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The cessation of frost in southern Africa marks the beginning of the growing period which occurs mostly during the months of September and October, while the onset of frost indicates that the growing period of most summer crops is coming to an end, which normally occurs in April, May, and June [ 5 ]. Studies by Tait and Zheng [ 10 ], Rahimi et al. Due to the interannual variability of frost, continuous updating of frost dates is necessary to reflect recent climatic events [ 12 ]. Different temperature thresholds are used for determining frost.
For example, Zenoni et al. Trasmonte et al. The threshold to use has to be specific according to a crop of interest but different thresholds can also be used to show severity of frost risk or to accommodate a variety of crops. The risk of frost varies greatly in Free State province due to the contrasting topography and geographical aspect.
Thus, there is a need to investigate the variability of the frost indices in the region and their impact on agricultural production. These thresholds have been determined based on previous studies in other regions [ 11 , 13 — 17 ]. Three main indices were determined for each threshold: onset of frost, cessation of frost, and duration of frost-free period. Trends in these indices were assessed at selected stations in Free State with data from to The daily minimum temperature dataset was used in the analysis for determination of frost risk assessment in Free State province.
The data used in the study was recorded inside a Stevenson screen elevated at 1. The data was arranged by agricultural season July to June. Where necessary, the data was patched with the UK traditional method which uses the nearest neighbour stations up to five to determine average temperature values in a particular month and then uses the differences target station long-term mean temperature — neighbour station long-term mean temperature in values to patch within that month [ 18 ].
A patched value is taken from the average of all the predicted values from each nearby station. The data was arranged according to agricultural season from July to June of the following year. For each season the last day of frost in spring cessation of frost and the first day of frost in autumn onset of frost were determined at each of the stations and for each year. The frost dates were converted to Julian days to facilitate statistical computations.
The frost-free period for each agricultural season was then calculated for light, medium, and heavy frost occurrences as the number of days between last and first frost. The frost-free period is the most important index which marks the length of the growing period for most crops in the high lying regions.
Minitab software was used to determine the appropriate probability distribution for all the variables onset, cessation, and frost period at each station. The onset of frost, cessation of frost, and frost-free period were determined for different probability levels based on the selected distribution to assess frost occurrence in Free State province. To clearly show the risk associated with the onset of frost, cessation of frost, frost period, and frost-free period, the exceedance probability was used for the cessation of frost while the nonexceedance probability was used for the other frost indices.
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Frost indices varied widely throughout Free State province. Onset of frost for all the thresholds mainly followed the Normal distribution in Free State province. Figure 2 shows an example of probability fitting for the Bethlehem station. The results of the Kolmogorov-Smirnov test yielded a value of 0. Relatively late cessation of frost is evident over the central parts and high lying areas in the southeastern, eastern, and northeastern parts vicinity of Wepener, Zastron, and Bloemfontein.
As shown in Figure 3 , the extremely late cessation of frost over Free State occurred between 26 October and 26 November 6 October and 26 November [26 September and 26 November] for light, medium, and heavy frost, respectively. This is an indication that frost risk is high in Free State province with a chance of frost damage well within the rainfall season with the potential of damaging crops at the vegetative stage [ 5 ].
Time series of cessation of frost shows a lot of variation from one year to another with an average standard deviation of 19 days for all the frost thresholds Figure 3. Onset of frost for all three thresholds varies greatly over the years with the average standard deviation across all the stations for light, medium, and heavy frost of 13, 14, and 16, days, respectively Figure 4. Extremely early onset of frost ranges from 18 March to 29 March 29 March to 14 April [1 April to 25 April] for light, medium, and heavy frost, respectively.
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This implies that frost-sensitive crops have to be fully matured by mid-March in order to avoid frost damage Figure 4. The frost-free period spatial pattern follows that of the onset and cessation of frost. Areas of short frost-free season are mostly over the southeastern parts, close to the escarpments along the border with Lesotho. Year to year variation of frost-free period is high over Free State province, as depicted in Figure 5 , with average standard deviation of 23, 25, and 26 days for light, medium, and heavy frost, respectively.
