Ogon Golden Variegated Sweet Flag Acorus gramineus

Acorus is a neutral grass. Evergreen or neutral grasses are usually plants that look like grasses but aren't actually classified as grasses, they are generally called grass-like plants.

Divide evergreen or neutral grasses and grass-like plants in spring only.

Evergreen grasses don't ever go dormant. Dividing plants wounds them to some degree. For evergreen grasses this wounding will really affect their ability to live through the winter.