From the RHS website:
"Roses in borders:
To encourage abundant blooms, feed twice a year:
in March/April before flowering ;
in mid-summer after the first flush of flowers (especially for repeat-flowering types).
Sprinkle general-purpose or rose fertiliser on the soil around the plant, at a rate of 70g per sq m (2oz per sq yd). Then mulch the soil with well-rotted manure or garden compost, leaving a gap of 10cm (4in) around the base of the stems"
https://www.rhs.org.uk/plants/popular/roses/growing-guide
So, if you're going for the Miracle-Gro brand anyway, perhaps this might be your best option?
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