The Winter Classic took its toll on the field at Citizens Bank Park. As such, the Phillies have used the event as an excuse* to replace the sod entirely, swithing from Kentucky Bluegrass to Riviera Bermudagrass, which, according to head groundskeeper Mike Boekholder will hold up better in both the cold and hot extremes of Philadelphia.
From a grass-nerd website:
The transition zone offers a unique weather pattern with periods of extreme cold, extreme heat, and drought. Bermudagrasses produced in the Southwestern United States typically suffer from severe winter injury and cool-season grasses produced in the Northwestern United States struggle with the summer’s heat, drought, and disease pressure. Riviera was developed for the turf manager who wants to manage grass and not constantly replant his grass.
*As a few in the comments note, it seems the NHL was on the hook to pay for the sod replacement.