Plant Hardiness Zones

          The new garden catalogs are here! And everyone has a new “super” variety of every species. “Out produces every other variety!”. Sounds great, doesn’t it? Until you read the very fine print or follow the ‘additional information” link if you are in the online version of the catalog. Then if you are looking carefully enough you may find that it isn’t recommended for any plant hardiness zone less than 7. But the big print says it grows anywhere! 

What is a plant hardiness zone? The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. They are often also divided into a and b segments of each numbered zone based on 5-degree F differences.

Plant Hardiness Zones are most useful for perennial or biennial plants. And they are not necessarily an absolute guide. Small shrubs and plants which regrow from a root crown every year can often be mulched with a layer of straw or leaves in the fall to help them survive. We can also have microclimates created by shelterbelts and buildings which may provide us with up to one zone higher than our normal 3b in this area. Conversely, a very exposed area with little to no ground cover or wind protection may need a zone 2 plant to survive.

If you are looking for annual flowers or vegetables growing season length is more important. Growing season length is based on the days between the first and last frosts. Here in Burke County, our average last frost is about May 20th and our average first frost is about September 14th or 110 total. Remember, this is an average and actual frost dates can easily vary by 2 weeks or more in both spring and fall. In addition, you can start many species indoors or buy started plants in the spring. You can also provide shelter to young plants by placing gallon jugs or another container with the tops and bottoms cut out around them over each plant. Commercially available shelters like “walls of water” or “low tunnels” can also be used.

If you want more information about which varieties do well in North Dakota check out the NDSU Home Garden Variety Trials at