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Protecting Your Plants

Frosty cabbage
graphicstock.com

There are some winters, like the one we’ve been experiencing, when the temperature suddenly drops. All of us intrepid desert gardeners race to get the raised beds covered.  Cool season vegetables need a little shelter from high winds and cold weather. These include all the leafy greens – spinach, lettuce, kale – as well as tubers and roots like potatoes or beets.

These go into the ground as either seeds or transplants during mid to late fall or very early spring. It’s chilly around then and that can be stressful to any plants, although these vegetables do better when it’s not hot. Still, even they can’t survive when temperatures get around freezing. Fortunately, although the weather may turn cold with only a little warning here in the great American Southwest, chilling such as we do get doesn’t tend to last for more than a couple of weeks.  

In some parts of this country, the weather’s so cold that gardeners need to use cold frames for months on end to keep their plants going. We use them here, too, which permits tender vegetables to survive during unseasonably cool weather. People who want to produce a salad in places like USDA zones 5 and 6 generally have to wait until long after spring equinox before getting started in their gardens.

In colder regions, like zones 3 or 4, the challenge is even greater. I had a friend from Vermont (not one of the warmer states), and she told me that in New England, “summer’s a month of bad skiing”. Sometimes, it’s so cold that only the most robust vegetables can grow well, and the summer’s often too short to get a satisfactory crop of tomatoes or peppers. You can forget heat-loving fruits like melons.

Southern Nevadans may not need to confront those kinds of problems, but we do need to pay attention to the weather, whether we’re going through the blistering heat of July or the teeth chattering cold of January.

The easiest way to tell how low temperatures’ll get is to look at the overnight lows. Nighttime temperatures are virtually always lower than those we experience in the daytime. On average, the coldest nights won’t drop much lower than 38° F, but we know that doesn’t mean it never gets colder. The record for this area’s about 17°. Granted, that’s a rare event, but it’s happened.

Since water expands as it freezes, cells burst when internal water becomes ice. Most cool season plants won’t actually develop frost injury, which does kill, until it’s a couple of degrees below freezing. Most of the time, this saves them. The reason they can survive is the salt dissolved in their cells. Just as we put salt over roads and sidewalks to melt the ice, plants maintain a concentration of salt within them that prevents water from freezing.

In the best cases, the temperature can reach 28° before killing plants. This gives growers a little window when they can protect their crops, but it’d be foolish to wait until it’s that nippy.

I recommend getting protection on the ground when it’ll be 35° or less. Shielding raised beds is easier than for most things. Since they’re above the ground, a sheet of heavy plastic draped over the bed can trap warmer air close to the soil. The soil loses its warmth more slowly under the plastic, and generally it doesn’t get as cold as the air above it during the night. It’s important to remove the plastic in the morning, since that permits warmer fresh air to reach the plants. If the forecast says it’s going to be cold the following night, replace the plastic. This won’t be necessary every winter night, only when it approaches freezing. You can also use heavy weight row covers from several suppliers. These maintain the planting bed about three degrees warmer, usually just enough to keep plants out of danger.

Information on which vegetables to plant when is available at the Extension website or the Master Gardener help line.

For KNPR’s Desert Bloom, this is Dr. Angela O’Callaghan of the University of Nevada Cooperative Extension.