coldest April in 20 years here) In spite of this, the apricot trees at the lowest elevation in the orchard had a good crop of Harlayne and Harogem apricots.

2012 -- Sprayed KDL April 4. April 6, apricots at late full bloom, temperatures dropped to 20F that morning. This should have destroyed our crop, but we had a very good crop of apricots in 2012.

2013 -- Apricots at full bloom March 31. Experienced a 23F on April 14. KDL not available, nor irrigation water to under-tree sprinkle. Lost all apricots below the 5-foot level on our trees, so crop was somewhat less than in 2012.

2014 -- Full bloom late March. Sprayed KDL April 2. On April 14, a 26F frost with apricots at late shuck stage. Small fruitlets killed from ground level to 4 feet above ground, but ultimately a much better crop than in 2013.

2015 -- Apricot buds not much affected by a -3F freeze on November 15, 2014. Very mild February resulted in a full-bloom date of March 20, earliest ever observed here. Sprayed KDL on March 24. April 2, apricots at fruitlet stage, 23F frost, loss of some fruitlets. April 3, 20F frost of 20F, much more loss of fruitlets. April 12, frost of 17F. Total apricot crop for orchard was only 3 lb; 99.9% of crop lost. This was also the third year of a 3-year drought with irrigation water shortages.

2016 -- March 18, apricots partly in bloom. Sprayed KDL March 25. March 26, 21F frost with apricot trees averaging 30% of the way from full bloom to petal-fall (shuck stage). Live blossoms 2 feet above the ground in spite of the frost. Apricot crop this year was 2,814 lb harvested with another 100-200 lb not harvested.

2017 -- We had a -20F reading January 7. Sprayed KDL on April 3. April 4 frost of 25F with trees a little past full bloom. May13 frost of 28.4F did not damage the fruits. Total harvested was 2,194 lb.

2018 -- March temperatures a bit cooler than average. Sprayed KDL on March 25. April 3, with apricots about 80% of the way from full bloom to petal fall, we had an 18F frost that lasted 1-1/2 hours. Without KDL we would have had 100% loss of our crop. It appears that there was no damage to the blossoms that were still out on April 3 although undoubtedly many apricots in the shuck were killed. Total crop harvested was 1,079 lb.

2019 -- Picture-perfect weather in March and most of April. April 29 we had a 28F frost with the apricots about the size of a dime or quarter. Sprayed KDL later on April 29. Another 28F frost on May 1. We had a bumper crop of apricots, but a severe hailstorm May 26 blemished many of them. Total harvest was 1,972 lb. Harvest would have been probably another 200-300 lb more, but many apricots were left unharvested because hail damage and lack of market outlets made it infeasible to harvest them.

To summarize the above, it appears that KDL can offer at least 7 and possibly 8-9 Fahrenheit degrees of protection when apricots are at full bloom to petal fall, probably about 3-4 Fahrenheit degrees of protection when apricot fruitlets are in the shuck, but probably no protection at all when the fruitlets are exposed to subfreezing temperatures below 28F. I have found that KDL applied at 4 weeks or 2 weeks pre-harvest does make a noticeable difference in fruit color, size, and sweetness on apricots, as has been claimed on the label. Japanese and hybrid plums bloom only a few days after apricots here, and it appears that my sprays of KDL have also helped greatly in preventing frost damage on them.

KDL is not registered as permissible for use by the Organic Materials Research Institute because the potassium is not derived from a “natural” source. Agro-K manufactures a product, 0-0-6, “clean” potassium, with citric acid as the chelating agent for the potassium and a small amount of seaweed extract, that is on the OMRI list of approved products. This product is slightly acidic. Preliminary research suggests the rate to be used is 6 to 8 quarts per acre, but more work needs to be done. Although I hope to test 0-0-6 alongside KDL to determine the relative efficacy of the two products, there is a need for organic growers to test 0-0-6 and provide feedback to Agro-K Corporation on how it works for them. If you are interested, please contact OFGA for more information.

About the Author

Bob Purvis owns and operates Purvis Nursery & Orchard in Homedale, Idaho.

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