Confused about SC Plant Hardiness Zone

Lady's Island, SC(Zone 8b)

I am hoping someone can clarify something for me. When I started gardening a year ago, the USDA Plant Hardiness Zone map said that my zipcode (29907) in coastal South Carolina near the Georgia border was Zone 8b. However, all summer the 2 sweet ladies at the local nursery kept trying to convince me that we are in zone 9a. I explained that everywhere I have looked online places us in Zone 8b. One of the ladies said some book they have at the nursery says we're in zone 9a. Of course, when I asked to see it, she looked everywhere for it and couldn't find it. (I was there watching her.)

Mind you, plants that I bought last year that were for Zone 9 survived our mild winter just fine. In fact, I have had a potted tropical Hibiscus for 3 years, which I never brought inside because the pot is stone and way too heavy, and it only died back the 2nd winter due to snow/freeze, but bounced back when spring came. ( I had actually tossed it into the compost bin in Feb of 2012 to find it leafing out 2 months later in April. So, I repotted it and it grew beautifully.)

Anyway, again this spring, they are still trying to convince me that we are in Zone 9a. This morning I looked up our Plant Hardiness Zone again and we are still 8b. However, and this is where I am confused, the city of Mt Pleasant, SC which is 90.4 miles (1 hour 54 mins) northeast of us is Zone 9a. I always thought the zones changed the farther north one went. So, why is it that a city 90.4 miles north of me in zone 9a and I am zone 8b?

According to the Weather Channel, the average annual monthly high temperatures for my city is higher or equal to Mt Pleasant and the average annual monthly lows for my city are still higher than that of Mt. Pleasant.

According, whose information was calculated from the historical data of 18,000+ U.S weather stations for the period of time from 1980 to 2010, I found the following information:

Annual Average Temperature:
My City: 65.8 F
Mt Pleasant: 65.0 F

Average Annual Precipitation:
My City: 49.30 inches
Mt Pleasant: 49.68 inches

Average Annual Snowfall:
My City: 0.20 inches
Mt. Pleasant: 0.33 inches

Annual Average Humidity:
My City: 80.32%
Mt Pleasant: 74.13%

Mean Minimum Temperature: (Both cities have their lowest mean minimum temperature in January.)
My City: 37.4 F
Mt. Pleasant: 36.2 F

Mean Maximum Temperature: (Both cities have their highest mean minimum temperature in July.)
My City: 91.4 F
Mt. Pleasant: 91.1 F

So, again I ask, why is Mt Pleasant in zone 9a and my city is zone 8b? Hope someone can explain this LOL

Northeast, WA(Zone 5a)

Doesn't matter if you plant for the coolest one. Plant for zone 8 and you won't have to worry about it. And don't get into conversations about it. :0) No arguments.

Lady's Island, SC(Zone 8b)

Jnette, the thing is, is that when comparing the two cities, the zone 9a city is considered the "cooler" city, and I don't understand why LOL I mostly do plant for my zone 8b, but some zone 9 plants I can't refuse, and they have done just fine from year to year. Heck, I even have Livingstone Daisy Trailing Mezoo Red that has been growing as a ground cover for 2 years now and its hardiness zone is 9, 10, and 11. Despite our low temps in the mid 30s, it did just fine. In fact, it did so good through our winters, it looked fake LOL I think what helped it perform so good through winter is that the area I have it in. During spring and summer it gets full sun from 10:30am to 2:30pm. Then in Fall and Winter, when the trees lose their leaves, they are in full sun from 10:00am until the sun goes down.

Anyway, one of the reasons I asked our local nursery ladies about our zone is because they have been ordering A LOT of plants for zone 9. When not-so-garden-savvy customers want to buy plants, they are often pointed to the plants for zone 9+, and usually convinced to buy them. Some of the plants are really expensive, too (Mandevilla & Bougainvillea)!

Oh well, I guess I will just keep doing things how I have been doing them despite what zone I'm in LOL

Liberty Hill, TX(Zone 8a)

My town is zone 8a but my mircoclimate is more zone 7b. The zones are just suggestions that don't mean a whole lot. I'd be more interested in the lady at the local nursery finding that book.

Opp, AL(Zone 8b)

Your decision about what to call your zone will make no difference to the plants if there's a harsh winter.

Cherry Grove, OH(Zone 6b)

USDA zones are based on the average annual minimum winter temperature in 10 degree increments and the A/B splits each 10 degree zone into two 5 degree increments.

