Its tolerance of salt and lime , and its dislike of competition from other plants , restrict it to limestone sea cliffs , like the chalk cliffs on both sides of the English Channel . Brassica oleracea , or wild cabbage , is a species of Brassica native to coastal southern and western Europe , where its tolerance of salt and lime and its intolerance of competition from other plants typically restrict its natural occurrence to limestone sea cliffs , like the chalk cliffs on both sides of the English Channel .