I found out the hard way today, and it's a fine line, I can tell you that. Only two spaces away from one another, one lived and one died. I suggest experiment before you place your campsite.
Do you have any current cedars in your town? Look around for them and see what point the lowest one is, where it's at is how far you can plant them in your town.
They may grow under the river too, as long as it's still on the top half of the map. So if the river curve is bendy, there may still be a 4 or 5 tile space that counts as "north"
I agree with everyone's suggestions of experimenting before you place the campsite.
Definitely experiment, but a good starting point is to find the one that's growing farthest down and go from there. That's what I've done in my towns. Just measure across the map if the southernmost pine tree is far from where you plan to plant the others.
If you look at the top screen where your map is, there's a little marker that shows the midpoint. As long as you are above that marker, your cedar should grow.
There are still random dead spots that could impact though, so keep that in mind.