To find out how much fencing material Bonnie needs for her square garden, you need to calculate the perimeter of the square.
The formula for the perimeter \( P \) of a square is:
\[ P = 4 \times \text{side length} \]
Given that each side of the garden measures 13 feet, the calculation would be:
\[ P = 4 \times 13 = 52 \text{ feet} \]
Therefore, Bonnie needs 52 feet of fencing material.