A short description of an Agroclimatic factor of interest: Frost-free period, frost dates

The agricultural season is primarily determined by the period of suitable temperatures for growing crops. Temperatures above zero are a necessary condition for crop growth. Frost has a devastating effect on crops in general. In light frost (between 0°C and -1/-2°C) the tender plants are killed. Moderate freeze (between -2°C and -4°C) is widely destructive to most vegetation and lower frost is already causing severe damage to most plants. [1][2] This is a general simplification, the effects also vary for different growth stages (see Figure 1 below) and different crops. Therefore, it is important for farmers to know frost-free period in their agricultural areas. Especially the last spring frost date for starting agricultural work and first fall frost date for the cessation of agricultural activities. The likelihood of frost and frost trends over the years will help effective planning.

Fig. 1: How Frost Damage Affects Crops [3].

The last spring date is usually called the last day during spring (more correctly from winter to summer), when the minimum daytime temperature is less than 0 degrees Celsius. The first fall date as the first day in the second half of the year (during autumn), when the minimum temperature is below zero. Usually this date is given with a 50% or 10% probability (statistically from several years) that it will freeze later (spring date) or sooner (autumnal date). For farmers, the knowledge of days with a low probability of frost is mandatory for agricultural planning and decision making, hence the last/first frost dates with a low probability. Sometimes frost dates are also given for other freezing temperatures e.g. – 2°C. [4]. The frost-free period is a period from the last spring frost to the first fall frost. It includes a period suitable for growing crops.

References:

[1] https://www.almanac.com/gardening/frostdates
[2] WMO. 1963.Protection Against Frost Damage https://library.wmo.int/index.php?lvl=notice_display&id=5363#.XZQ_Wkb7SUk
[3] Feed Central, https://www.feedcentral.com.au/frost-damage-affects-crops/
[4] Agroclimatic atlas of Canada, 1976 http://sis.agr.gc.ca/cansis/publications/manuals/1976-acac/index.html