Would That It Were That Simple
The USDA Plant Hardiness Zone Map was originally compiled at the Arnold Arboretum, first published in 1938 and most recently updated (for only the second time) in 2012. * The previous update was in 1990. The map can be useful in helping to decide whether or not to place a particular species in the landscape as a potential perennial, but temperature is just one limiting factor. Moreover, the temperatures used in creating the zone boundaries (the thermoclines) of the USDA map are merely the range averages for the winter low. The average DOES NOT determine hardiness. Extremes are what damage or kill specimens. For most plants, especially those with living tissue above ground overwinter, it may take only a brief period at the extremely low temperature to cause damage. Besides, unlike what most people believe, winter temperature is not the only possible limiting thermal factor (e.g., hot humid summer nights are tragic for some cold tolerant montane species). Frequently it is several interrelated factors. For example: soil moisture (especially too much or too little) which is directly related to soil temp, age of the specimen (i.e., established or not), stress history (esp. how recently planted), windiness (thus amount of evaporation and transpiration), where planted (substrate and lighting), cultivation techniques employed, topography and elevation, species/cultivar being considered, how quickly and when the change occurs, duration, snow cover when cold, et cetera.
The USDA zone hardiness minimum temperature for over 90% of Indiana is -10 to -15 degrees F (6a to 5b). My research found the average maximum low for central Indiana during the last 45 years is -6.3, yet in ten of those years the coldest winter temperature did not get to negative numbers, but for six years the lowest temperature was at least -20. Moreover, one-third of the years the lowest temp was outside the range (-5 to -15) and only four times has the maximum low actually been -6 or -7. Furthermore, there is little to no consistency regarding the annual fluctuation, and little to no causal relationship between the winter condition and the summer maximum or average. And, when compared to summer, the winter temperature range is more than twice as variable (39 to 17 degrees). What about this year? With spring right around the corner, I consider it fairly safe to predict the rating for greater Indianapolis likely will be 7a (central Tennessee). Additionally, the cold duration was minimal (just hours on three widely spaced dates) while January was the warmest on record. In case you are interested; 2018-19 was 5b. [Unlike other seasons, winter spans two years] During the past 40 years the central Indiana zone has ranged from 4a (typical of the southern half of North Dakota) to 8a (typical of central Mississippi). Ignoring the potential extreme will imperil inappropriate choices, recent plantings and those plants otherwise stressed..
Another problem with the USDA zones is that they overlap. So, is -10 in Zone 5b or 6a? Most plants labels and websites you encounter will not be that specific, i.e., they will only indicate zone 5 or 6. There is a huge difference (-20 to 0) and the conditions experienced within the zones are not uniform or certain. What if you are zone marginal? SHOCKER: many people are inclined to fudge--zone denial.
While the USDA zone rating is useful, it IS NOT the only consideration. Would that it were that simple. Just because you want something to be true DOES NOT make it so--both gardening and life in general. Attend one of my classes or lectures and you are guaranteed to hear this advice repeated. Chapter 12 (Weather & Climate) in my book has lots of additional interesting and useful information for gardeners.
* Unfortunately, climate change from global warming is so dramatic the recent 2012 revision is already inaccurate; e.g., the following year (2013-14) was brutal for many midwestern plants. What should we anticipate in the future? In addition to a higher mean temperature, we should expect more variability and a continued movement away from historical norms. Traditional seasonal boundaries are changing and will continue to migrate and differentiate. Perhaps you have noticed how truncated spring and fall have become. Extreme variation--the stuff that causes the most damage--likely will continue to increase. Note that the damage to plants is worse when it occurs at the vulnerable state of entering and leaving dormancy. As a hardcore gardener who relishes the seasonal transformation, I do not like the change. But that my friends is reality. We can try to ignore or pretend it is otherwise. It is called denialism--irrational behavior attempting to avoid uncomfortable truth. Some of us are so inclined. THE GOOD GARDENER WILL COMPENSATE AND MAKE WISE DECISIONS.