Cold Hardy zones, Heat Zones and Climate Zones

Selecting the right plants and plant varieties for our gardens makes the difference between planting whatever looks good and building a garden that gives beauty and satisfaction for years with little trouble to the gardener.

Most plant tags and plant catalogs provide the USDA cold hardiness zones, along with standard information such as the ideal sun or shade location, mature plant height and width, and some cultural tips such as water needs. They are provided in order to help gardeners be successful with their plant selections and gardening success.

The USDA cold hardiness zone map, at http://1.usa.gov/znDthG, is widely used by growers and gardeners to decide whether a plant will survive a normal winter outside. The ten zones are based on average annual minimum winter temperatures. Each zone represents 10-degrees difference from the next zone up or down the scale. The zone number refers only to an average winter temperature range.

If you have been gardening for a few years, you have no doubt planted a few items that failed to thrive or even make it through the summer. In addition to the cold hardiness zones, there are other helpful reference points for plant selection, including the American Horticultural Society heat zones and Sunset climate zones.

In our climate, heat tolerance can be an important factor when selecting plants. Too much heat can be just as destructive as too much cold and some plant growers have started adding heat zone information to their tags and displays.

The American Horticultural Society’s heat zone map is at http://bit.ly/YXjKo0. Researchers used data from 7,831 weather stations in plotting the map to make it as accurate as possible.

There are 12 Heat Zones that indicate the average number of heat days over 86-degrees F or 30-degrees Celsius. It has been determined that 86 degrees is when plants begin to suffer damage to their cells. Zone 1 has no heat days and Zone 12 has 210 or more heat days. Oklahoma is entirely in heat zone 8, with 90 to 120 days above 86-degrees.

The full-color Heat Zone maps are available for $10 from http://bit.ly/15N8EFB and (800) 777-7931 ext. 133, or gardenshop@ahs.org.

The American Horticultural Society’s 1100-page reference, “A-Z Encyclopedia of Garden Plants”, provides specific growing information and plant details. Plus, each plant description includes the Heat Zone range, the hottest zone in which the plant will grow, and the minimum amount of heat necessary for the plant to produce flowers and fruit. The 2002 edition of the book is about $10 at online book sellers.

Sunset Magazine has its own way of defining their 45-Climate Zones. The reason Sunset has so many distinct Climate Zones is that their system takes into consideration snow, rain, wind, air movement patterns, latitude, elevation, days between last and first frost, and wind patterns.

The “Sunset National Garden Book” is a solid reference of over 650 pages in a paperback, black and white print format. Published in 1997, this book sells for a penny at online booksellers.

Each plant description states the Climate Zones in which the plant can be expected to thrive, assuming that the gardener provides the necessary culture (location, water, etc.).

For example, Zone 35 includes the Ouachita Mountains, Northern Oklahoma and Arkansas, Southern Kansas to North-Central Kentucky and Southern Ohio. The Central Plains zone map is at http://bit.ly/c34Nw3.

Zone 35 climate is a combination of how much air comes from the Gulf of Mexico, the region’s latitude, and how much arctic air comes through during the winter. Those combine to produce a climate with hot humid summers and winters with arctic masses arriving every few years.

We cannot control the weather but we can plan and plant for it.