I guess it would depend on your existing soil conditions and whether or not copper is needed and in what amounts.

I did ruin some beautiful blue hydrangeas with pennies.
I heard that I could change the color from blue to purple by putting a few pennies in the ground. They only managed to turn a sickly splotched lavender color.

Never could find all of the pennies and the hydrangeas were never again any one brilliant color - just a half-way attempt and two different colors.

I didn't try it yet..I wanted to ask on this forum first..
I heard that Hydrangeas are blue or pink based on whether the soil is acidic or alkaline..

To get Pink aluminum MUST NOT be present in the soil...

To get blue aluminum sulphate can be added to the soil...

There is a web site www.hydrangeashydrangeas.com it has a section titled "Changing The Color Of Hydrangeas" This site is VERY helpful.

Pennies have been less than 2.5 percent copper for over 20 years. FYI, sort of interesting.

Pennies have aluminum and other "filler" metals in them now..Yep it was about 20 years or more when I observed my Mother doing this...

SOOOO pretty much, pennies are not so good for plants anymore unless they are old..

I was just wondering about it...

Pennies (old ones, from 1982 and later) have copper in them which deter slugs, snails, and some cutworms. It actually sends a slight electric shock to them so they stay away from anything copper. Old pennies, copper tape or sheets of copper, copper wire or pipe, etc.