Depends on what you've done up to this point... if you used slow-release fertilizer when you planted out, you should be fine.

Most dahlia growers fertilize through the month of August, with strong differences in opinion in what is the accepted amounts of nitrogen and other fertilizer components.

As long as you don't blast the plant with tons of fertilizer, it should do fine in any scenario. It is really easy to go overboard and cause problems with too much, though.

Thanks!