Can you move it off to the side one way or another? Nandinas don't have tap roots so that's not an issue but the remains of the stump will decompose over time and planting directly above the stump will run the risk of the soil settling as the stump decomposes and the shrub sinking down too deeply. Planting too deeply - intentionally or not - is a primary reason for plant failures. This is why it is not recommended to dig a planting hole any deeper than the container or depth of the rootball, even if you fill back to the proper level with soil.

Nandina 'Filamentosa' is one of my favorite cultivars of nandina and not an easy one to find. Congrats on your new purchase!

Armillaria, which appears to be rather common here in this forest region may grow from an infested stump onto garden plantings and kill them. British sources have long recommended removing stumps from woodland gardens before planting. So, you may be gambling with planting near buried wood. It depends on if the stump is infested or not and if the fungus moves onto your nandina at some point.

Thanks for your comments.

The underground tree stump is/was perfectly healthy; there's a sad story why the tree (a 12' high Japanese maple) was taken out in the first place (my husband misunderstood and gave wrong instructions to the tree pruners who came early before I got home). It happened so recently that I don't think that the stump will decompose in my lifetime!

This particular nandina is one I have been looking at longingly every time I've seen it at the nursery where I got it. So it's kind of a consolation prize I got for myself - and since I love looking at it, I really want to put it where I can see if often - which happens to be in this particular spot. Any movement to the side would be unbalanced because of the position of the other shrubbery bordering the open space in this border.

Now that you've told me there is no tap root, I will probably give it a try. It's in such a deep nursery pot now that I was afraid it had a huge, long tap root and I knew that wouldn't work, but maybe most of what's in the bottom of the pot is just soil. On the other hand, if, once I get it out of the pot, I see it really won't fit, then I'll have to find another place for it.

You should wash the roots and pull them open anyway, much container stock is potbound and needs to be corrected - and you don't want to plant in pockets of potting soil surrounded by natural soil or other differently textured material.

A stump from a 12' high Japanese maple shouldn't be all that hard to remove with a shovel and a small chainsaw. I would think 20 minutes max. The relatively small amount of work to remove it seems well worth it given the disadvantages listed above as well as some others.

It won't matter if the stump was clean at planting if armillaria gets into it later, then spreads to the nandina.

It also seems you can't now know if the stump is being attacked by a fungus without digging it up and looking at the whole thing, pulling the bark off.

I would just LOVE to be able to get that stump out! There was another tree (a large, probably 40+ year old rhododendron) nearby that I also took out when all these changes happened, and it has taken me three solid days of 3-4 hours each day to get that particular stump out. I don't have a chain saw, so I had to use a combination of hand saws, a gravity-drop splitting maul, and other non-powered garden tools. I tried using the maul over the buried stump and it was like trying to crack a buried cement sidewalk about a foot in diameter. (There is also a sprinkler system and rock wall nearby so I don't have a lot of room to maneuver tools.) The guy who did the stump grinding asked if he could quit once he got down 8 inches and nothing showed above ground, and I said Yes because I wasn't planning on planting anything there at the time. So I'm pretty discouraged about getting the stump out now, even though that's what I'd really prefer. I guess I'll give it another shot trying to chip away with the gravity maul at the extremely hard, flat surface that's underground, but since it was so recently cut, it doesn't seem to be very much affected by that heavy wedge dropping on it again and again.

To remove a larger stump, you really have to dig it out. If it's solid, any hits to try to break it up are not going to do much. If you dig around it and then cut the roots maybe a foot or a little more from the stump, it should come up easily. If it were me and I didn't have a saw, I'd either think about getting one, borrowing one, borrowing a friend or relative that has one, or renting one. I'm not big on the idea of "beating" a stump out of the ground.

You won't necessarily have a problem with leaving a stump. If you have to move Heaven and Earth to get one out it might be as well to leave it and take the risk.