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- Author or Editor: Katharine B. Perry x
A BASIC microcomputer program, FROSTPRO, has been developed from existing literature (3) to calculate irrigation rates for frost/freeze protection of orchards based on given atmospheric and crop parameters. An energy budget approach was used to determine the rate of heat lost by the susceptible plant part through radiative, convective, and latent heat transfer processes at the actual plant part temperature and at the critical temperature, below which damage will occur. The difference between these 2 rates of heat loss is the rate at which heat must be added by the latent heat of fusion liberated as the applied water freezes. The model developed follows that previously described (2), with the addition of the relative humidity parameter.
Basic meteorology as it applies to frost-freeze events and a discussion of the methods of frost protection are included in this article. The presentation of basic meteorology includes descriptions of heat transfer, energy exchange, inversion, frost, freeze, microclimate, air versus crop temperature, and forecasts and warnings in the context of how each of these in involved in frost-freeze events. The second part of the paper describes the major methods of frost protection for commercial crops. The methods included are site selection, irrigation (overhead, undercanopy, man-made fog, flooding), wind machines, heaters, covers, and sprayable materials.
A heat unit model developed in a previous study was compared to the standard method (average number of days to harvest) for ability to predict harvest date in cucumber (Cucumis sativus L.). Processing and fresh-market cucumbers were evaluated in 3 years (1984 through 1986), three seasons (spring, summer, and fall), and three North Carolina locations. The model predicted harvest date significantly better than the standard method for processing, but not for fresh-market cucumbers.
The use of a previously developed model for predicting harvest date in cucumber production systems is described. In previous research we developed a new method using daily maximum temperatures in heat units to predict cucumber harvest dates. This method sums, from planting to harvest, the daily maximum minus a base temperature of 60F (15.5 C), but if the maximum is >90F (32C) it is replaced by 90F minus the difference between the maximum and 90F. This method was more accurate than counting days to harvest in predicting cucumber harvest in North Carolina, even when harvest was predicted using 5 years of experience for a particular location and planting date.
‘Jackpot’ tomatoes (Lycopersicon esculentum Mill.) were grown in 6 plant protection systems with or without black polyethylene mulch. Mulching increased early and total yield of large and marketable fruit and increased total yield of cull fruit. Row covers reduced early yield of large fruit and average large fruit weight. Use of large transplants from 7.5-cm square peat pots tended to increase early marketable yield but decreased early and total average large fruit weight as compared to performance without a plant protection system.
Fourteen methods of calculating heat units from planting to harvest were applied to daily maximum and minimum air temperatures taken in a standard weather shelter for 2 growing seasons (spring and summer) over 5 years of cucumber (Cucumis sativus L.) plantings in North Carolina. The coefficient of variation (cv) was used to determine which of the methods was most reliable in predicting day of first harvest. The best method was to sum over days from planting to harvest the difference between the daily maximum and a base temperature of 15.5°C; but if the maximum exceeded 32°, it was replaced by 32° minus the difference between the maximum and 32°, before subtracting the base. This method had a cv of 3%, compared with 10% for the standard method—numbers of days from planting to harvest.
A commercially available cryoprotectant (50% propylene block copolymer of polyoxyethylene, 50% propylene glycol; trade name FrostFree) and an antitranspirant (96% di-1-p-menthene, i.e., pinolene, a terpenic polymer, 4% inert; trade name Vapor Gard) were evaluated for their ability to protect `Pik Red' tomato (Lycopersicon esculentum Mill.) and `Keystone Resistant Giant #3' pepper (Capsicum annuum L.) plants during frost and freeze occurrences in the field. Tests were conducted during four spring and two fall seasons. Protection from these products was not observed under field conditions when minimum air temperature reached -3.5C and -l.0C on separate occasions. Yields for treated and untreated plants were similar. Neither cryoprotectant injured the foliage in the absence of cold events.
The radiant fractions of four currently available orchard heaters (spot, large cone, return stack and auto clean stack) were determined under orchard conditions using observed surface temperatures, emissivity and the Stefan-Boltzmann Law. The contribution of the flame extending above the spot heater stack to its total radiant output was determined. Stack surface temperatures increased with burning rate on the large cone, return stack and auto clean stack heaters but decreased on the spot heater. The position of the highest temperature was near the bottom of the large cone, return stack and auto clean stack heaters and near the top of the spot heater. Surface temperatures were lower in general on the spot heater than on the other three heaters. The radiant fractions of the large cone, return stack and auto clean stack heaters were greater than 20% and relatively constant with burning rate. Even though the flame contribution is purposely overestimated the radiant fraction of the spot heater was less than 16%. For low wind speeds under which most orchard heating occurs the effect of wind speed on the radiant fraction was negligible.
Adjustment of the sprinkling application rate to existing atmospheric conditions to conserve water may be accomplished by turning systems on and off. The maximum off period that is tolerable is calculated. It is the sum of time required to freeze the applied water plus time during which the ice coated plant parts cool to the critical temperature. Values of the off period for typical frost conditions are proportional to wind speed and wet bulb temperature. Field test results indicate intermittent sprinkling provides a method to reduce water consumption in sprinkling for frost protection.
Access to weather information for planning and implementing horticultural practices is an important component of the production system for growers. Advances in meteorological instrumentation, data acquisition and storage, and communications technologies have improved greatly the potential for applying sophisticated weather information into daily on-farm decisionmaking. The North Carolina Agricultural Weather Program seeks to provide weather information to the horticultural interests of the state. It has developed over the past 13 years. Recently, budget reductions near 50% and the loss of two-thirds of the extension full-time equivalents have necessitated significant changes. Through regional cooperation and the use of electronic communications technology, the program has sustained these negative impacts and emerged as an improved program. This paper describes the evolution of a state agricultural weather program into what is now a regional cooperative project to provide the weather information horticultural producers require.