Growing money on tree definitely a very trivial Proverb but its was a 17th century practice in Scotland.
As you come across the Scottish highlands you could find tree or barks of timber trees embed with coins of silver and copper. It almost looked like a treasure or the tress have grown these coins.
But question comes why these coins were hammered into the trees?
This story dates back to 1700 in Britain, there was a tree present in Scotland which had a florin stuck to it. According to legend it was believed that any sick individual if places a coin inside the bark would get healed. While if anyone trys to pull the implanted coins would become sick.
Another reason people placed coins to the trunks to bring good luck for their travel and please deities, which they believe used to reside inside the trees.