roses need potassium so can you extract this from a banana?

Just put them round the plants as a mulch - they decompose quite quickly.

or chuck them in your compost bin and then the goodness will be shared throughout the compost.

Coincidence this.
I had some bananas today and was about to bin the skins and then thought.."Roses" so that's where they're going, round my roses and I've also got a couple of skins for my daughter for her compost bin, lol.