There is only one way to propagate an existing species or cultivar: vegetative reproduction AKA cloning or dividing. And what about tissue culture ? Well, that's just a more sophisticated way of dividing, but, if you take a spade and cut your H. June in half, basically it's the same principle.

And what about sowing as a way of propagating a cultivar?  Doesn't work. 

You want to propagate, lets say, Hosta 'June' by sowing.  You'll get a kinds of different seedlings, green, yellow, blue, but not one of them will be anything like H. 'June'.  Maybe there will be on or two special seedlings, that have all the good qualities to become new cultivars.

No, if you want to multiply H. 'June', dividing or tissue culture is the only thing that will work.

There is, however, one exception: H. ventricosa, the only species that comes true from seed.