Negative trends show that cessation of frost is becoming earlier over time implying that planting of crops can be early depending on the soil water content status. In most places, cessation of frost becomes earlier by an average of 2, 3, and 4 days per decade for light, medium, and heavy frost. In contrast, significant positive trends are obtained in Virginia and Ficksburg whereby cessation of frost is becoming later by an average of 3 days per decade for all the frost thresholds.
Onset of frost mostly shows nonsignificant positive trends in most parts of Free State with the exception of Virginia and Ficksburg showing a significant negative trend. Positive trends are an indication that minimum temperatures are increasing with time over the region, implying an increase of favourable conditions for crops that are sensitive to low temperatures.
In contrast, significant negative trends are evident in Ficksburg and Virginia at a rate ranging from 2 to 6 days per decade. Frost analysis is important because knowledge of the frequency and timing of frosts will help reduce the risk of frost damage in vulnerable areas [ 19 ]. Probable dates for the first frost and last frost of the growing season can also help farmers in preventing or reducing the damage to agricultural produce caused by frost [ 11 ]. Consequently, knowledge of the frost-free period can be used as a proxy for the length of time available for crop production [ 20 ].
The assessment of frost occurrence in Free State showed considerable variation from one place to another owing to the vast differences in topography over the province.
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The results of the frost risk assessment show earlier frost onset, late cessation of frost, and shorter growing period over the northeastern, central, eastern, and southeastern parts. These areas are more vulnerable to frost especially if planting is early and this forces farmers to delay their planting resulting in shorter growing periods suitable for short or medium cultivars [ 5 ].
Late planted crops in these areas are also in danger of not achieving full maturity before suffering from damage resulting in poor quality yield or production loss in extreme cases. The first frost at the onset of a frost period causes damage to crops at their later stages of development, while the last frost at the cessation of the frost period damages seedlings and young plants [ 11 , 21 ]. Farmers in these regions are advised to plant short season varieties taking into consideration all the frost indices.
Trends in frost indices are mostly favourable for increasing frost-free period implying that the planting of relatively longer cultivars might be suitable in the future depending on the rate of frost-free period increase. The length of the frost-free period denotes the growing period, and, in Free State, it is mostly between days and over days, depending on the temperature threshold and locality. Long frost-free periods are evident over patches in the far western and northeastern Free State.
These areas have low frost risk but it does not mean they are the most desirable places for planting maize or any other crops sensitive to frost damage under rain-fed conditions as there are other climate risks like rainy season length and agricultural drought that have to be considered [ 22 ]. In Free State, the frost differences in the risk levels between onset and cessation of frost were mostly 10 days from one risk level to another at all the temperature thresholds which does not denote vast temporal differences to the advantage of crop management in the province.
Targeting of maturity before the onset of frost should be the practice without compromising on crop yield.
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In some places, like the eastern Free State, careful consideration of the cultivar which has a high potential but requires relatively lower heat units accumulation is desirable. Overall, very late cessation of frost occurs over the east and southeastern Free State. These areas are also characterized by very early onsets resulting in shorter frost-free periods as compared to other regions.
In these areas, planting early would definitely harm the seedlings while planting very late would result in crops not attaining their full maturity before the risk of frost increases. The window for planting is therefore relatively short in these places. In contrast, most parts of the northern, western, and southwestern Free State experience relatively long growing periods making them ideal for planting long-season crops providing that other climate requirements have low risk in those areas.
In general, the results show low frost risk in the far western parts of the province, increasing gradually towards the east where the late frost cessation and early onset risks are high. Trend analysis showed a tendency of positive trends of frost-free period in some areas implying an increase in minimum temperatures with time.
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The rate of increase in frost-free period ranges from 1 to 5 days per decade depending on the location and severity of frost. The authors declare that there is no conflict of interests regarding the publication of this paper.
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The authors are grateful for funding by the Department of Agriculture, Forestry and Fisheries Project no. The authors thank Dr. Thomas Fyfield of the Agricultural Research Council for editing the manuscript. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We will be providing unlimited waivers of publication charges for accepted research articles as well as case reports and case series related to COVID Review articles are excluded from this waiver policy.
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