My wife and I have 6 extractors now. We put 3 per root and when the root runs dry we move to another. I haven't been back to a previously dried up root to see if it has recharged but I will now. One thing is for sure once dried out with 3 extractors in them they definitely do not recharge.

We've finally found enough skulls to craft magic gear and all staves for both of us. Now for magic porridge.