Some winters, temperatures drop so suddenly that intrepid desert gardeners must start running to get our raised beds covered with some kind of protection for delicate cool season vegetables. Cool season vegetables include all the leafy greens, tubers and bulbs. These plants go into the ground as seeds or transplants during late fall or very early spring when it is certainly chilly, but they do not thrive in freezing temperatures. Fortunately, although the weather may turn cold with little warning here in the great American Southwest, such chilling does not tend to last for any great length of time.
In many parts of the country, gardeners must use cold frames for months on end permitting tender vegetables to survive through autumn. People attempting to produce a salad in places like USDA zones 5 and 6 must generally wait until long after spring equinox before getting started with their gardens. In colder regions, like zones 3 or 4, the challenge is even greater. A friend from Vermont (not one of the warmer states) told me that in New England, "summer is a month of bad skiing." Most of the time, it is too cold for leafy greens or root vegetables, and the summer is often too short for a satisfactory crop of tomatoes or peppers.
Southern Nevadans do not need to confront those kinds of problems, but we do need to pay attention to the weather, whether it is the blistering heat of July or the teeth chattering cold of December or January.
Nighttime temperatures are virtually always lower than those we experience in the daytime. On average, the coldest nights will not drop lower than 39°F, but that does not mean it cannot ever become colder. The record for this area is about 17°F - a rare event.
Since water expands as it freezes, cells burst when internal water becomes ice. Most cool season plants will not really develop frost injury and die until it is a couple of degrees below freezing, which saves those most of the time. The reason is the salt dissolved in their cells. Just as we spread salt over sidewalks to melt the ice, plants have a concentration of salt within them that prevents water from freezing. In the best cases, the temperature must be below 28°F before killing plants. This gives growers a small window where they can protect their crops.
Protection for raised beds is easier than most things. Since they are above the ground, a sheet of heavy plastic draped over the bed will trap warmer air close to the soil. The soil loses its warmth more slowly under the plastic, and generally will not get as cold as the air above during the night. It is important to remove the plastic in the morning to permit warmer fresh air to reach the plants. If it will be cold the following night, it is important to replace the plastic. This is not necessary every night, just when it approaches freezing. I recommend protection when it will be 35°F or less.
Dr. Angela O’Callaghan is the Social Horticulture Specialist for University of Nevada Cooperative Extension. Contact firstname.lastname@example.org or 702-257-5581.