Do not feed either at all until springtime. You can use a specialised rose fertilizer in the spring - there is one called Rose Grow but I guess there will be others. It would be useful if you added your county to your profile - advice for north Scotland sometimes differs from that for eg Cornwall for lots of plants.
Do not leave your dahlias outside in pots over the winter. When they have died down or when the frost has blackened the tips of the leaves you can dig them up and dry them off. Store the tubers in a cool dry place over winter,. You can put them in paper bags or cover them with dry sand or dry compost. Keep them here mice can't get a them. Then when growth begins repot in fresh compost. Sorry this isn't what you asked but its what you should do...

Thank you Steragram, Definately, what I needed to know, and first class advice.

I dry off my Dahlias in the winter in their pots and repot them in new compost in March when I see signs of growth.They winter in the greenhouse.

Good ploy if you have room!