When does your area experience the last spring freeze? The vernal equinox signals the astronomical beginning of spring. Meteorologists often divide the seasons into equal three month periods that align with calendar months. By their definition spring starts on 1 March. While both of these definitions work for their communities, neither meets the need of most gardeners. Whether you’re planting vegetables such as tomatoes or annual flowers like pansies, you need to avoid frost.
Dangers of a ‘Soft’ Freeze
When the temperature drops below freezing, water crystallizes into ice. The white frost you see on the ground comes from water vapor in the air. As the air temperature drops, the vapor condenses onto exposed surfaces such as rocks, car windows, and plants. This process will inconvenience you with the need to scrape your windows and, over time, will ‘weather’ rocks; the crystals damage plants’ flowers, leaves, and stems. Depending on the species, even a soft freeze is life-threatening.
Hard Freeze
In a hard freeze, defined as several hours at or below 28°F (-2°C), the survival rate drops to near zero. When this happens, the water inside the plants freezes. Because of its molecular structure, water expands as it freezes. This expansion creates powerful forces. I forgot about a can of soda that I put into the freezer to cool. Several hours later, we heard a loud pop. When we opened the freezer, we found a ripped can and soda ice covering the inside. The same thing happens inside plant cells during a hard freeze. As the water ice expands, it shatters the cell membranes, effectively destroying them and killing the plant.
That brings us to our featured map courtesy of NOAA.
If you’re interested in a more detailed interactive map, please visit Climate.gov. Note that these maps depict the average date of the last spring freeze. To be safe, most gardeners should allow add an extra week.
As always thanks for reading.
Armen
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