Prevention of Temperature Damage Part 2

UC Davis – Avg. Energy Fluxes During a Radiation Frost Energy Transfer Flux Density Watts per square meter Conduction (from the soil) +28 Convection (from the air) +39 Downward Radiation (from the sky) Upward Radiation (from the orchard) Net Energy Loss from the crop TOTAL +230 -315 -18 -36 W/m2 Frost Protection – Top Gun Style  You may have noticed that heat can be gained from the sky  This might seem confusing given we were losing heat to the sky  Often that lost heat will form a layer of warm air above a field.  A large fan or helicopter can push this warm layer back down onto the crops  Wind machine for kiwi crops  Helicopter & Ice Frost Protection Wind Machines & Helicopters  During the day, the air temperature at the surface of th soil is warmer than the air temperature 10 meters above because light energy is converted to heat energy when it strikes the ground  On a calm night, the air 10 meters above the ground is warmer than the ground itself because radiation inversion  Radiation inversion: a layer of cool air at ground- level created by rapid loss of heat from the ground after sunset  A helicopter or wind machine can push this layer of warm air back down Minimizing Temp Swings  Mulching is a very effective method for minimizing temperature fluctuations  For example, it is a very good idea to provide mulch around the base of perennials  During a winter thaw, the melting and refreezing of water around the roots of the perennial can push it out of the ground, damaging the roots and possibly killing the plant  Mulch can reduce the temperature fluctuations that cause this problem Minimizing Temp Swings  Wind buffers are also a good idea at very low temperatures  Wind Chill is the phenomena wherein something will cool at a faster rate at the same temperature due to the wind  The item losing heat cannot form a ‘heat island’ that slows the loss of heat, and more heat is lost more quickly  Windbreaks can reduce the rate at which heat is lost  The rate of temperature change is as important as the total change itself! Growing Degree Days (GDD)  Agriculturalists are able to determine plant requirements      using a measure called the Growing Degree Days (GDD) GDD is based on the idea that plants have a minimum need at which growth and development will occur. That said, there are three crucial temperatures for plants: 1. Tmin - The minimum temperature at which growth occurs 2. Topt – The temperature at which max growth occurs 3. Tmax – The temperature above which growth stops Topt Tmax Tmin GDD Formula  GDD is calculated by –  Min Daily Temp + Max Daily Temp = Daily Avg 2  Daily Avg– the Tmin = GDD E.g. Corn  For example, the Tmin for corn is 50o F  Below 50, corn does not really grow at all  Tmax is 86o F  Topt is in the upper 70s  Growing degree days can then be used to calculate the growing needs of corn in terms of temperature  Example: If a low temperature was 60°F and the high was 90°F, the GDD would be 60 + 86 = 146 divided by 2 = 73 – 50 = 23 GDD.  86 + 60 = 73 – 50 = 23 Growing Degree Day units 2 Corn  Each GDD accumulates each day  If a variety of corn requires 2450 units GDD, it would require 107 days to mature if each day had a low of 60 and a high of 90  2450/23 = 107  If the high was 100 and the low was 70, what would our GDD be? New GDD  86 + 70 = 78 2  78 – 50 = 28 GDD  2450 / 28 = 87.5 Days to Maturity Forecast for Tuesday, 4-27  High – 57  Low – 36  57+ 36 = 46.5; 46.5 – 50 = 0 degree days 2  This is why we would typically wait to plant until April!  On the other hand, lettuce as a base temp (Tmin ) of 40 46.5-40 = 6.5 GDDs Most GDD calculations use 50 as a base temp. Vernalization  Some plants require a cold treatment for physiological processes to occur.  This is known as vernalization.  Tulips and narcissus require vernalization to flower.  Some cereal grains, including winter wheat, also require vernalization.  Apples require 1,000 to 1,200 hours of temperatures between 32°F and 45°F to break their rest period. Stratification  Seeds of some plants have a dormancy mechanism that is broken by a cold period.  The seeds do not germinate until the seed has undergone a cold period.  This cold requirement for seeds is known as stratification.