Understanding Frost Probabilities - March 21, 2018
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Deciduous tree fruit growers and vegetable gardeners are usually interested in when the last spring freeze may occur. Fruit growers want to know how likely it may be for a freeze to occur that will damage or kill developing fruit crops following bloom. Vegetable gardeners want to know when it will be safe for them to plant warm season crops such as tomatoes, green beans, and squash without using frost protection.
The critical temperature for developing deciduous tree fruits is usually about 28 degrees F. There are a few frost protection methods that can partially protect developing fruit from freezing and subsequent losses, but the best strategy is to plant later blooming fruit varieties that are known to successfully produce fruit in your neighborhood. The protection strategies range from wind machines (too expensive for home gardeners) to covering plants with frost cloth or sheets and having an incandescent light bulb to heat the trapped air under the fabric.
Freezing temperatures (32 degrees F and below) are critical temperatures for warm season vegetables. However, it is also important for the soil to be warm enough for germinating seeds and active root growth. Ohio State University Extension recommends that warm season crops only be planted after soil temperatures reach 55-60 degrees F. Local Verde Valley folk wisdom says to plant warm season crops only after the native mesquite trees have leafed out in your area. This is actually pretty reliable because it integrates air and soil temperatures.
Many vegetable gardeners will gamble with the likelihood of frost. They may use season extension techniques such as floating row cover, low tunnels or “walls of water”. Those gambling without using season extension techniques often need to replace transplants or wait for weeks for seed germination. I’ve often used season extension on warm season crops only to find that waiting and planting after June 1 allows the plants to grow/germinate rapidly and often outgrow the plants that I got in the ground earlier.
The best way to gamble with critical temperatures is to be aware of local climate records and use them in decision making. Fortunately, the Region Climate Centers have compiled local climate data and have on-line information available to inform local growers of historic temperature trends that help make better informed decisions.
Arizona has 375 weather stations where the historic data has been gathered and analyzed. In the Verde Valley, the locations are: Cottonwood, Tuzigoot, the Sedona Ranger Station, Jerome, Montezuma Castle, Rimrock, and the Beaver Creek Ranger Station. Precipitation and temperature records are available along with many tools which include fall and spring freeze probabilities.
I have included several helpful links with the online edition below. However, I included a few web navigation instructions that may help you find the specific information you seek. The Arizona weather station data can be accessed at: https://wrcc.dri.edu/summary/Climsmaz.html. At this site, you will see a map and a navigation bar on the left side of the screen. The little red squares on the map are weather station locations and you may click directly on these for that station’s data. Similarly, the stations are listed by name on the navigation bar.
Once you’ve found the station you are interested in, you will see a table with temperature and precipitation data. You will also see the period of record which is not the same for all stations listed. Again, there is a navigation bar on the left. Scroll down through the information and under the word “Temperature” you will see spring and fall frost probabilities. Click on one of these and you will see a graph with data for multiple temperatures. Below the graph is another link called “Tabular Output”. I prefer this data format.
Here you will see probabilities across the tops of the columns and temperatures at the head of each row. To interpret the data, select a temperature and look across the row to see how the freeze probabilities change with date. For example, in Cottonwood, there is a 50% chance it will freeze (32.5 degrees F) on or after April 8 and a 10% chance it will freeze on or after May 3. The latest freeze date in the period of record is May 14.
Number crunchers and data geeks will enjoy accessing and using this information. Others may think interpreting the data is too complicated. You can always watch for the mesquite trees to leaf out.
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Arizona Weather Station Data
Western Regional Climate Center
Critical Temperatures for Frost Damage on Fruit Trees
Utah State University Extension
Season Extension: Introduction and Basic Principles
North Carolina State University Extension
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Last Updated: March 15, 2018
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