Planting high is most commonly suggested for heavy or clayey soils, though some settling of the root ball occurs regardless of soil type. The goal is to make sure that the top of the root ball NOT be buried in backfill.

You don't want to leave feeder roots exposed if you can help it :-) Those that may appear on top of the rootball are less of a concern - really shouldn't be in that location anyway and usually indicate the root flare was buried too deeply by the grower/nursery. But those on the side of the rootball should be covered. Generally this is accomplished by mounding up the backfill soil to the top of the rootball.....a sort of mini-berm or mound. Or you can do the same with whatever mulch you may be using. And there us really no limit to how high the rootball can be.....depending on soil conditions, it could be completely above grade (as long as stability was provided), but with an appropriate amount of soil mounded to cover the exposed sides.

While I don't necessarily agree with everything they recommend with planting trees, the attached link has a very good diagram of what a high planted tree should look like.

Here is a link that might be useful: high planted tree diagram

Thank you for your responses, and also (gardengal) for the very interesting link.

I understand about planting high for heavy/clayey soils, but I don't correlate planting high with not covering the top of the root ball with back fill (after all, you could plant low and still not cover the top of the root ball).

I see many plants in their nursery pots with the very top-most roots exposed, and I just can't imagine why leaving them exposed after planting (on the top only) would be a good thing. Therefore, it makes more sense to lightly cover any exposed roots on the top after planting while still keeping the heavy mulch away from the base.

We have heavy clay soil and citrus trees that have thrived for decades that I'm sure weren't planted high -- so did they survive in spite of that, or is "planting high" a precautionary method for the worst conditions? Perhaps our soil drains better than I assume it does.

Planting 'high' is now a pretty well accepted and recommended practice for ANY soil condition, poor drainage or not. There's a variety of reasons supporting this - settling (collapse of the soil surrounding the roots - common with containerized trees and shrubs), less chance of overwatering, improved drainage, minimizing soil interface issues between native soil/backfill and the rootball soil and to maximize soil oxygen levels (lowering risk of transplant shock). It has been established that trees planted with at least a quarter to a third of the rootball above grade establish faster than those planted at grade level.

Feeder roots should not be present on the top of the rootball - that is not their 'normal' location on a healthy, well-grown young tree. If they are present, it is an indication that tree has been grown too deeply, either in the field (for B&B stock) or in a container. Larger, structural roots are not so much of an issue as they are a) a better indication of a proper planting depth and b) not inclined to dessication like feeder roots would be. Regardless - virtually any respected and current tree planting resource will advise you NOT to cover the top of the rootball with backfill soil. Mulch is quite different but even then, mulch very lightly over the top of the rootball (feather out to a greater depth further away from the tree) and avoid any mulch touching the trunk.