Climate Zones for horticulturists.

Climate conditions in a geographic region are a mixture of different weather characteristics. Elements considered in designating a climate include the amounts and levels of sun, snow, rain, wind, and humidity. These elements vary according to the physical terrain (flatlands vs mountains), latitude, and proximity to oceans or major waterways.

A climate zone is a designation for an area based on the average minimum temperature throughout the year. These zones are published in almost all gardening books. Plants are given a zone rating indicating the region(s) in which that particular plant will best thrive. A zone may not only affect whether a plant lives or dies but will affect how it grows: tall vs short or bushy vs thin. For example, cherry trees need winter frost in order to produce flowers and foliage in the Spring.

Boundary lines between Climate Zones are not absolute and may vary over time. Temperatures of adjacent zones will be very similar at their boundaries. Also within any given zone the annual temperature may vary up to 5 or more degrees.

The zones of North America are set by the U. S. Department of Agriculture. They divided North America into 10 climate areas.

Zone 1
The coldest zone with winter lows of down to -50 degrees F. (-46 degrees C). According to the published maps, this zone is located in the northern-most areas of Canada.

Zone 2
Annual minimum temperatures ranging from -50 degrees F up to -40 degrees F. This zone is also in Canada positioned throughout the central part of the country. This zone actually covers some areas more northerly than some of the Zone 1. The reason for this is the location of Hudson Bay. The large bay raises the ambient temperature so that Zone 2 surrounds the bay, even though further inland, both on the East side and the West side, Zone 1 temperatures prevail.

Zone 3
Annual minimum temperatures ranging from -40 degrees F up to -30 degrees F. This is the zone covering the central part of Canadian North America. It snakes across the continent from the west side of the Rocky Mountains though not bordering the Pacific Ocean, dipping south over most of North Dakota and northern Minnesota in the U.S.A., then flowing back north over the top of Lake Superior. It continues east but does not border the Atlantic Ocean either.

Zone 4
Annual minimum temperatures ranging from -30 degrees F up to -20 degrees F. Zone 4 covers most of the American Rocky Mountains from Montana to as far south as the north part of New Mexico. It also covers all of South Dakota, the lower 1/2 of Minnesota and all of Wisconsin to the southern coast of Lake Superior. It begins again on the Canadian side of the Great Lakes, flowing eastward along the St. Lawrence Seaway. It is the zone for the east coastal areas of Canada.

Zone 5
Annual minimum temperatures ranging from -20 degrees F up to -10 degrees F. T

Zone 6
Annual minimum temperatures ranging from -10 degrees F up to 0 degrees F. T

Zone 7
Annual minimum temperatures ranging from 0 degrees F up to 10 degrees F. T

Zone 8
Annual minimum temperatures ranging from 10 degrees F up to 20 degrees F. This is an interesting zone due to the extensive area it covers. The zone starts on the west coast as a thin strip covering most of Vancouver Island and portions of the Canadian coastline nearby. It extends like the letter "U" from Seattle, Washington down along the inland regions of the Pacific coastal states to Arizona. It covers the central 1/4th of Texas and widens out as it flows over the southern states curving northward to a point on the southern tip of Maryland on the north side of Chesapeake Bay.

Zone 9
Annual minimum temperatures ranging from 20 degrees F up to 30 degrees F. Most prevalent along the Gulf of Mexico coastal states of Texas, Louisiana, Mississippi, Alabama, and Georgia. It also covers most of the central portions of California except for areas of higher altitude in which zone 8 conditions exist. Zone 9 also extends as a thin strip along the Pacific coastline as far north as the western edge of Washington State. The Japanese Current journeys across the Pacific Ocean and warms the northern most states. It is the reason for the Olympic Rainforest that exists in the west- and north-most part of Washington State.

Zone 10
Annual minimum temperatures ranging from 30 degrees F up to 40 degrees F. Only a very few spots in North America experience Zone 10 conditions. These include the southern 1/3rd of Florida, the very southern tip of Texas, and a couple of areas in southern California.

Cli"mate (?), n. [F. climat, L. clima, -atis, fr. Gr. , , slope, the supposed slope of the earth (from the equator toward the pole), hence a region or zone of the earth, fr. to slope, incline, akin to E. lean, v. i. See Lean, v. i., and cf. Clime.]

1. Anc. Geog.

One of thirty regions or zones, parallel to the equator, into which the surface of the earth from the equator to the pole was divided, according to the successive increase of the length of the midsummer day.


The condition of a place in relation to various phenomena of the atmosphere, as temperature, moisture, etc., especially as they affect animal or vegetable life.


© Webster 1913.

Cli"mate, v. i.

To dwell.

[Poetic] Shak.


© Webster 1913.

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