Hey man, that sucks to hear.. it really depends on how well you know your grandparents and their views on marijuana. How old are they? Did you use the word marijuana to him? Has your grandmother see them? In this situation if their pretty cool and lenient about alot of things id tell them the truth (that is if they already know you smoke it, which im guessing they don't.) Or if you think that they would snap on you, save yourself some trouble and scrap it, i know it might be a hard thing to do but you could just say to them "yea i was trying this stupid experiment that didnt work." Did you have alot of grow equipment in there when he saw it? I seriously think the stupidest/lowest thing to do would be to keep growing them and lying about what they are lol, now that he/they saw them they're gonna want to know whatsup. So yea if your pretty sure that they wont banish you, id just break the news to them, see what happens.. but beware, once they know they might start considering you just another "lazy stoner" and trust me its no fun when your familys always thinking your high. my 2 cents.