It wouldn't be hard to tax marijuana at all, all you would have to do is put the tax the large scale grow operations. You do that by setting a number of plants that would classify a large scale grow operation. For example, only tax people with more than 10 plants in a non-residential building. Allow residential buildings 6 plants, and fine the violators.

A decent large scale corporation would build massive warehouses for their grow operations, they wouldn't be able to hide from the tax, and the large quantity they would produce would far outweigh the costs of the tax.

And another good thing is they would be in constant competition with personal grows, so the professionally made cannabis would have to be far better quality than most personal grows.

I'm still curious what downsides TomStoner is talking about, I can't imagine any.