Hi NickRose: My 50+ roses here in Chicagoland also love banana peels: I chopped up with a scissors and soak the peels overnight in used kitchen water .... that way they decompose faster. Sometimes I put a tiny bit of molasses in the bucket for its chelating action in drawing out NPK of banana peels 0-3-42.

My roses in zone 5a are very small due to own-roots and winterkill yearly, so I use 1/2 cup per month ... no problems whatsoever. Since you are in CA, warmer climate, IF it is a big root & big rose, then 1 cup per month is OK. NPK of coffee grounds is 2.1 - 0.3 - 0.3. Nitrogen of 2.1 is about the same as alfafa meal.

I put 1 cup of espresso grounds on my tiny rhododendron (1.5 x 1) and the leaves got pinkish stripe .. that was too much, and espresso grounds is higher in caffeine than regular coffee.

Roses, esp. own-roots prefer regular smaller doses, so if it's 1 cup in one shot, that might be too much. But if it's 1 cup broken down in 2 applications per month, then it's safer.