If we know the area, we can first find the side length of this garden. The area of a square is the side length squared. So, we can write:
x^2 = 128
Square root both sides, and you get
x = sqrt (128)
The diagonal of the square is the side length times sqrt (2) because the diagonal plus two side lengths make a 45-45-90 triangle. So:
sqrt (128) * sqrt (2) = sqrt (256) = 16.
The length of the diagonal is 16 meters.