Dear Scott,

Thank you for contacting us about rhododendron care. Most landscape plants, including rhodies, do not need routine  fertilizing if the soil is healthy. 

Recent research indicates most gardeners over-fertilize. Ideally, fertilizers should be used only if specific nutrients are deficient in the soil. (Any surplus is an unnecessary expense that degrades the environment.) The only way to know which nutrients are needed, and in what amount, is through a laboratory soil test. 

The best approach is to enhance the health of your soil by applying up to 1-inch of mature compost annually, followed by about 2 to 3 inches of mulch year-round. (Note that too much compost can also be detrimental to the environment.) Compost is safe to add at any time but is usually applied in late autumn so that it can break down during the winter. 

If you do need a fertilizer, it is best applied soon after flowering in the spring. Organic fertilizers are a bit more costly, but are released slowly and need to be applied less frequently than chemical fertilizers. Avoid the use of lime or alkaline fertilizers, and do not fertilize after mid-July. Follow all directions exactly as indicated on the product label since every product is different.

I hope this information is helpful. Please write again if you have other questions.