Rhododendrons and azaleas have shallow roots so don't cultivate the soil around them. The standard procedure is to mulch them to protect the shallow roots from heat, cold, drought, sun, and prevent weeds. Worm casings would be better as a soil amendment when planting. Peat humus would be better than worm casings since it is acidic. If you want to pull back the mulch and use this as a top dressing and then put the mulch back, you could do that. However, peat humus would still be a better top dressing.

I bought a 10# bag a couple of years ago at our Farmer's Market (when I say 'our', I mean an hour from here) - and I remember it as being very expensive.

I used a little as top dressing here and there and some when amending soil for new plantings...the recommended amount for azaleas/rhododendrons as a top dressing was 1/2" if I remember right.

It was a good product, but I don't know if I could say for certain if it had the disease resisting properties the worm farmer promised. I did put some around roses, but you have to understand this cool damp climate isn't ideal for roses no matter what they are treated with :)

I have one bed where I'm not happy with the soil this summer and I plan to lift plants, amend and replant this Fall....I may buy some more of the castings as one of the amendments.