I'm sorry if this has been asked and answered before... perhaps some newbies could benefit from a new post on this subject alone.

Basically, it's the point where the plant transitions from trunk/stem tissue to root tissue.

You don't want to bury the trunk tissue because it can soften the wood/bark and promote rot.
That's also why we say to not allow mulch to sit directly against the trunk.

And you don't want to over-expose the roots because they can dry out, overheat, or chill.

Josh

A couple years ago I planted 3 Meyer lemons in my front yard, and I too had read to plant the collar just above the soil level. From what I'd read, one does this so as to not develop rot at the trunk/root interface. Additionally, especially for those of us on clay soil, it is recommended to plant on a mound. Below are photos of one of my trees - immediately following planting, at 1 yr, and at present (just shy of 2 yrs.).

Meyer lemon 3: Planted just above the horizontal soil level (you can see a bit of mounding that rises like a volcano from the surrounding soil level).

Meyer lemon 3: 1 yr. after planting. You'll notice that the mounded soil around the trunk has fallen away exposing some thicker roots - not to worry, this is fine. I've also moved the mulching pebbles, expanding outwards. When I did this, I had to add a small amount of topsoil, but you'll see that it was at a level that NEVER covered those thicker base roots.

Meyer lemon 3: 1 yr, 10 months after planting. You can definitely see some of the thicker roots exposed - these will continue to thicken over time. I've groomed the tree a bit by pruning the lower branches; the tree seems to want to grow more wide than tall at the moment.

Tim

Thank you both for your replies.

The reason I think they are two different things is this statement I found online about planting citrus:

Plant the tree so the root collar is above the soil line and the top of the root crown is barely below the soil.

You can't plant one thing both above and below the soil... can you? :-)

To me it actually does make sense: I suspect they mean the root collar to be that portion of the trunk just above the beginning of the root layer (what they're calling the root crown). This (the root collar) is indeed above the soil level in my original planting. The root crown is that mounded part that is just below the soil and the very tippy top of the crown later becomes exposed in my 2nd and last photos. Hope this helps, and best of luck on your planting!

Tim

nice site on how to properly plant trees w/ pics
http://www.fotitree.com/treeservices/rootcollar.htm

good video about potted trees and too much soil covering the root crown.
http://www.youtube.com/watch?v=YKXhLvfVIB8

In this image of my Moro blood orange, you can clearly see the transition from the trunk
to the root-flare. You can see the point where the green on the trunk abruptly ends,
and the root-flare immediately below. In bonsai, a great deal of attention is paid to
developing and displaying the root-flare, which is called the 'nebari.' When I first
potted my tree, the top of the roots was covered slightly, but now that the roots
have toughened up, I've pulled back the mix to show off the root-flare.