I've been growing ferru in 1.106 (updating to a newer version soon, so perhaps this is outdated information) and I can tell you that I've had plants die. Granted, I'm growing mostly in terrible conditions (desert biome, underground), but it seems that any plant with growth or gain 9 or over will stop growing in this setting and may disappear (die) while growing unless they are watered and fertilized at times. I assume that the higher the growth/gain stats are, the more nutrients they suck up from the ground, sort of like an overclocked machine requiring more and more power. But from what I can tell, if you keep your plants stats all at 8 or below, you can continually harvest without hydrating/fertilizing. At least that's the limit for me in my current surroundings.
I started breeding plants for higher resistance, assuming it would lower the nutritional requirements for high stat plants so they won't disappear on me, but from what I can tell resistance doesn't help in that way. Although I really, really wish it would. It would actually make resistance a desirable, if not needed statistic. For the moment, I'm just trying to breed plants at lvl 6-8 for maximum growth/gain without the chance for death. I'm planting somewhat of a large community field of ferru, so everyone hydrating/fertilizing before every harvest is just impractical.
If anyone actually has insight on how to grow higher stat crops without fertilizing/hydrating, I know I'd like to hear. I haven't entirely ruled out a high resistance trait as a possible way to do this (I guess I need more testing), but for now I'm just going for quantity of plants over quality. My people need their iron.