Encores are azaleas and use the same fertilizer as other azaleas or rhododendrons. It is best, if you fertilize at all, to fertilize when they first bloom in the spring and then fertilize at half the rate on the package. I like HollyTone because it has organic nitrogen, a real plus on plants that benefit from mycorrhizae.

The product you saw was probably Dynamite Select ACR Encore Azalea & Acid Loving Plant Fertilizer. It is a slow release 18-8-8 fertilizer distributed by Black Gold EnviroSafe, Inc. of Oxford, FL. It contains Sulfur and Magnesium. The name is to help it ride on the marketing budget of Encore and allows Encore to get royalties in return.
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Thanks rhody, just what I needed to know!