The farm of eight-hundred mana roots bore benefits that a handful of mages could not fully harvest by themselves. The roots were fullgrown, they were adult roots, so the amount of mana they bore each was within the millions, so they wouldn't even be able to harvest them even if they were all Arch Mages!
Odds were that a mana bracket would hit them ages before they could harvest the roots. Genta had planted many of them, because she had expected to harvest them once they were just a few months old, where the yield would have been about ten times less, if not lower. She didn't account for a giant jellyfish to come and boost the roots into later adulthood, though, she wasn’t complaining either, because now they didn’t have to be savvy with the roots.