FROST PREVENTION By Linda De Wet

WHAT IS FROST? Frost is defined in the Glossary of Meteorology as the condition that exists when the temperature (Ta) of earth’s surface and earth-bound objects fall below freezing, i.e. 0  C (Rosenberg, 1974).

CONDITIONS FOR FROST FORMATION RADIATION FROST –Calm, cloudless skies –No (or very light) wind –Inversion ADVECTION FROST –Cloudy skies –Strong winds –No inversion

FROST FORMATION Frost forms through sublimation White or hoar frost –Inversion Black frost –No inversion Above-ground formation first

NIGHT (inversion) DAY Height(m) Temperature ( o C) Temperature variation with height above earth surface, during day and night.

Note On earth’s surface at different altitudes: Windy day or night – Ta is lower at a higher altitude  Advection frost Calm day or night – Ta is lower at a lower altitude (valleys)  Radiation frost

Course of air and soil temperature and wind speed during radiation frost conditions (Rosenberg, 1983)

Results for an experiment measuring minimum temperature profile for 4 different locations at different altitudes as compared to the measured Tmn (SAWS) for 22/8/03 and 10/05/03 for a suburb in Bloemfontein.

FROST DAMAGE Frost damage influenced by: Differences in climate during the season Cultivar types Morphology or growth stage Yield size Rate of Ta decrease Minimum Ta (Tmn) Time duration of Tmn

Frost probabilities

HOW TO PREVENT FROST Consider energy balance of soil surface at night Aim: to maintain energy status of soil- plant-air volume to be > critical Ta in 3 ways: Retard energy loss from system Redistribute existing energy in system Add new source of energy to system

1. Site selection Cold air drainage to valleys Local weather records Soil types- Dark soils - Light - Gravel Windbreaks – frost pockets Fences/shadenets Heavy vegetation Water bodies Wind direction.

2. Radiation Interception Artificial clouds  back-radiation -Spraying water into the air -Burning oil or tyres for smoke Environmental authorities Smaller burners  labour intensive labour intensive

3. Thermal insulation Clear day  solar radiation warming soil  radiation frost Small plants covered with black plastic or brown paper Trees – wrap trunks with fiber-glass, cardboard, fiber-glass, cardboard, polystyrene polystyrene

Cover cold-tender plants e.g. basil, peppers, tomatoes, geraniums and impatients with blankets, sheets, newspapers, etc. (excluding plastic). Protection at -6.7 o C using frost-cloth. Drape plants completely – no openings 3. Thermal insulation

Above reduces rate of Ta change and  decrease in time period where T < 0  C Disadvantage = disease / pest problems 3. Thermal insulation

4. Air mixing Fans Wind machines HelicoptersHeaters 5. Direct air- and plant-heating

6. Soil manipulation Irrigation and compaction or rolling soil  of 0.4  C in soil Ta Rolling soil + irrigation   > 0.6  C in soil Ta (1.1  C to 1.4  C) Soil covers hold soil heat in, e.g. plastic but should be Soil covers hold soil heat in, e.g. plastic but should be opened early morning to release heat to the plants.

Factors influencing soil surface Ta at night Soil surface Ta HighLow Good exposure to sun Open to sky at night Cloudy night No clouds Windy weather No wind Compact soil Loose soil Moist soil Dry soil Bare soil Cover crop or mulch

7. Application of water Water temperature > 0  C Water can give MJkg -1 + heat of melting (by 0  C) = MJkg -1 Sprinkler or flood irrigation Non-sprinkled and sprinkled apple trees

Average bud temperature for non-sprinkled and sprinkled apple trees, as compared with air and wet-bulb temperature in May at Logan, Utah (Rosenberg, 1983).

In the same time period (15:30): Ta = 25 o C and Tw = 15 o C Non-sprinkled apple trees – Ta = 45 o C sprinkled apple trees –Ta = 15 o C Difference of 30 o C for sprinkled and non- sprinkled!

a)Sprinkler irrigation -At onset of freezing T, water freezes on plants and heat of melting is released -As freezing process proceeds – ice stays at 0  C, but there – ice stays at 0  C, but there must also be enough heat for back radiation, evaporation and convection, sprinklers must continue after sunrise till all ice has melted has melted b)Flood irrigation: -gives heat to soil -Can increase Ta by up to 3  C. 3  C.

8. Chilling to prolong dormancy Use sprinkler irrigation during the day in winter and early spring to cool air and to delay bud development. Must be for minimum time period under specific Ta. Can delay development by 8 to 17 days for apples and peaches.

9. Other methods In Vineyards: –Insurance –Frost Alarm –Fog machines –Vine guards –Physical coverings –Electric currents - ViniCable.

The average number of frost-free days per year is projected to rise across much of the globe by the 2080s, with the largest increases (red and orange) across the western fringes of North America and Europe. (Illustration courtesy Gerald Meehl and Claudia Tebaldi, NCAR.)

CONCLUSIONS Frost is a great threat to most crops Effects of frost –Differences during season –Cultivars –Morphology stage –Yield size –Rate of temperature decrease –Minimum temperature –Time duration of minimum temperature

High value fruit and vegetables Little data available on yield decreases relative to minimum temperatures  scope for research in this area is relatively wide There is no cure for crops or plants damaged by frost, so the only other alternative left is to practice the prevention methods described. Prevention, after all, is better than cure CONCLUSIONS