I'm experimenting with P. aquatica in pots.
It doesn't need much care to survive, but I can't say it thrives without care. It's been very dry here (no rain for 2 months or so) and that coupled with the heat is not helping. Perhaps summer is not the best time for it.
Harvesting is as easy as cutting it. Mowing could work, but you may want to make sure you're only mowing the Phalaris and no other grass or weeds, although if you're going to extract it, it probably doesn't matter.