I put some Siberian irises in last year. Should I fertilize them and with what?
Good idea to fertilize after spring growth has begun. I like to use low nitrogen (ie. 6-10-8) for ones in the ground. Potted bulbs and iris get Miracle Grow which is higher nitrogen.
I sprinkle Plant Tone around them in early spring. For a vigorous cultivar like Caesar's Brother, that seems to be enough.
This year, I may try a second feeding with something stronger for some of the less vigorous ones.
I have never heard SI described as heavy feeders. However, some of the varieties I have just seem like they need a little kick. Probably just after they bloom.