The area of a square garden is 169 square feet. Find the length of one side of the garden, in feet.

Let x be the length of one side of the square garden.

The area of the square garden is x^2 = 169.
Taking the square root of both sides gives x = ± √169.
The positive value of the square root is 13, so the length of one side of the garden is 13 feet. Answer: \boxed{13}.