One excuse I've heard (I've got a feeling it was just a rumor) was that things are changing and that they felt like putting out a new map would be a waste of time because it would quickly change. It may be that a continuously changing map (constantly updated from information from the previous 10 years) would be the best answer. I've also heard a few other reasons (that sound more like conspiracy theories). Someone should write the USDA and ask them what the deal is!

A map that raises everyone around 2/3 of a zone would not only be useless, but would be wrong. Some areas have actually gotten a little colder! Some areas have increased as much as about 1.25 zones (Knoxville was in zone 6b, but the last decade has been a low 8a). A one-size-fits-all solution would obviously not work.

Does that mean that we are now considered warmer than usual?

So are we fully inside of zone 7 then?

Kelton

Kelton, in Knoxville, the actual average low temperatures of the past decade have us in a literal very low zone 8a. Our average low temperatures are definitely much warmer than they were a decade and especially a few decades ago.

I choose to only plant trees hardy in zone 7a and perennials hardy to 7b to be on the safe side, but planting zone 8a perennials (especially in a warmer microclimate) wouldn't be as daring as it might sound. There's always a chance that our weather pattern could gradually change back to colder temperatures, but that doesn't mean that that is what will happen.

I plant for 7b. Brandon, I cannot even think of tree generally considered 7b. Do you know some?

Myrtleoak, I'm not sure I understand your question. If you are asking if there are trees rated for 7b and warmer, there are many from many different genera. Even many oaks (a genus that I don't usually think of as "subtropical"), are limited to the warmer zones. Dirr's Trees and Shrubs for Warm Climates is one source that could give you a great number of candidates.

I was just asking this bc it seems most trees I see are rated 7a. I dont see many b divisions for trees for some reason. I was just looking for a few specific examples. It is also important to remember that different sources will often say different things and that hardiness range may vary based on precipitation, length of growing season, etc. Apparently one common observation is that sometimes a plant will be less hardy in areas with less intense growing seasons (ie Pacific NW, UK). I receive a catalog from oregon. Multiple species rated 8 in this catalog are generally considered 7 in the SE. If only hardiness was more black and white!

Since the PERSISION of the zone rating system is predetermined (by the USDA), eliminating the b designations would result in a less ACCURATE rating. There are many variables that go into hardiness, and letting a number represent something this complex may not really work well. But, choosing to only give whole zone number ratings certainly does not result in making the ratings more reliable. Saying the hardiness is "somewhere between 7 and 8" is different than giving a rating of 7. I think you will find reputable sources will give about the same number of a and b zone ratings. At least they should if they understand how the system works.

Some sources will supplement their ratings or even choose to use other descriptions. Dirr frequently goes as far as giving actual hardiness temperatures and locals on many plants.

Why are you being so defensive? I was EARNESTLY asking for some examples bc I couldn't think of any! In no way did my post doubt the significance of the subdivisions! I am HONESTLY asking for a few examples.

Myrtle, you are taking me very wrong. I was being defensive at all. If the capital letters made you think this, I was only capitalizing them because I wanted to differentiate them from each other. I wasn't trying to say them loudly or anything.

Oh, sorry. Thats one thing I hate about these boards; its difficult to display tone. I am curious, brandon7. What are some species that you would place in 7b?

What about Chattanooga? Am I considered in Zone 8b or 9? All the usual sites note I'm in zone 7a. Do you know?