I would imagine the maintainence required for a mining farm is far more than for a cannibis farm. Sure, growing eats up power with lamps, but its on/off (usually on an auto timer), with replacing a bulb here and there - hardly any 'intensive' upkeep. Plus most of your investments can't 'break' (soil, pots, seeds). In addition, growers don't need to worry as much about ventilation, especially in regards to cooling. Plus the yield of space/energy in growing is much more lucrative than mining.
I'd imagine if you have access to the funding, space, ventilation, and energy to do something illegal, I don't think 'mining bitcoins' is very high on the list.
edit: less->more
Wrong...I have known people who run grows...if you are not there doing work every day things come out like crap. I have not had to even look at my mining rigs in weeks...I know they are working by the heat.
There is massive intensive upkeep in grows...like pruning, insects, mold, fertilizer levels, not to mention pumps, bulbs, fans, charcoal filters, etc.
I would not get into either one expecting no work.