Orchard growers across the United States know all too well the toll of frost damage. Combating frost and controlling its overall effects could mean the difference between a fruitful harvest or sluggish season. We recently spoke with Steve Hammersmith, chief executive officer of Shur Farms Frost Protection and author of “Cold Air Accumulation and the Grower’s Guide to Frost Protection.” Hammersmith provided some insight on what growers need to know to fight this ongoing concern. Frost Protection
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GROWING: What are the most vital concepts that every grower must know when it comes to frost protection in the orchard field?
Hammersmith: While there are a lot of vital concepts to understand as each will work in conjunction with the others to provide protection, an understanding of how frost develops and how cold air flows along the ground and accumulates is first and foremost. Also critical to understand is how micro-climate and temperature differentials within the growing area develop. Armed with this knowledge, a grower can then seek to obstruct and slow down the buildup of cold air in the orchard and in specific “frost pockets.”
While every grower should know the concept of barriers and diversions and how they may benefit, actual design and implementation should never be done without the aid of air flow models, which can be obtained through Shur Farms or other geographic information system (GIS) service companies.
It’s of critical importance to understand that there is no “conventional wisdom” as to the placement or removal of barriers or obstructions. A heavy vegetation barrier along the low side of an orchard while seemingly blocking drainage of cold air flows out of the orchard may also be a barrier to a greater mass of cold air coming in from the other side and entering into the orchard. A barrier placed on the high side of an orchard may cause a severe negative effect on a crop growing above it. Basically, when cold is obstructed or diverted from a growing area the cold air mass remains and the question remains, “Where is it going?”
Growers must understand the process and effects, both intended and unintended, of any frost protection system that they use. It serves no purpose to simply push a problem from one area to another. Growers must also be aware that not all frost protection methods can be used together. For instance wind and over tree water will cause evaporation and the temperatures will get much colder than with no frost protection at all, but the use of one or the other may be beneficial.
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GROWING: Is there a particular myth of frost protection that is more detrimental for a grower to believe?
Hammersmith: Additional damage can occur when a grower believes that simply blowing wind over plant tissue will protect against frost. This leads to growers starting wind machines late or calling in helicopters after the plants become super cooled, which can cause ice crystallization and damage to occur where otherwise there may not have been any by doing nothing. This also contributes to growers blowing wind on plants during non-inversion conditions (advective freezes) or when the plants are still wet from a recent rainstorm that just cleared out. Starting frost protection early is critical and most frost protection systems fail because it is either started too late, turned off too soon or not turned on at all. Another detrimental myth is to customarily remove downhill obstructions without examining the repercussions as discussed above.
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GROWING: Can you elaborate on growers providing the right balance of frost protection: active versus passive measures?
Hammersmith: Passive frost protection methods focus mainly on obstructing cold air fluxes coming into the orchard and encouraging cold air fluxes to exit the orchard. Barriers blocking out cold air flows, cover crop and soil management encourage good flows of cold air along the ground. Other passive measures seek to limit cold air being generated within the orchard itself, generally falling into the obstructing cold air coming into the orchard category. Keeping soil hard packed and not tilled is one of these. Since it is the ground that chills the air that causes radiation frost damage and loose or sandy soil loses heat at a faster rate than hard packed soil, keeping the soil hard packed and slick will not only allow cold air to flow more rapidly across, keeping the depth of the cold air mass lower and hopefully below the fruiting zone where it causes no damage, but it also reduces the rate at which cold air is added into the orchard, thus helping to accomplish the same goal of keeping the cold air mass below the fruiting zone.
Some passive measures such as removing cover crop to enhance cold air flows also have the additional benefit of reducing the incidence of ice nucleating bacteria near the plant, in theory helping to retard ice crystallization in and around the plant cells.
Passive frost protection seeks to restore the natural balance between cold air coming in and cold air flowing out, reconfiguring the growing area to be as naturally frost free as possible. Successful implementation of an optimization plan can result in far fewer actual frost nights per season, and since the cold air mass is less than what it would have been without these measures, when there is a frost event it will start later in the night, build up less deep of a cold air mass, and be less severe than otherwise.
After passive measures have been taken, then the remaining cold air masses can be removed or protected against with less active measures. A word of caution: Not all active measures are compatible with all passive measures.
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