I personally wouldn't buy any plant food or do anything to my plumerias at this time. With the help of others and researching I learned that our plumerias are close to going dormant for the season.

Not sure what part of Southern California you are from but mine are all starting to loose their leaves slowly as the temps go down in the night.

A friend told me not long ago that "plumerias almost thrive better when you neglect them"

Bryan