The USDA site says Zone 8b (min. 15 to 20 (F)) for your zip code, but you are saying 37.4 (F). That's a significant difference.

Russell, KY(Zone 6b)

Zones are influenced by factors other than latitude- things like mountains, valleys, stands of large trees, bodies of water, jet stream patterns, general elevation- it's a whole science. That's why we need these complicated charts to guide us. Anything that changes air flow patterns and how layers of air settle can impact how low the temperatures get. So that should help explain why a location to your north could be "warmer."

For example I am smack in the middle of 6a, but I am less than a mile from the Ohio River which somehow mediates the lower temperatures a bit, so I am 6b even though a city directly south of me is still 6a.

And the charts are still just a guide, not an absolute. Micro climates exist everywhere and zone pushing can be successful or not based on the tiniest of differences. Go ahead and try plants not rated for you but only if you are prepared to lose a portion of them. I have a group of azaleas rated up to zone 7 that are thriving and blooming every year here with no protection and other plants rated through 6 don't make it.
You never know.
So my advice is to pay attention to the zones and ratings but understand that everything in nature is fluid and surprises are the rule, not the exception.

central, NJ(Zone 6b)

and didn't they just change it? didn't they drop everyone down a zone warmer? I def don't trust's supposed to be 37 tonight!!!!!

Lady's Island, SC(Zone 8b)

Quote from SteveOh :

The USDA site says Zone 8b (min. 15 to 20 (F)) for your zip code, but you are saying 37.4 (F). That's a significant difference.

SeteveOh, I was looking at the average minimum low that both cities had over the last 10 years or so. But even going by the USDA website, you are correct about my zone's average extreme minimum being 15F to 20 F & the other city in question which is a 9a has an average extreme minimum being 20F to 25 F. What I am questioning is why that city is 20-25F when it is north of me and when its average lows for the last 10 years or so has been lower than my city. Just confuses me on how this zoning stuff works. I'm just trying to educate myself :)

As for the "mysterious book" the local nursery claims it has that states my city is Zone 9...if it does exist, I bet that they saw that this city north of us is a Zone 9, so they figure if zone 9 plants can survive there, they'd be able to survive here in our Zone 8b, which has been true for me over the last two years.

Cherry Grove, OH(Zone 6b)

Savvy, yeah, it can be confusing, many things mitigate temperature extremes. Proximity to large bodies of water is a major factor and microclimates are common. The USDA zone maps do take some of that into account, as you can see around the major river valleys and great lakes, but the zone maps are not perfect. Confusion is definitely understandable. The zones aren't exact and many factors can create small areas that don't conform to the regional norm. I suspect this is the case in your area.

So, just like has always been the case, the expertise and experience of local gardeners, about what will and will not grow in your area, is probably more important than the zone listing. Use the zone as a starting point, but don't be afraid to experiment a little, and by all means talk to other gardeners in your area. But, as Cearbhaill said, In nature "surprises are the rule". If you grow beyond your zone listing a little, you have to be prepared for losing a plant every now and again.

Here (SW Ohio, Ohio river valley) I am rated as half of a zone warmer (6b rather than 6a) than the surrounding areas due to the Ohio river and river valley affecting the regional climate. Also, due to a southern exposed and my uninsulated stone foundation wall with a heated basement I have a micro-climate around one bed that allows me to "step up a zone" and grow thriving perennials that normally don't survive here. On the shady and cool north side of the house the basement is finished and insulated and Hostas grow like weeds against that wall.

Good luck and have fun with your gardening!

Prairieville, LA(Zone 9a)

Hi Savvy...It is a puzzlement for sure. USDA zones are for the most part referential..... When the new USDA zone map came out a couple years ago, my area moved to 9a....but areas 5 miles from me remained zone 8b....(and I am in the northern part of our parish (county)) . As SteveOh said, there are factors that vary within an area that can change the growing conditions....I have plants that are rated for zone 8 and lower that have never done well here and plants that are rated zone 9b and higher that thrive for me....bodies of water (including ponds in your garden), windbreaks, elevation, location within the garden and even large trees that afford a bit of frost protection can make a difference. I have plants that are growing on the south side of my yard/house that do not do as well on the north side....same amount of light, etc, but subject to cold winter winds that tend to knock them back, whereas the heat and bulk of my house protects the ones on the southside.

