Whyizzit that we feel compelled to plant just exactly where a tree's been removed?
Save yourself work and later woes by shifting the new plant to one side.
My neighbor's daughter wants to plant some of the zebra grass (Miscanthus sinensis 'Zebrinus') I gave her on top of the ground-up stump of an ash tree as soon as the village grinds the stump. Do you think the grass will grow there? If so, how should she prepare the soil? - F.K. -
If it's sunny and the soil is well drained a Miscanthus can grow there. However, it's not going to grow well in the what the grinder will leave -- nearly pure sawdust and wood chips. Without regular additions of fertilizer plants growing in sawdust are often puny and malnourished, at least in the first year or so. They may suffer from root rot, too. Dig out the woody material at least a foot deep and replace it with proper soil.
If she can't dig a foot deep, that's no surprise. Stump grinders are big, noisy, powerful machines so we assume once they're done, the stump is history. Most of the time that's not true. All that machine did or could do was to grind the stump to be level or a few inches below the surface.
Whenever you plant over or very near an old stump, mound the soil so the new plant sits at least a few inches high in anticipation of settling. Settling is unavoidable as the wood of the trunk or main roots that remains in the ground loses mass in decomposing. If it was a very large tree so that the grinder left intact a wide section of the trunk base below ground, shift the ornamental grass so it sits beyond or at least at the edge of this shelf, where drainage will be better than at its center.