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The USDA recently released their latest hardiness zone map that provides essential climate information to our country’s gardeners and nurserymen. Climate zone maps help us determine what plants are adaptable to our local environment. If you see a hardiness zone in a plant description, in a catalog or plant tag it is referring to this map. The country is divided into 26 separate zones, each 5 degrees warmer or colder than the next. This map was last updated in 1990 and there are significant changes to many areas and some states (like Ohio) have seen dramatic changes. (A good portion of Ohio changed from zone 5a to 6a)
The USDA is attributing the zone modifications to better data and technology. The new map also takes into account topography, slope, elevation, prevailing winds and proximity to water - for the first time. There are some significant climatic conditions the map cannot account for. The benefit of a snow cover over perennial plants for example. The regularity or absence of freeze-thaw cycles, soil drainage during cold periods and extreme elevation changes (especially in the western US) greatly affect the winter hardiness of plants but are not reflected on this hardiness map.
"The map is not a good instrument for determining climate change," said Kim Kaplan, a spokeswoman for the USDA's Agricultural Research Service. "In some cases where areas changed zones there was less than a one-degree change in temperature."
Now that these changes are official the conversation around the “horticultural water cooler’ has become “what can I plant now that our hardiness zone has gone from 5 to 6?” Considering that hardiness zones are based on a 30-year span of weather the simple answer is “the same things you could have been successful planting for the past 10 years!” The fact of the matter is that there are very few “new” items to add to your planting palette and proper planning and planting remain key for success with any plant, regardless of hardiness rating. For example I have had a Cunninghamia lanceolata (China-fir Hardiness zone 7) growing in a garden here for over 15 years. The planting site is right next to a building facing directly south allowing for maximum heat reflection and protection from winter winds. The soil is very deep and fertile allowing for a greater than average root structure. Also in this garden is a Cedrus altlantica (Blue Atlas Cedar) that was never supposed to grow here but has done very well for 12 years. Proper site selection and soil conditions will be necessary for plants rated for zone 6 that gardeners may now consider “new”.
Following are a few of the items listed for zone 6 that may not have been previously considered suitable for our local climate:
Cedrus Atlantica Blue Atlas Cedar
Cercis canadensis ‘Hearts of Gold’ Yellow-leafed Redbud
Magnolia grandiflora ‘Edith Bouge’ Southern Magnolia
Styrax japonica Japanese Snowbell Tree
Abelia grandiflora Glossy Abelia
Buxus sempervirens 'Variegata' Variegated Boxwood
Camellia (Many selections)
Cytisus scoparius Scotch Broom
Hydrangea ‘Blue Billows’, ‘Lemon Daddy’ (and others)
Ilex crenata ‘Sky Pencil’ Upright Japanese Holly
Ilex x 'Sadezam' Santa’s Delight Variegated Holly
Lagerstroemia Crepe Myrtle (Hardiest selections)
Leucothe ‘Rainbow’ (and others)
Prunus laurocerasus Cherry Laurel
Roses (Many David Austin Hybrids as well as others)
Agastache Anise Hyssop
Corydalis ‘Blue Heron’ Blue Bleeding Hearts (and others)
Gaura Butterfly Flower
Lithodora diffusa ‘Grace Ward’
Mentha requienii Corsican Mint
Some of you may find many “new” plants here and others will consider this old news. The fact is there are a lot of really fun plants that perhaps have limitations on where and how they are situated in your landscape but armed with a little bit of good information you too can have great success and enjoy some of these unique and different plants.
Now go outside and have fun in the dirt!