Unless, someone knows something specific in contrary, in my experience harvests can be "effected" upon, but not "controlled".
It appears to me that, the higher the average loyalty of your provinces, the better the harvests.
This is what adjusting your tax rate is all about. My habit is, in the autmun, to set the rate at either Punitive or High; and after the harvest, in the winter, set the tax rate to very low.
Also, I think I've noticed that relative stability ("peace") and/or victories have an effect on your harvest as well.
I don't believe the above effects harvests in a great way, but in a "key" way; and strongly feel it effects situations such as this:
>>Sometimes I'll get like -25%, -25%, -25%, -50%, -25%, -50%, average, then really good.
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