Let radius of the garden is r

Given, perimeter of the semicircular garden = 108

=> Diameter of the garden + semi-circumference of the garden = 108

=> 2r + πr = 108

=> r(2 + π) = 108

=> r(2 + 22/7) = 108

=> r(14 + 22)/7 = 108

=> 36r/7 = 108

=> r/7 = 108/36

=> r/7 = 3

=> r = 3*7

=> r = 21

So, the diameter of the garden = 2r = 2*21 = 42 cm