[regularfry]

I think it's more likely that there's a chemical effect from the copper, somehow. Maybe it nukes algae that would otherwise be competing for nutrients?

[k__]

They also said that artificial lightning strikes improved mushroom growth.

So, it doesn't seem to be directly related to plants at all.

[regularfry]

That could easily be a different effect. Lightning strikes are going to give off all sorts of interesting chemicals that you wouldn't get just with electric fields.

[_Microft]

The comment on mushrooms and lightning strikes stood out to me as well. My first idea was that lightning arcs might make nitrogen more available by generating nitric oxides?

[trilbyglens]

It might also simply sterilize an area, allowing fast spreading fungi to temporarily dominate in the absence of normal biological competitors.

[ChoGGi]

I've heard of using copper coins in the water of cut flowers to spruce them up.