Hmm ... 0-0-50 sounds like Potassium Chloride, which as the name indicates contains chlorine -- a substance the plant only likes in very tiny quantities. Also this compound is 50% potassium by weight, so 1 pound of it contains 1/2 pound of potassium -- an entire year's dosage for a 1-meter or taller ornamental banana plant. However, the plant also needs potassium going into the winter so you did great
on that account!
The bottom line is that I would not feed a banana plant more than 1/2 cup of potassium chloride per year. Next fall, consider a winterizing formula like 10-20-30.
Bananas in Vancouver!