Question on Phlox as ground cover

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The math looks good but that is a whole lot of creeping phlox. Off hand, I think that unless you have a full acre plot with a wide vista, that's a bit much. Here in northern Illinois creeping phlox is used as a spring time accent plant. Only commercial locations or botanic gardens have 625 square feet of phlox in one plot. I agree that it looks beautiful but it sounds a bit excessive. Just a thought. It's your garden.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.