Principles of Freeze Protection for Fruit Crops
When it comes to temperatures, not all farming sites are equal, even when located in the same general area. There are a number of factors that can affect minimum temperatures during freeze events. And the temperature differences in rather small areas are commonly referred to as microclimates. In general, microclimates are created naturally and are associated with the topography of the area, caused by man made structures, developed from farming practices or a combination of all three. Small microclimates may only include portions of one farm while larger microclimates may include an entire county. An example of the latter is the Brewton area which is in extreme south Alabama but because of natural cold air drainage is often as cold or colder than areas 200 miles north of it.
Natural Terrain/Topography Effects
During cold nights temperature differences are quite common in areas of hilly terrain. As air near the surface is cooled on radiational frost nights, it becomes more dense and flows down hill to lower areas where it collects. These locations will become much colder than higher up the terrain. Dense stands of timber and other plant growth may slow down or block movement of cold air resulting in a cold air dam. Such obstructions create the same blockage effect if they are located anywhere along areas where cold air is draining downhill, i.e., near the bottom of the slopes or near the top.
During advective (windy) freeze events in winter/early spring, the winds are predominately from the north/northwest. Consequently, the presence of a natural wind break of tall pines or hardwoods on the north and northwest sides of an orchard is highly desirable. The value of such windbreaks in reducing damage to fruit buds and flowers of crops such as peaches has been readily demonstrated in portions of central and north Alabama. If windbreaks are used for this purpose, special attention must be given to the creation of cold air dams that could cause cold pockets. This can usually be managed by opening up portions of the lower several feet of windbreaks to allow cold, dense air to move through. For obvious reasons, the ideal place to have a windbreak is on top of a hill where it can provide maximum benefit in breaking the wind while not impeding movement of cold air downhill.
Bodies of water such as lakes are quite helpful in modifying temperatures and reducing crop damage, especially when located on the north and northwest sides of orchards. The Great Lakes are well known for their beneficial effects on protecting fruits grown along the southwest coastal area of Michigan. The many individual lakes scattered across portions of central Florida are also well known for reducing freeze damage to citrus grown in those locations. Experiences in the state have also demonstrated that even rather small bodies of water such as one to ten acres can reduce freeze damage in peaches and other crops during radiational frosts/freezes when located very close to the crop on the north and northwest sides of plantings.
Highly Elevated, Windy Locations Are Usually Best
Within a given area of a farming region, the most elevated sites tend to be the warmest during freeze events. For example, if a county has areas where elevations may range from 400 to 800 feet above sea level, those sites with 700 to 800 feet elevations are nearly always the warmest during freezes. Because these sites are the most elevated in an area, they also tend to be the most windy. Experienced growers realize the value of such windy sites because they tend to be warmer on radiational freeze nights when other less elevated locations become quite calm and colder much quicker. What is so amazing is that even very light winds of only 2 to 4 mph, that are much more persevering on elevated sites, can keep temperatures several degrees warmer than on less elevated locations.
South Slopes Are Valuable
If one reads many horticultural publications on locating fruit orchards, most indicate northern slopes are preferred over southern because trees planted on the former tend to remain dormant a few days longer and therefore are more likely to escape freezes. However, most of these authors are describing orchards being grown in more northern states. In Alabama, stone fruit orchards (such as peaches) located on the southern slopes of highly elevated areas tend to escape freeze damage much better than similar blocks of trees located on northern slopes. The primary reason for this difference is that time of budbreak does not vary greatly between slopes and the trees on northern slopes are damaged much more severely by the extremely cold and dry winds during advective winter/spring freeze events. In many cases, dormant as well as active buds are literally desiccated by the strong, dry winds that generally occur during the first night or two of major freeze events.
Effects of Soil Types
Soil characteristics can exert a microclimate effect. Grower experience has clearly shown that orchards may become slightly active sooner in late winter on heavier, clay type soils and/or darker colored soils (such as reds and blacks) than on lighter colored, sandy soils. Although the latter tends to warm faster, they reflect more heat during the day (trap less heat) and lose it faster during the night. This effect is somewhat like the differences recognized in the earlier flowering and cropping of plasticulture strawberries because of the retention of warmer soil temperatures. However, if the orchard floor is allowed to become covered with grass and other vegetation, the differences among soil types becomes minimal to none. The soil must be exposed to the sun to allow the differential daytime heating to develop between soils with different texture and color.
Effects of Man Made Topographical Features
There are several other factors over which a grower has little control but which can have varying effects on providing a form of freeze protection. Growers in north Alabama have been the beneficiaries of extra warmth on freeze nights because cotton land adjacent to orchards is tilled in late winter-early spring for May plantings. Large areas of clean, firm soil, full of moisture from winter rains, provides somewhat of a "lake effect" by releasing substantial heat that may drift across orchards during freeze nights. Airline pilots can attest to the tremendous jetties of heat released into the atmosphere from large areas of freshly tilled farm land versus flying over the much calmer skies located above areas covered by plants. When these recently cultivated but firm and wet areas are located on the north and northwest sides of orchards, a small but sometimes beneficial effect may be realized during freezes.
Large areas of paved roads, such as interstate highways release substantial heat on cold nights, and this combined with heat released by vehicles and air currents created by traffic can sometimes provide a beneficial effect to several rows of trees located close to such highways. This has been experienced in some states over the past twenty years.
Effects of Cultural Practices - Orchard Floor Management
One of the greatest impacts the producer can have on creating microclimates effects through cultural practices is management of the orchard floor. The condition in which the orchard floor is maintained can add or reduce minimum temperatures during radiational frosts/freezes by several degrees. This may be sufficient on freeze nights to make a difference between making a crop or losing it.
In tree fruit as well as small fruit plantings, maintaining the area beneath plants free of vegetation such that the majority of the soil surface is exposed to the sun is highly advantageous. This practice permits the maximum of radiant energy to be stored n the soil during the day. To absorb this energy most efficiently, the soil must be firm and moist. During a freeze event when radiational cooling occurs at night, the long wave radiation being lost from the soil surface moves upward through the plants providing a warming effect to the crop and the surrounding air. This warming effect may easily increase the temperature in a planting by one to three degrees during a calm, radiational frost/freeze situation.
If the orchard floor is mostly covered with a cover crop or other plants, there will be less heat stored in the soil because the ground cover reflects radiation and transpires to cool itself during the day. Recent work in Florida where high density pine bark growing systems are being used to grow blueberries (several inches of pine bark mulch are used in developing growing system) has shown such plantings become 4øF colder than similar unmulched plantings. Thus dead or live plant material that creates a mulching effect (traps heat in the soil) develops a cold microclimate.
It is a common practice for orchards to be maintained with a weed- free strip of two to six feet (depending upon age and type of crop) on either side of plant row. The orchard middle is usually maintained with natural or planted sod for ease of movement of equipment and erosion control. Mowing the orchard floor grass to two inches or less before freeze problems develop greatly adds warmth to the orchard through daily heating of the soil and release at night. Cultivation of the soil should never be done just before a freeze. Growers should always be mindful that a weed-free, firm soil with good moisture content provides the most efficient manner of providing natural warmth to a fruit planting on a cold night.
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