I would use Hollytone in the spring at half the rate on the package. It is mostly organic and normally a safe material to use. Actually rhododendrons and thuja don't need much fertilizer. I never fertilize my thuja and fertilize my rhododendrons about every 5 to 10 years. I need to augment the soil with sulfur and phosphorus due to a tendency for the pH to rise and a shortage of phosphorus in my soil. I use sulfur, super phosphate, MagAmp, and Hollytone.

Here is a link that might be useful: Epsoma's web site