Author Unknown

An important factor in choosing what to plant in your garden is its hardiness zone. What this means is how cold a temperature the plant can survive in. From it you can determine what plants will ‘winter over’ in your garden and survive for years. The United States Department of Agriculture (USDA) has, since 1960, provided this information to interested citizens. The most recent map provided by the USDA was in 1990 and because of climate changes they are working on another update. The updated version will use new mapping technology and an extended set of meteorological data. There’s no information on when it might be available.

Basically the USDA Hardiness Zone Map divides North America into 11 separate zones with each zone having a 10-degree warmer (or colder) annual minimum temperature in an average winter than the adjacent zone. Some zones have been further divided into A and B regions which represent only 5 degree differences in minimum temperature. Since 1990 both Moscow and Pullman have been in zone 5B, which is the warmer of Zone 5. Zone 5 minimum temperatures range between -20 to -10 degrees F. Zone 5B is in the upper part of that range, from -15 to -10 degrees F.

It’s being suggested that gardeners use the Arbor Day Foundation Maps until the USDA maps are ready. The Arbor Day Foundation, which inspires people to plant, nurture, and celebrate trees, felt that the climate changes were dramatic enough to make a difference in what trees people should plant in their regions. In 2006 they released a Hardiness Zone Map that divides the country into ten different temperature zones. The new map shows that many areas have become warmer since the last USDA map was published and that significant portions of many states have shifted at least one full zone. Some areas of the country have warmed two full zones. The Foundation developed the new zone map based on information from the National Oceanic and Atmospheric Administration’s Climatic stations across the U.S.  According to the Arbor Day Foundation Press Release, “The new 2006 arborday.org Hardiness Zone Map is consistent with the consensus of climate scientists that global warming is underway.” They go on to say that tree planting is one thing that we can do to reverse the trend.

Using the arborday.org hardiness zone map, Moscow and Pullman are now in zones 6-7. The range of average annual minimum temperature in these 2 zones is -10 to +10 degrees F. This is very important information for gardeners. Nearly all American references books and nursery catalogs describe plants using USDA zones. Changes of 1 to 2 zones will have important ramifications for all of us. With each zone change plant choices change. We will be able to grow species that were marginal, at best, before. Additionally, we may start seeing some of our local plants failing to thrive and even begin to struggle. And a consequence of that may be that certain native animals will no longer be able to find the food they depend on for survival.

Stay tuned for the new USDA Plant Hardiness Zone Map and until then, follow the advice of the Arbor Day Foundation and plant a tree!