Well, in Generation 5 and 6, a female Hidden Ability Meganium must breed with a male Pokemon of the same egg group in order to pass it down to the offspring.
However, in Generation 6 only, a male Meganium with it's Hidden Ability may pass it down to it's offspring, if it's bred with a Ditto.
So, you're Meganium needs to have the Hidden Ability in order to pass it down.
And even then, it's not a 100% chance it will pass down. The real chance is currently unknown, since nobody has been able to access the hidden data.
Hope I helped. :)