Answer:
 100%
Step-by-step explanation:
When the dimensions double, the perimeter doubles.
The percent change can be found from ...
 percent change = ((new value)/(old value) -1) × 100%
Filling in your values, you have ...
 percent change = ((2×(old perimeter))/(old perimeter) -1) × 100%
 = (2 - 1) × 100%
 = 100%
The perimeter increases by 100% from the first garden to the second.