To say that money doesn't grow on trees means that it is not plentiful or easily obtained.

eg: "Be careful how you spend your money David.  It doesn't grow on trees you know!"  

    Note:

    Money doesn't grow on trees的意思是说钱来之不易或者钱很紧张，不够花了。

    例：“大卫，花钱小心点。钱不是树上长出来的。”