Hi Diane, Knockouts are roses, so this is the right place. The canes of most roses will die back, to some extent, in a typical zone 5 winter. That is totally normal, and pruning off the dead canes is exactly the right thing to do. It would be fine to give the roses some fertilizer now, as you mentioned they are actively growing.

Thank you KS. The winter was pretty harsh this year which is probably why so many canes died back this year in particular. I'll have to research how to protect them over the winter time.

Fertilize during the season, not at the end of the season. What are you planning to use as a fertilizer?

For Winter protection, I mound compost over the bud union when I put the rose to bed for the Winter. I remove it in Mid-March providing the weather is warming up. Just scratch it into the soil.

I feed monthly from Mid-May through Mid-September. I am in Zone 6 on Cape Cod. The ocean temperature delays Spring and lengthens the Summer.

You can fertilize after the pruned down KO starts growing back a decent amount of leaves... No need to fertilize if no or little leaves are on the bush...