Answer: Poor drainage is the probable cause for decline in your hydrangeas. Any organic material, including mushroom compost, will loosen your clay soil and help it drain well. You can use aged manure, compost, shredded leaves or peat moss to amend the soil prior to planting hydrangeas. The easiest way is to spread a 4-5" layer of organic matter over the soil surface and dig it in to a depth of 8-10". Plant your hydrangeas and then mulch over the surface of the soil with an additional 2-3" of organic matter. This mulch will moderate soil temperatures, slow water evaporation and suppress weeds. As it breaks down it will leach nutrients into the soil. Over the years, as you add more and more organic matter as a mulch, your clay soil will eventually improve and most plants will be happy with the results.
Best wishes with your landscape!
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