The most common question of these past few days has been "What has this cold weather done to my plants?" The query is important, and it is more difficult to answer than you might imagine. Before I can give a reply, I have to ask more questions.

The first is, where do you live? Temperatures vary dramatically throughout Utah, and except for the St. George and Moab areas, the counties bordering the Great Salt Lake or Utah Lake are the warmest in the state during the winter.

Higher valleys to the north, east, west and south get much colder. From having lived in Cache Valley for a couple of winters, I can personally attest to the cold there. Heber Valley is also much colder, and the reports from the Uintah Basin sometimes make it seem like the thermometer is in the deep freeze. Selecting cold-hardy plants is your greatest protection against winter damage.

Although it is too late to take any action to prevent damage from the recent weather, it is always helpful to do a reality check on our plants. That usually starts with looking at their zone rating. We are not reviewing Super Bowl strategy here, but we are determining what plants are suitable for our playing fields.

Plant hardiness zones are based on cold temperatures. The most common zone designation is the U.S. Department of Agriculture's Plant Hardiness Zone Map, which was revised in 1990. It shows in detail the lowest temperatures that can be expected each year in North America.

These temperatures are referred to as "average annual minimum temperatures" and are based on the lowest temperatures recorded in the United States for each year from 1974 to 1986. The map shows 10 different zones, which represent a winter hardiness area for plants.

Map zones 2-10 are subdivided into light- and dark-colored sections (a and b) that represent differences of 5 degrees F (2.8 C) within the 10 degree F (5.6 C) zone. The light color of each zone represents the colder section; the dark color, the warmer section. Areas above an arbitrary elevation are unsuitable for crops, so they do not have specific zone designations.

The map shows that much of Salt Lake and surrounding counties are Zone 7a. That designation is based on average low temperatures of 0 to 5 F. As recent temperatures plunged well below that mark, more tender plants are bound to suffer.

Plants do not recognize averages. The ultimate low temperature is more critical, so what guidelines should we use? Microclimates might add a zone or two, but most experienced nursery professionals consider Zone 5b to be the safer designation. It has a range of minus 15 to minus 10 degrees F, which is where the mercury recently dropped in a number of locales in Utah. Of course, if your garden got colder, look for the appropriate plants.