USDA Plant Hardiness Zone Map Updates

[Gary350]

TV said, our Zone is being updated by ½. We are in zone 7a and 45 miles east is zone 7b they are 2000 ft elevation.

Are we changing to 7½a or 6½a?

I never can remember which way zone numbers run. Is zone 6 hotter or colder than us?

USDA says our average winter low is 2½°f warmer so that means 15° is now 17½° average low.

[applestar]

Hi @Gary350,

I just posted about the update the other day. Take a look at here

…basically ‘a’ and ‘b’ represent 1/2 zone and indicates 5°F difference in the average coldest temperature. ‘a’ is coder than ‘b’ … and lower whole number zone is colder (by 10°F).

[pepperhead212]

It looks like my place changed to 7a or 7b - no sign of 6b nearby any longer. Doesn't surprise, even though I had a mild summer, compared to most recent summers.

[Gary350]

TV says middle TN in my low 650 ft elevation area is 7a and east TN higher 2000 ft elevation is 7b. 7b is about 5° colder than 7a.

[imafan26]

All that really matters is what works for you. The map only averages the temperatures over the last 30 years, but micro climates and the local temperatures will matter more for what you are able to do in your area.

[Gary350]

I think zones are confusing. I remember what our weather has done in the past 45 years. Our average winter low is 15°f but that is up & down year after year, low has been from -4° to 85° in Feb. Our average last frost is April 20 but I have seen last frost April 1st and May 10. Our first frost is Nov 1st but I have seen frost 1 week early and 1 week later.

If Ohio is zone 5 that means colder weather is lower zone numbers. I think we are in zone 8a now instead of 7a but so what that does not change anything. We had 85° last Feb all my onions bolted, we had -2° Dec 23 all my garlic froze and died. Extreme weather changes are bad for the garden.

Zones say nothing about having 47" of rain Jan to May then no rain and 100°f desert June to Sept. Living in tornado alley we have lots of wind, sometimes 75 mph wind.

Zones are probably intended for new gardeners that have no clue what their weather might be. My mother use to say, zone map on seed package says I can't grow this here but I have been growing these for 20 years.

[applestar]

Primary use for using zones as guideline is for determining which perennials including asparagus, strawberries, other berries, fruit trees, ornamental perennials, shrubs and trees, etc. are likely to be able to survive the winter in your garden.

It also provides some idea of whether you can grow overwintering vegetables like lettuce, spinach and brassicas, or two year crop like artichokes with or without protection.

…

I think mid-North American continent regions are seeing the most volatile effects because there are no moderating effects from the oceanic mass to buffer like the coastal regions, and the blasts of heat from the south/Gulf or cold from the arctic/north seem to pull the temperature directly up or down….

[imafan26]

Correct. Over the span of the last ten years. The climate has gotten warmer and average temperatures are 3-4 degrees higher. I can plant the same crops, but now I have to look for cultivars with more heat and disease tolerance and grow mainly tropical vegetables in summer. I may have to modify other things like watering. I usually don't have to water much this time of the year, but because of El Nino and many years of drought, I have to water more than usual and I have changed my potting mix for some things so that it holds water longer and for some crops I may consider summer shading to try to keep things cooler.