Garden

[SeaMaiden]

I live in the Seattle Area.

[Sweetbay magnolia]

Only if necessary.  I generally recommend after flowering, if  needed at all (leaf mold, compost, fish emulsion, etc would be my preference - soil condition is the utmost in importance).  Lots depends on lots of other factors!

I would guess you have lovely rhodos, given your location.

[Carmie]

They should be fertilized right as the buds start to swell to bloom.  They can also be fertilized again after the blooms have died. If you need to trim them back, do so right after the blooms have wilted and the petals drop or you will not have flowers next year.

I keep mine trimmed back or they get out of hand easily and get a big as a tree.  

[JustJazzmom]

I agree with @Carmie & @Sweetbay magnolia. As long as the ground isn’t frozen go ahead. Use a slow release fertilizer.