Spring frosts & snows
When snow falls late in the spring, gardeners often worry about the effects it will have on their early spring flowers. Actually, snow is a great insulator and may protect hardy flowers from frosty temperatures.
Tolerance to freezing temperatures depends on the plant. Spring bulbs, such as tulips, crocus and daffodils, tolerate a light frost or temperatures of 29 degrees Fahrenheit and above. Flowers are susceptible to hard frost when temperatures drop below 28 degrees. Plants located close to the house may receive some protection. Bulbs located on the south side of buildings and south-facing slopes are more at risk, because these areas warm up earlier in the spring.
Perennials that emerge early in the spring are rather cold hardy, and tolerate light frost. Pansies, snapdragons, pinks and dusty miller are examples of spring bedding plants that tolerate cold. Most new transplants, however, don't tolerate frost. Young petunias will tolerate some frost. But impatiens and tomatoes won't tolerate any. In fact, they won't even tolerate cool weather.
You may need to cover hardy perennials and bulbs on unusually cold nights when a hard frost occurs. To delay spring growth, mulch perennial and bulb flower beds in the fall after the ground freezes.
For more information, see the following Colorado State University Extension fact sheet(s).
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Updated Tuesday, November 19, 2013