The USDA has just released a new map of plant hardiness zones, the first since 1990, and Mid-Missouri has been moved from zone 5b to 6a. The zones are defined by the average annual minimum temperature, with zone 6a meaning winter lows are -10 to -5˚ on average.
When you read about plants, a zone rating helps you judge whether a perennial can make it through the winter where you live. A lot of the most popular home gardening vegetables are annuals in most of the U.S. because they can’t make it through our winters. But, plant hardiness is something you want to consider if you’re buying plants you hope to keep around for several years. Of course, the details do matter, and taking steps like planting in a sheltered area or covering plants in the winter can help some more tender plants make it through a tough Missouri winter, if you’re a more adventurous gardener.
The new USDA map reflects calculations of average temperature from 1976 to 2005, and I think it’s fair to say that most people are not surprised to see a warming trend over the past 30 years. Looking further back into the history of changes to the map however, it seems that in the ’60s Mid-Missouri was in the -10-0˚ zone, too.
There are also alternate zone maps to consider, including some, like the AHS Heat Zone Map, that describe how much heat a plant will have to survive in the summer (zone 7, 60-90 days over 85˚ in Boone County). Heat zone descriptions have started to appear on some plant tags in garden centers recently alongside hardiness. There’s a great set of interactive maps by state showing heat and hardiness zones and other useful information like average first and last frost dates at www.plantmaps.com.