Answer: The answer to your question will depend on several factors, so the best way to know for sure is to run some basic tests on the mushroom mulch and find out if it is terribly alkaline or fairly neutral. Many gardeners prefer to use a mildly acid mulch such as pine bark on acid-loving plants such as azaleas, and this is fine, but the real secret to healthy azaleas is the overall soil and mulch pH rather than the mulch material itself. (Azaleas are shallow fibrous rooted plants and really need a few inches of organic mulch -- you could also use rotted leaves or straw for that matter, but I would not use a mulch that tested above neutral.)
Annuals, on the other hand, are heavier feeders than azaleas and may suffer if a bark type of mulch "ties up" available soil nitrogen. (Annuals also do best with a layer of organic mulch -- it feeds the soil as it decomposes and eventually you will see a marked improvement in the soil where it has been mulched year after year.) They would also be more sensitive to a fresher, less decomposed type of mulch. It may also be that the mushroom soil contains additional nutrients beyond what would be found in plain bark; if this is the case, you might consider adding some balanced fertilizer to the planting bed when you use bark mulch. This again could be determined by running some basic soil tests. Your County Extension (350-2540) should be able to help you with the tests and interpreting the results, or may even be familiar with the mushroom compost sold in your area.
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