The problem
If a plant looks a bit yellow or drooping, someone might suggest putting a penny in the pot. The idea is that the copper will leach into the soil, liven up the plant and maybe even ward off fungi. It is one of those tips that refuses to die, passed on like family folklore.
The hack
The promise is simple: pop a coin in the compost and let chemistry do the work. Supposedly, the copper acts as a mini-fertiliser and a mild fungicide.
The method
Take some pennies and press them into the top layer of soil and water as normal. That is it. No measuring, no idea how much metal is dissolving, and no control over where it ends up.
The test
I buried pennies in two pots and left their counterparts on a standard feed. Months later, the penny pots looked exactly the same, except for a greenish coin. Any change in the plants came from light, water and photosynthesis, not the loose change.
The verdict
A penny in the pot won’t hurt, but it won’t feed your plant either. Modern pennies are barely copper at all, and even old ones corrode far too slowly to act as a useful feed. Save the coins for the piggy bank and give your plant a decent, balanced fertiliser instead.