A root plate is doing more work than it gets credit for
While a tree is alive, its root system is quietly holding a section of slope together - not dramatically, just by threading through soil that would otherwise be free to move whenever water pushes on it. Chestnut oak and black cherry, common on our upper ridge lots, tend to anchor with a spread that reaches well past the visible trunk once the tree's had a few decades to establish itself against the pull of gravity on a grade.
Grind that root plate down to six or eight inches below grade and that anchoring effect doesn't fade gradually the way it would if the tree simply died standing and slowly decayed in place - it stops the day the grinding finishes. The soil that root system was holding together is suddenly on its own, and how it holds up depends entirely on what happens in the weeks right after, not months down the road.
The exposure window is the real risk period
The stretch of time between a stump coming out and new ground cover establishing over that spot is when a hillside is most vulnerable, since bare, disturbed soil on a grade has nothing left holding it against a hard rain. This matters more on a steep upper-slope lot than it would on a gentle rise, and it matters more right after grinding than it will six months later once grass or ground cover has had a chance to root in and start doing some of the same work the old root system used to.
Timing a stump job for a season when you can get seed or sod down quickly afterward, rather than heading into a wet stretch of the calendar with bare dirt exposed, makes a real difference in how that window plays out. We'll flag it if a scheduled removal is landing right before a stretch of forecasted heavy rain, since a short delay on seeding can sometimes matter more than the removal date itself.
Where the grindings themselves fit into the picture
A ground stump produces two to three times the chip volume most homeowners expect, and on a slope that material isn't automatically waste - spread thin and even over the exposed root zone, it can slow water crossing bare ground while grass reestablishes, acting almost like a temporary mulch blanket over the most vulnerable patch of soil on the property. Piled loose in a mound instead, especially above a low point or a retaining wall, it becomes exactly what a hard rain moves first, turning a helpful byproduct into the next problem downhill.
The decision to spread versus haul isn't cosmetic on a hillside lot, it's part of managing that exposure window directly. We look at where the stump sits, which way the ground around it drains, and how soon new cover is going in before deciding which way to go, rather than defaulting to one option regardless of the site.
What actually helps once the stump is gone
Getting some kind of ground cover established quickly - grass seed with an erosion blanket on a steep spot, or a fast-rooting groundcover where turf won't easily take hold - closes the exposure window faster than leaving bare soil to fill in on its own timeline. On a slope directly above a retaining wall, we'll sometimes recommend a slightly more conservative grinding depth specifically to leave a bit more root mass in place temporarily, trading a small amount of future replanting clearance for a season of extra soil stability while new cover takes hold.
None of this is complicated once it's laid out, but it's easy to skip if nobody mentions it - a homeowner focused on getting an eyesore stump gone doesn't always think about what the bare patch underneath it needs next. We'd rather bring it up before the grinder ever starts than after a storm has already made the point for us.
The short version
Once a hillside stump is ground out, the root system that was quietly holding that patch of slope together is gone with it. The real risk sits in the weeks between grinding and new ground cover establishing - manage that window with seed timing and a deliberate choice about where the grindings themselves end up.