Well, this is a long shot if you get caught by the fuzz, but...

Tell them you didnt even know it was weed. Tell them it just appeared a few months ago, and you thought it looked and smelt nice so you kept it. Even imply that a bird may have eaten a seed and shat it out (it happens!), or that a neighbour threw the seed out of a bag of weed.

Deny, deny, deny. Can I ask, did you get your parents permission to grow? If you did, then thats cool If you didnt, then thats definately not cool