I have used those and banana peals on roses to give them more pottasium but I have not tried them on hydrangeas. I have not heard of anyone doing it either but it may not hurt. Of course, in my household, it would probably "hurt" (the smell would attract the dogs to the hydrangeas and make them dig where I spread the shells). Brother! LOL!

If that might be a problem for you, you could use composted manure (they love it), organic compost or cottonseed meal. Apply them in Spring and that is it. Optional - coffee grounds, liquid fish, liquid seaweed but stop all ferts by the end of June so the plant goes dormant at its proper time. If you live in FL or places where hydrangeas go dormant in Jan/Feb, you could use a low-nitrogen fertilizer in the Fall too.

A chemical fertilizer that is labeled as slow-release, general purpose, with a NPK Ratio of about 10-10-10 would also work fine in the Spring.

Also any fertilizer that is recommended for acid loving plants will work just fine.

Stop by a Starbucks every now and then. They give away huge bags of used coffee grounds for free. Free acidic fertilizer!!

The 'Starbucks' comes up often enough that Sunset actually picked some up and had them tested. Developed for Sunset by Soil and Plant Laboratory Inc., Bellevue, WA.

"Use of Starbucks coffee grounds in amending mineral soils up to 35 percent by volume coffee grounds will improve soil structure over the short-term and over the long-term. Use of the coffee grounds at the specified incorporation rates (rototilled into a 6- to 8-inch depth) will substantially improve availabilities of phosphorus, potassium, magnesium and copper and will probably negate the need for chemical sources of these plant essential elements.

The availabilities of nitrogen, calcium, zinc, manganese and iron are quite low and in some cases deficient. Thus, the coffee grounds will not supply appreciable amounts of these essential plant elements when used as a mineral soil amendment.

The pH or reaction of the coffee grounds is considered slightly acidic and in a favorable range at 6.2 on the pH scale."