The USDA map is a guideline and is compiled from hundreds of temperature readings taken over a period of time and averaged out to determine what zone boundaries an area for using it as a planting guide, you can assume that certain plants should thrive in your area, but that does not definitively limit what will actually thrive in your garden. That is determined by many smaller factors and in some cases, plants that are rated as say, zone 9 and above, may actually grow well in zone 8. But, should a hard freeze that lasts for an extended period come through, the zone 9 plants may not survive it. I have succulents that say they are hardy to 30 degrees, but have lost them when we had sustained temps of 32 degrees.....the length of time plants are exposed to cold is as important as the temperature range.

I have Phiaus terrestrial orchids that I planted 12 years ago (when I was considered to be in zone 8....) and they have done quite well....but, I moved some of them to the north side of the house when USDA said I was zone 9....and they were fine the first year....but none of them on the North side came back after a rather extended freeze (6 to 8 hours) below 32 degrees.

Opp, AL(Zone 8b)

Go to (or the info source of your choice for historical weather data) and look at the historical records for your location. You'll see that record highs and lows are wildly variable regarding what year they were set. For example, one particular day's record low could have been recorded in 1937 while the next day's record low could have been recorded in 2008. Any info that doesn't take into account the entire SHORT (less than 200 years) of temperature records is ridiculous IMO.

Also, not all ZIP codes have a national weather station. Our little town does not and the info from 7 miles away can be quite different from what happens here, especially with rainfall amounts, but I've also noted temp difference as much as 5 degrees.

Regardless, winter survival depends on so much more than ambient temp. Basing decisions for any particular spot solely on that without consideration for drainage/moisture amounts, leaf or snow cover, the presence or absence of mature trees, proximity of large bodies of water, windblock, direction of exposure, etc... is just far too narrow. Not to mention other microclimate factors like structures and pavement. Or that some plants are much more likely to die if trimmed in the fall vs. waiting until spring.

Then there's provenance to consider. It's easy for many plants to creep north (or south) especially when the plants aren't moved far. Like from the south side of town to the north side. But a plant purchased from another state might be much less likely to handle the radical change. If you're in OH and you buy a marginally hardy shrub with a root ball of red GA clay, that could be much more vulnerable than one propagated from plant material that is already established locally.

The best indicator I know of what can grow in an area IS the area. If you can't find any examples of a particular plant in your neighborhood or town, your odds that you're wasting your $ to try it are pretty high.

But if you did want to, your chances of success would likely greatly increase if you make the effort to investigate the origins of the plant, so you could do whatever is within your power to replicate the optimal conditions. Like if you had a woodland plant, you'd want to cover it with plenty of leaves. If you had a plant from the African desert, you'd want to leave it exposed and NOT put it in a wet spot. You might be able to turn a bucket or something over it if there are extended periods of rain. If your gardens are packed full with mature trees overhead, that's a more likely location for survival than a sparse, new bed in the middle of an exposed yard. These kinds of little cultural factors can have more of an effect than ambient temp in a lot of instances.

I guess I'd just tell people to try it if you don't mind the risk but don't blame the map if it doesn't work. ...or take a loss as a definitive answer. Try to figure what could have been done differently and do that if you want. The examples of plants surviving out of bounds are incredibly numerous. By reading tons of them, you start to see patterns and get ideas of things you can do. And if it's only the rare winter of condition that's going to cause a problem, just go into it knowing "one day I might have to replace this."

Caladiums aren't reliably hardy here but the ones I got 3 and 4 years ago have come back so far. Too early to call it yet this year but I have a strange feeling that the mild yet especially WET winter may have rotted them. Ironic but that's gardening for ya!

After glancing back again at what's been said, some of this is redundant, I just got ta' babblin'...

Opp, AL(Zone 8b)

Moon, excellent point: "the length of time plants are exposed to cold is as important as the temperature range" !!

...and how wet they are at the time can be huge.

Prairieville, LA(Zone 9a)

Same here with the caladiums....mine did just great for years and then I lost most of them to a very wet, but not particularly cold winter.

Opp, AL(Zone 8b)

Yeah, I'm about to give up on checking to see if they're sprouting yet. We just had so much rain for so long recently. I should have bought more sooner - that would probably have increased the chances of survival for the originals. LOL! Definitely got my moneys' worth, no qualms about replanting them and enjoying while they last! ...If everything came back looking great, perfectly spaced, what in the world would I do with myself?

Prairieville, LA(Zone 9a)

LOL Amen! I think I am gonna scarf up the End Of Season caladiums....and raise the planting area for them.

Post a Reply to this Thread

Please or sign up to post.