ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Wednesday, 3 March 1999

START OF THE GROWING SEASON & SPRING PLANTING

By early spring, backyard gardeners and farmers alike begin thinking about the upcoming growing season. In fact, crops have already been planted in many areas in the Southeast and the Gulf Coast states to include Florida and Texas, as well in the Southwest, from Arizona into California.

Many crops are sensitive to the weather, in particular to extremes in temperature. Several factors must be considered:

• If temperatures drop below the freezing point of water, considerable damage can be done to the living, non-dormant plant when the water within the plant or its fruit freezes. A "killing freeze" occurs when wide spread damage to crops terminates the growing season. Some plants are more susceptible to frost than others, as frost sensitive plants, such as tomatoes, peppers, and cucumbers, freeze when the temperature falls to 28 degrees Fahrenheit, while apples are damaged when the temperature drops below 20 degrees.
• While most plants may not be damaged when temperatures are above freezing, they will not grow unless the temperature reaches some threshold temperature. For example, corn will not grow until the air temperature is at least 50 degrees Fahrenheit. Germination of the planted seeds will not occur until the soil temperature reaches a particular temperature. Cool weather vegetables such as endive and peas can be planted and germinate when the soil temperature is about 40 degrees, while warmer crops such as sweet corn and cabbage favor soil temperatures of 70 degrees.

THE SEASONAL AND GEOGRAPHIC FACTORS

In most mid latitude climates, the growing season is often used synonymously with the frost-free season, loosely defined as the length of time between the last killing frost in spring and the first killing frost in the autumn. While the exact time span would vary by plant type, the growing season is often related to the interval when the daily minimum temperature remains above 32 degrees Fahrenheit. Maps of the average dates of the last spring freeze and the first autumn freeze have been prepared by the Joint Agricultural Weather Facility (NOAA/USDA). These charts are based upon 30 years of data collected at both National Weather Service offices and cooperative observers for the 1961-1990 interval.

The average date of the last spring freeze depends upon the latitude, the elevation and the proximity to large water bodies. Typically, the locations along the Gulf Coast have experienced their last spring freeze by mid March, while the average date of the last freeze in Michigan's Upper Peninsula is at the beginning of June. In the West, the topographic effects are such that high altitudes even in relatively southern locales often have a late season freeze.

The occurrence of the last freeze, coupled with soil temperatures, means that spring planting of various crops typically does not begin in areas outside of the relatively frost-free regions along the immediate Gulf Coast and southern California until mid to late March.

ADVISORIES AND WARNINGS

The National Weather Service issues several non-precipitation weather advisories and warnings to inform the public of the potential for damage to agricultural crops and other vegetation as a consequence of cold weather. These advisories and warnings are only issued during the growing season when the greatest threat to agriculture would occur.

• A freeze warning is issued during the growing season to warn the public when the near surface temperatures are anticipated to fall well below freezing over a wide spread area for a significant length of time. These warnings are issued regardless of whether frost is expected to form.
• A frost advisory is issued during the growing season to alert the public that wide spread frost is expected. Typically, overnight low temperatures in the 30s can result in the occurrence of widespread frost, especially in conjunction with light winds and clear skies.

A special thanks is extended to Steve Davenport of Madison, WI for his help in preparing this supplement.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.