From the record
“The Celadon Slope Garden, also known as Hays Station, in Payette National Forest in Idaho County, Idaho in the vicinity of Warren, Idaho, was listed on the National Register of Historic Places in 1990.”
Wikipedia article Celadon Slope Garden, CC BY-SA 4.0.
One of 42 National Register listings published here for Idaho County · listed in 1990 · category: Site.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service