I'm having trouble finding low nitrogen fertilizer, but I do have tomato tone which is 3-4-6. Would this work well for dahlias?
Translated into fertilizer numbers your 3-4-6 would be the same ratio as 15-20-30 and would be ideal for dahlias in my opinion.
I use Espoma Tomato Tone on all of my flowering plants. Makes life easier to just get a big bag of that at the beginning of the growing season.