Gardening zones

For guidance on what to plant in different climates as we've moved around the country, we have consulted the U.S. Department of Agriculture's Plant Hardiness Zone Maps as many other gardeners do. By using the map to find the zone in which you live, you can what plants are most likely to "winter over" in your garden and survive for many years. The map was first published in 1960 and updated in 1990 to reflect changes due to the changing climate.

But cold isn't the only factor determining whether our plants will survive and thrive. The effects of heat damage are more subtle than those of extreme cold, which will kill a plant instantly. Heat damage can first appear in many different parts of the plant: Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing. Plant death from heat is slow and lingering. The plant may survive in a stunted or chlorotic state for several years. When desiccation reaches a high enough level, the enzymes that control growth are deactivated and the plant dies.

The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. You will see the heat zone designations joining hardiness zone designations in garden centers, references books, and catalogs.

Sunset Climate Zones

The U.S.D.A. maps tell you only where a plant may survive the winter; Sunset climate zone maps let you see where that plant will thrive year-round.
Sunset's Climate Zones consider temperature as well as other important factors:
Latitude
Generally, the farther an area is from the equator, the longer and colder are its winters. Closer to the poles, the number of daylight hours increases in summer and decreases in winter.
Elevation
Gardens high above sea level get longer and colder winters, often with intense sunlight, and lower night temperatures all year.
Ocean influence
Weather that blows in off the oceans and the Great Lakes tends to be mild and laden with moisture in the cool season.
Continental air influence
The North American continent generates its own weather, which ― compared with coastal climates ― is colder in winter, hotter in summer, and more likely to get precipitation any time of year. The farther inland you live, the stronger this continental influence. Wind also becomes a major factor in open interior climates.
Mountains, hills, and valleys
From the Rocky Mountains to the Appalachians, continental and arctic air dominate, with moist air from the Gulf pushing north during the warm season.
During winter, Arctic outbreaks are most intense between the Rockies and the Appalachians. Both ranges act as barriers that limit the influence of the cold beyond them.
Microclimates
Local terrain can sharply modify the climate within any zone. South-facing slopes get more solar heat than flat land and north-facing slopes. Slope also affects airflow: warm air rises, cold air sinks.
Because hillsides are never as cold in winter as the hilltops above them or the ground below them, they’re called thermal belts. Lowland areas into which cold air flows are called cold-air basins.
Microclimates also exist within every garden. All else being equal, garden beds on the south side of an east-west wall, for example, will be much warmer than garden beds on the north side of the same wall.

ZONE 43. Upper Mississippi Valley, Upper Michigan, Southern Ontario and Quebec
Growing season: late May to mid-Sept. The climate is humid from spring through early fall; summer rains are usually dependable. Arctic air dominates in winter, with lows typically from -20 degrees to -30 degrees F/-29 degrees to -34 degrees C.

When deciding when to plant annuals so they won't succumb to possible frost, I consult maps indicating average dates for the last spring frost and the first fall frost, give some consideration to the recent weather patterns and hope for the best.