I see no one seems to have an answer.
I am guessing you are in the middle.

I asked my navigator son if there was a scientific answer.
He said "go outside and listen to how people speak. If they have a twang you are South". Well that was a big help. LOL

Traditionally the "South" is any state below the Mason Dixon line.

"Most Americans know the Mason-Dixon Line as the divider between North and South; freedom and slavery. But the line's origins have nothing to do with slavery and actually predate the United States.

The line is, in fact, the result of a bloody land dispute between proprietors of Pennsylvania and Maryland when the country was just a collection of British colonies."
I am pretty sure it has to to with the sun and temps. Many plants listed as full sun, are NOT full sun here in TX and need late afternoon shade.

Here is a link that might be useful: {{gwi:646611}}

If you are in zone 5, then your plants could take some shade...if they say part shade. When they say north, they mean us really cold zoners and most part shade plants can take full sun in these cold zones.

Yes it refers to the temps. In my zone we don't get very many hot days here.....we don't use air conditioners and we hope we don't get to many frosts mid summer.

Sierra

I had to laugh when I read your son's answer, natvtxn, because this part of Missouri is known for "twanging" worse than some areas further south. And I guess I do sound as confused as Missouri seemed to be during the Civil War. But, thanks, sierra, that answer of "we don't use air conditioners and we hope we don't get to many frosts mid summer" brought it home and made it clear. I have been scorching some of my plants. Are you serious about frosts in mid summer? I had already clicked on your name while reading other posts to see who gardened in zone 2 and where it was. When I turn my ac on in the spring it doesn't go off until well into fall.

Hi, I'm in a northern location in Canada. If you want to see who gardens in zone 2...come say hello in the Far north garden forum here on garden web. There are a lot of cold zone gardeners there.

And yes I am very serious about frosts in mid summer. Every year we get frosts end of July or early to mid Aug. We have to watch and cover things or we lose the tender plants like beans. The first killing frost is usually the last day of Aug.....the occassional year its a week or two later. Last year I got an extra three weeks out of the growing season. :-)

Sierra

When they say part-shade in the south for plants that require full sun in the north, they are not just talking about heat. Although that is a factor depending on the plant. Some plants, of course, don't tolerate heat as well as others and should get some afternoon shade in really hot areas because of their "dislike" for the heat. BUT, part of this also has to do with the sun being more powerful in the south. It takes less sunlight in the south for a plant to get it's required "light accumulation" which is sometimes measured in PARs (Photo synthetic Active Radiation)than it does in the north.

So, without getting too scientific, it depends on the plant. For instance, most roses will need full sun in the north and probably in your area as well. Roses, though they love the sun, can often tolerate some shade in the south. This is largely true of most sun lovers. In these instances, in zone 5, it would be safe to give your sun lovers as much sun as possible. But, then there are those plants, like heuchera that can take full sun in the real "north" (My mother's Heucheras in Michigan are in full sun) but in the south the NEED afternoon shade because of the heat and since they get all the "sun engergy" they need by just a little moringing sun, they are happy. So in your case, I would judge that based on your typical summers. She is also in hardiness zone 5 but I imagine that you are probably not the same heat zone.

Does that make any sense at all? Like anything else in the gardening world, nothing is an exact science. The yankee in me is still constantly being surprised at how LITTLE sunlight most plants need here in the deep south. If I were you, and a plant called for part shade, I would err on the side of caution and try not to give it too much sun. You could always move it to a sunnier site if it doesn't thrive.

Theresa

Thank you. That explains why some of those plants, including mums and roses, that I planted against the east side of the house that are just getting morning sun have done so well even though they aren't getting the full days sun the books say they need. I planted them my first year gardening and I have been meaning to move them, but they have been doing well so I have left them. I have learned a lot. And thanks, sierra, I think I will come over and visit your Far North forum. I can't quite imagine gardening like that!

As Theresa just said, the intensity of the sun makes a huge difference and as Kathy mentioned earlier, "full sun" on a plant label does not mean full sun in Texas. So true!

If you are not sure where to plant a new purchase, keep it in its pot and just move the pot around until you find the spot where its happiest. If you have a yard like mine ( full sun, dappled shade and full shade) I often have to move pots around until I find just the right spot.

Marilyn