I'm looking to kill the grass on a hill that I'm tired of cutting. A
local garden center here suggested phlox as a good ground cover for
the area (southeast Wisconsin).

However, the hill is 25 feet by 25 feet. The planting suggestions I
found on a garden website say that the plant grows between 12 to 18
inches around, so plant them 18 inches apart.

With my area, I come up with 17 rows and 17 colums roughly. That
would equate to 289 plants? Seems like a lot for this area, though
the math seems right.

Am I missing something about figuring area and planting requirements?

Thanks

J