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February Meeting
3/08/2012
9am refreshments
Hostess Team: Nancy Stone/Charlene Simmons
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A review of the physics of cold damage to a plant Plant Hardiness Freezing Water freezes at 32 degrees Fahrenheit (0 degrees Celsius). Plant sap contains water and when it freezes and crystalizes it expands breaking and killing the plant cells that contain it. However, the sap of a plant is not just water, but is also a mixture of sugars and nutrients and other components which lower its freezing temperature below 32 F. Just how low temperatures have to go to freeze the sap of a plant is dependant on the type of plant. Plants that are more cold resistant are refered to as being more hardy. Tropical plants, being from climates that are warm year round, are the least hardy, but few if any will be damaged right at 32 degrees, because all plants have some sugar in their sap. Any article stating that a plant cannot handle temperatures below 50 F. or 40F. is highly likely to be incorrect. It is possible that certain plants will become sick and eventually die if temperatures are consistently that low, day and night for weeks, but that is different than saying a plant will be damaged if temperatures drop that low at night. It is important to make this distinction when researching what will grow where you live. Plant Hardiness Frost A plant can still be damaged at 32 F. even if its saps freezing temperature is lower. The reason this can happen is that frost forming on the the leaves of a plant can pull moisture out of the cells on the leaf surface, which dehydrates and kills those cells. Frost dehydration damage is typically superficial and only affects parts of the plant most exposed to the open sky. A plants ability to resist this kind of damage depends on the anatomy of the surface of its leaves, and once again tropicals are the most sensitive. Interestingly, frost can actually occur when thermometers read temperatures above freezing. Heat Transfer Heat flows from objects at higher temperatures to objects at lower temperatures. On a cold night the warmer objects are the ground, buildings and bodies of water and the colder objects are the air and the night sky. Warmer objects can be refered to as heat sources and colder objects as heat sinks. Plants are not large enough to be a heat source or sink so they are caught between the two, and heat will simply flow through them. The closer a plant is to a heat source the warmer it will be. So, the goal of keeping a plant warm is to actually have it physically close to a heat source and to insulate it from the heat sink. Insulation slows down the transfer of heat through the plant and makes its temperature closer to that of the heat source. Techniques for protecting tropical and subtropical plants from frost and freezing weather. Covering Plants Unlike people, plants do not have an internal heat source, so they must be covered in a way that insulates them together with a source of heat. This source is most commonly the ground. Simply wrapping a plant in a blanket will do little to keep it warm, because one would just be insulating the plant itself against everything around it. Yes, the insulation will slow the transfer of heat to the surrounding environment from the plant itself, but there isn't a whole lot of heat in a plant compared to other sources like the ground. Instead, the insulation should be put over the top of the plant and attached to the ground around it, creating a tent, with the ground as a floor. The less that air can move in and out of this tent the better. With particularly tender plants, an additional heat source can be added to the tent. Incandescent light bulbs give off a significant amount of heat. For safety, use grounded outdoor cords. For a socket, the light bulb cages that come with a hook and extension cord, for hanging under the hood of a car while working on it, are a good choice. The old style outdoor Christmas light strings, that have hot incandescent bulbs, are also a good choice because they will distribute the heat over a larger area. One will also have to decide what to use for insulation. A bed sheet is a good start but a more insulating fabric will provide more protection. However, a very heavy blanket can create problems because the weight of it can break limbs, and it can be hard to keep on top of the plant. Also, an ideal cover can be left on the plant during the day, in the case that temperatures are going to drop down at night for several days in a row, which is usually the case. Frost cloth is a special fabric made to address all of these problems. It is light yet insulating, but it still allows some sunlight to come through during the day. For smaller plants or large areas one might also consider building a structure over the plant on which to mount the cloth. Using 1.5 inch pvc pipe is an inexpensive and easy way to create a temporary scaffold. Having a structure to hold up the fabric also helps the plant to sustain less damage because leaves and branches that come into contact with the covering will be damaged as if they were unprotected. This damage occurs because contact with the fabric allows heat to be more easily transfered through these points. Watering Plants Water is a substance with a high heat capacity, meaning it stores a relatively large amount of heat compared to other substances at a given temperature. For this reason, it is a good idea to water ones plants the day before an especially chilly night. Not only will the plant have a little bit more heat stored for the cold night, but more significantly the ground around it will be wet and store more heat as well. It is not a good idea to this night after night because if the ground is kept too wet it can cause a plants roots to rot. Spraying Plants With Water All Night When a glass of water is being frozen it will stay at 32 degrees Fahrenheit until all the water in the glass is solidified. This is an interesting concept from Physics called a phase change. Since most plants actually freeze below 32 degrees, they can be saved from freeze damage by keeping their temperature at waters freezing point. Spraying water on a plant when temperatures are below freezing will keep the plants temperature at 32, because the warmer liquid water is constantly being supplied. Another way to think of it is that the process of freezing water requires the colder surrounding air to suck heat out of the water. So, constantly supplying liquid water means constantly supplying heat. The downsides of this method are that ice will build up and its weight can crush the plant, one could waste a lot of water, it can create a muddy mess, and all of that soaking might not be good for the plants roots. Microclimate Planting frost sensitive plants near heart sources to begin with can go a long way in protecting them. Significant sources of heat are buildings, concrete walls, and bodies of water. Well developed neighborhoods will always be warmer at night than outside of town. The eave of a house reduces heat loss from the space below it to the sky .
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Mexican Bush Sage (Salvia leucantha)
Mexican Bush Sage is a late summer and fall blooming tender perennial from Mexico, as its name implies. This drought tolerant Mexican native prefers sunny, well-drained sites and will survive with no supplemental irrigation is most parts of the state. In order to promote an abundance of late summer and fall flowering spikes, the plant should be sheared several times during the summer months. The subsequent regrowth produces a sturdier more compact plant as well. Deer Resistant |
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Oleander (Nerium oleander)
Ideally, select a site with full sun to light shade and well-drained soil. However, oleanders are adaptable and will withstand dry conditions as well as marshy soils. Deer Resistant |
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Russian sage (Perovskia atriplicifolia)
It requires full sun, but is hardy and cold tolerant. It is also tolerant of dry, chalky soils with a high pH, salt tolerant and drought tolerant. Deer Resistant |
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Mexican Bird of Paradise (Caesalpinia pulcherrima)
Plant Mexican bird of paradise in the fall in full sunlight exposures for best establishment. This large shrub is evergreen in mild climates, but where frost occurs, it will shed foliage. Although extremely tolerant of sun and reflected heat, it requires good drainage to thrive. Established Mexican bird of paradise plants require little water. Mature plants may reach a height of 15 feet tall. Regular pruning will help control size. Deer Resistant |