Why are Wrigley Field's outfield walls covered in ivy?

Chicago Cubs Wrigley Field Hanging of Ivy on Canvas Wrigley

478 Wrigley Field Wall Stock Photos, High-Res Pictures, and Images

Wrigley Field still 'home' 100 years later

As Many Exceptions As Rules: Ivy League Climber

Wind and Worry Community Foundation of Grant County

478 Wrigley Field Wall Stock Photos, High-Res Pictures, and Images

Dan Kolko takes a closer look at the ivy at Wrigley Field

AEP Energy powers Wrigley Field, buys space amid ivy-covered brick

wrigley field,rooftop bleachers at wrigley,ivy covered outfield

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Ivy covered Outfield Wall,Distance marker for Wrigley Field Wall, Left Field Wall Wrigley, Wrigley Field, 353 feet, Left field foul pole, | Throw