Prevention of Temperature Damage

Human mechanisms  Horticultural crops are aided by humans when fighting off the effects of chilling and freezing damage  To understand why human responses to cold weather work, we have to better understand how temperature works. 2 Kinds of Frost  There are two kinds of frost that can damage plants:  1. Advection Frost: a cold front brings lower temperatures with the new weather pattern  2. Radiation Frost: these are characterized by clear skies, calm or no wind, and gradual drops in temperature as the heat of the previous day is gradually lost  More heat is radiated away than received, so temp drops  Without anything to slow its loss (such as clouds, downward wind, or a cover), the temp continues to drop Temperature  Temperature is the measure of degree heat  This is a really dumb definition because it’s like saying moisture is a measure of the degree wetness – it doesn’t mean that much to the average person  A better way to think of temperature is as a type of energy that enables a plant to do what it does, including…  Photosynthesis and Respiration  Enzymatic Activity (biochemistry, sorta)  Maintain a liquid state where needed (vs. solidification)  Uptake water via transpiration, xylem, and stomata Temperature = Energy  The energy of heat can be transferred to a plant in one     of 4 ways 1. Radiation – transfer of energy through open space (e.g. those red lights that keep your food warm) 2. Conduction – transfer of energy by contact (e.g. touching a hot kettle) 3. Convection – transfer of heat through mass movement (e.g. a hot air blower in your car) 4. Latent Heat Exchange – chemical transfer of heat (e.g. sweat absorbing heat to evaporate cools you) The 4 Methods in nature  All four processes naturally affect temperature changes     in plants 1. Radiation from the sun strikes plants and warms them 2. Warm soil conducts heat into the plant cooled by the night air 3. A light breeze blows warm air from high places into low areas where cold air has settled 4. For water vapor to condense, it must give up energy; it will transfer latent heat to a plant to form dew in the a.m. Horticultural Strategies  These four methods are actively employed in      horticulture to minimize heat loss from plants Warming: greenhouse, cold frames, hot beds, hotcaps Cultural: sloped beds, waterfronts, plastic sheeting Frost protection: avoidance of low areas, covering plants, moist soil, air movement, sprinklers Cooling: misting; wrapping trunks in white wrap Minimizing temp swings: mulching  See next slides for explanations Warming Methods  Greenhouses – Mr. Blake, on a tour, asked how much it cost to heat the greenhouse on that particular day; I replied “Nothing”. How was this possible if it was almost freezing outside and balmy inside the GH?  The greenhouse effect is when radiation (light energy) from the sun or other source is absorbed by an object and transferred into heat energy.  The panes of the greenhouse allow light energy to pass through but limits the flow of heat energy. Greenhouses aside…  Most people do not have their own greenhouse, but this does not mean they cannot benefit from the radiation benefits of the greenhouse effect  Hot beds: covered boxes with heating elements to warm the soil and plants inside  Cold frames: a frame with a southern-facing clear covering that traps solar radiation and converts it to heat  Hotcaps: sort of a one-plant greenhouse; e.g. cutting the top off a clear 2-liter bottle for a tomato seedling Cold frames Hotbed Hot cap Cultural Practices  A cultural practice in this case is the method of raising crops  One cultural practice for temperature maintenance is to plant your crop on a slope  For example, orchards and vineyards are usually found on the side of a hill  This enables the cool air to sink below the crop, reducing the likelihood of chilling/freezing damage Cultural Practices  Location near water is also key.  Water has a high specific heat  Specific Heat: the amount of energy (calories) required to increase the temperature of a substance  Water has a specific heat of 1 calorie/gram of water  This is why we have the term “cooler by the lake” – the temperature of the water will rise slower than the temperature on land  In the fall, this reverses to “warmer by the lake”, as the land around the water cools faster than the water does. Peninsulas Are Ideal for Fruit  Much of the nation’s fruit production occurs on peninsulas for this reason  The temperature fluctuations are minimized when you are surrounded by water because it takes the temperature of water does not change as easily  Water forms a buffer against severe temperature swings  This is why Door Co. by Green Bay is known for its apple and cherry orchards and why Bayfield along Lake Superior can grow fruit Frost Protection  Plastic sheeting works for frost protection because it traps the heat radiating from the earth  Like water, the earth will cool more slowly than the air around it; it will radiate heat, which will be lost to the sky and eventually space unless it is trapped  Plastic sheeting can trap the rising warm air and form a buffer between the plant and the cool night air. Frost Protection  Sprinklers are also effective in frost protection.  When the ice forms on the surface of the plant, it must give up energy to become ice (a solid is at a lower energy state than a liquid)  The energy is transferred from the liquid water to the plant so that ice can form  This both provides additional energy to the plant while insulating the plant against more severe temperature drops  This would be latent heat exchange Moist Soil  For the same reasons previously mentioned, wet soil will maintain a warm temperature longer than dry soil  Watering your plants before a cool night can offer protection both in terms of  A) reduced heat loss (which would also be increased heat radiation into the air and conduction from the soil) and in terms of…  B increased gains from latent heat transferred as the condensation forms around the plant