December 15, 2008

Zonal Issues

Here in Syracuse we gardeners are challenged every winter by temperatures that often fall well below freezing. We tend to be grouped into that category of cold-climate gardeners or northern gardeners who, as a cultivating demographic often face an exaggerated “deprivation” in our inability to grow certain plants that simply cannot survive below a minimum temperature. It is generally accepted that winter hardiness is the limiting factor on whether we can have a plant in our garden, period.

The United States Department of Agriculture (USDA) hardiness zone map is usually used as the default reference for this important factor. The map breaks the U.S. into zones that represent areas of average winter low temperatures. The current map is based on data collected from about 1970 to 1990. Syracuse is located within zone 5 on the USDA map. Zone 5 sees average winter lows between –10 and –20 degrees Fahrenheit. This basically means that plants rated for zone 6 and above simply should not survive here. Here is the USDA hardiness map:

Well, there’s some good news (depending on your perspective). Syracuse is now in zone 6. According to the National Arbor Day Foundation, which has collected its own data from 1990 to 2003, the United States has generally gotten warmer (a few places cooler) and the USDA map is now obsolete. Here is the Arbor Day Foundation’s new map:

Due to rising global temperatures, we here in Syracuse now have a wider palette of tender plants to choose from when planning our gardens, like southern magnolia (Magnolia grandiflora), which is rated to zone 6. In fact, I’m experimenting with this very species in my own garden.

Zone 6 is not all that bad. There are plenty of folks in zones 3 and 4 who would give their left arm to be able to plant some of the things that we can grow here in Syracuse (like Japanese maple). So get out there next spring and try some zone 6 plants; you may be rewarded in upcoming years with blooms that you thought you’d never be able to have in your garden.