Climate Zones and Chill Hours

Understanding and knowing your Climate Zones and Chill Hours will prove beneficial for the best gardening experience.

Climate Zones are divisions of the Earth’s climates into general climate zones according to average temperatures and average rainfall. The three major climate zones on the Earth are the polar, temperate, and tropical zones.

Zone 3 Zone 4 Zone 5 Zone 6
Zone 7 Zone 8 Zone 9

Chill hours are the chilling requirement of fruit computed from the minimum period of cold weather after which a fruit-bearing tree will blossom. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.

Climate Zones - USDA Cold Hardiness Zones

The Climate Zone (USDA hardiness zones) offer a guide to varieties that will grow well in certain climates. Each zone corresponds to the minimum winter temperatures experienced in a given area. For best results, make sure that your hardiness zone lies within the zone compatibility of the variety that you are considering. Find My USDA Cold Hardiness Zone

Great for the East

The USDA map does a fine job of delineating the garden climates of the eastern half of North America. That area is comparatively flat, so mapping is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles or so as you move north. The lines tilt northeast as they approach the Eastern Seaboard. They also demarcate the special climates formed by the Great Lakes and by the Appalachian mountain ranges. (Citation: Website – The National Gardening Association)

Zone Map Drawbacks

But this map has shortcomings. In the eastern half of the country, the USDA map doesn’t account for the beneficial effect of a snow cover over perennial plants, the regularity or absence of freeze-thaw cycles, or soil drainage during cold periods. And in the rest of the country (west of the 100th meridian, which runs roughly through the middle of North and South Dakota and down through Texas west of Laredo), the USDA map fails. (Citation: Website – The National Gardening Association)

Problems in the West

Many factors beside winter lows, such as elevation and precipitation, determine western growing climates in the West. Weather comes in from the Pacific Ocean and gradually becomes less marine (humid) and more continental (drier) as it moves over and around mountain range after mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low elevation, coastal Seattle are much different than in high elevation, inland Tucson, Arizona, even though they’re in the same zone USDA zone 8. (Citation: Website – The National Gardening Association)

Chill Hours
These are the number of hours between 32°F and 45°F in your climate zone from November to mid-February. Temperate fruits need go through anywhere from 100 to 1,400 chilling hours to bear fruit the next season. Gauging cumulative chill and matching varieties for your area is more of an educated guess than an exact science, as low temperatures vary considerably within a climate zone and from year to year. Chilling requirement is a concern for USDA zones 9B and 10, predominately in Southern and coastal regions where chilling hours average 100 to 600 per year. If you are within this area, take note of the chilling requirements listed for fruits and choose accordingly. Most of Northern California receives between 800 and 1,500 chilling hours per year, which is sufficient for most fruits. Persimmons, almonds, olives, berries and pomegranates all have low chilling requirements. Low chill varieties are available for apples, pears, apricots, nectarines, peaches and plums. Use the map to see if you are in a low chill area and estimate your amount of winter chill. Chilling Hours Map
Texas and NE Texas Weather Historical Data
Really good historical Data can be found here at the Texas A&M AgriLife Extention.

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