Knowing how a vegetable grows tells you the proper way to boil it. The rule is a simple one. If the vegetable grows underground then you put it in cold water and bring it to a boil ( i.e. potatoes, carrots, parsnips) and if a vegetable grows above ground you add it to boiling water (i. e. string beans, corn).
Added tip: Do not put a lid on a pot that is cooking any green vegetable, it will turn grey.