- Author: Marian I Chmieleski
FLately we've had some unusually cold weather and I've heard several different warnings in regard to overnight temperatures. According to the National Weather Service, a frost advisory is issued when the temperature is expected to drop to 36ºF or lower. Frost is formed when the earth itself loses enough heat that the temperature at ground level drops to 32ºF. A freeze warning foretells a coming temperature of 32ºF or lower and a hard freeze warning indicates that temperatures are expected to drop to 28ºF or lower. When there is a freeze it is because cold air is blown in from somewhere else (like the Arctic air we're getting right now). Freeze temperatures, clearly, can go much lower and it's those freeze warnings that need our immediate attention.
Freeze damage to plants can be anything form leaf and fruit damage to complete death of the plant. If you've had lingering tomatoes and basil this year, you've seen it first hand. They're finished, but we expected that. However, lots of people have citrus trees in their yards. In the citrus family the lemon, limes and citrons are the most sensitive to freezes, some of them suffering damage at 28ºF. Following them are the grapefruits and regular lemons. 'Meyer' lemons, oranges and mandarins are the most hardy and will be affected when temperatures get closer to 20ºF. Avocados, guavas and loquats can also suffer in cold temperatures.
So what to do?
Generally, if the tree is mature and healthy it will survive. Newly planted or young trees should be protected by wrapping the trunk with cardboard or other insulating material. Be sure the soil around the tree is irrigated and free of mulch to let the water and the earth radiate to the tree the warmth that they have. Hanging a low-watt light bulb or a string of Christmas lights in the interior of the tree will also provide just enough warmth to protect it. Still, the fruit and the leafy canopy of any of these trees may suffer damage. Fruit like lemons or oranges that are hit by a heavy freeze will quickly dry out. If the fruit is picked within the first few days after a freeze it may still be usable, but left on the tree will begin to rot.
If you do see freeze damage on your fruit- or nut trees, don't rush to prune. Be patient. Some years ago the temperature in our area dropped to 19 on two consecutive December nights. My 'Eureka' lemon was hit very hard. It lost all of its leaves and appeared to be dead. I was on the verge of taking it out the following spring, but Sunset Magazine recommended waiting to see if anything sprouted. It took quite a while, but by May we saw the first little green shoots. We now have a magnificent lemon tree that at times produces fruit the size of a small grapefruit. I suspect it is no longer a Eureka, but it sure does produce great lemons!
The University of California website http://homeorchard.ucdavis.edu/8100.pdf can give you even more details on how to protect your garden.
- Author: Sharon Leos
Is it a frost or a freeze? Honestly, it really does not matter. It is cold! But just in case you are curious, the difference is technical and meteorological.
A freeze (also called an advective freeze) occurs when a mass of cold air brings freezing temperatures into the area. For us, that normally means arctic air coming down from the Gulf of Alaska. It is usually cloudy and windy during a freeze.
A frost (also called a radiation frost) occurs under clear skies with wind less than five miles per hour which allow a temperature inversion to form near the ground where the temperature drops to freezing. Normally the temperature increases with altitude as you leave the ground. An inversion occurs when the temperature above the ground begins to cool at increasing height. If the air is very dry during a frost, no ice forms and it is called a black frost. A white frost forms when the air is holding water that condenses and freezes on surfaces forming ice.
Plants do not care why it is cold and the damage freezing temperatures cause depends on the species and age of the of plant and the amount of time it is exposed to the cold temperature. Severely frozen citrus may drop off the tree while less affected fruit may look normal but be dried out inside. Tender growth on unprotected flowering plants may turn black once thawed and bamboo (Bambusa spp.) leaves may desiccate and fall off. Other plants such as mondo grass (Ophiopogon japonicus) may be completely undamaged.
If a plant is damaged, the best advice is to wait until spring before pruning out the dead material. Pruning now may lead to further damage from later frosts since damaged branches and leaves may provide some protection. Be patient and let new growth on the plant show you where to prune so you do not cut back too much. The last frost in our area is normally around the third week in March, but Mother Nature can be unpredictable!
For more information on protecting your plants from freezing weather, visit the UC Integrated Pest Management website http://www.ipm.ucdavis.edu/PMG/GARDEN/ENVIRON/frostdamage.html. A search for “frost” on the UC IPM website will also help you find examples of frost damage to many types of plants including fruits and vegetables.