Answer:
15 feet is the length of one side of the fenced-in garden.
Step-by-step explanation:
Diagonals of rhombus bisect each at 90 degree angle.
In rhombus ABCD
OA = OC =
OB = OD = 
Since, the diagonal intersect at 90 degree angle.
Applying Pythagoras theorem in ΔAOB

AB = 15 feet = Length of side of the rhombus fenced garden
15 feet is the length of one side of the fenced-in garden.