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Utah's spring weather is anything but predictable. In the past few weeks we've gone from hot to cold, calm to windy, dry to rain or snow -- and just about everything else you can imagine.
Most experienced gardeners have a time that they routinely plant certain vegetables. This might be May 1, Mother's Day, May 15 or any of a number of other systems for determining the last frost date but the reality is nature, not gardeners set those dates.
Successfully planting tender vegetables takes some experience, some dependence on past weather data and a little luck. Predicting the weather more than a few days in advance is difficult. The best you can hope to do is to look at long-term averages.
With warm-season vegetables -- beans, celery, sweet corn, cucumbers, New Zealand spinach and summer squash -- you must observe the frost-free growing season. This is the number of days from the last killing frost in the spring to the first killing frost in the fall.
The easiest way to get information on specific microclimates in our area is to log onto http://climate.usurf.usu.edu/reports/freezeDates.php. There you will find data on the last spring and first fall freezes at many stations in the state.
The Web site offers three spring dates -- the average date of the last spring freeze and the earliest and latest dates on which the last spring freeze has occurred. Similar dates are given for the fall freeze.
The site gives the freeze-free season for each station and the shortest and longest freeze-free seasons during the period of record. Since areas vary considerably, look for the weather station with similar conditions nearest your home.
Remember you are dealing with long-term averages, not fixed dates. For example, in some seasons, temperatures drop to near freezing in many locations much later than the usual date of the last frost.
Even though the tomatoes and other warm-season crops may not have been frozen last year, they were severely stunted by the cold temperatures in late May and late June.
Another good indication of when to plant tender vegetables is to measure the soil temperature. Put a thermometer a couple of inches under the soil. Check the temperature at 9 each morning. When the temperature reaches 55 degrees for three successive mornings, it is usually safe to plant the tender vegetables.
Larry Sagers
Horticultural Specialist
Utah State University Extension Service
Thanksgiving Point Office
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