Pogostemon stellatus "octopus." I planted 20 stems, each about 4 inches long, on 12/15/19. It was slow at first, but then took off. By week four, the tops were floating across the tank and, though it looked kinda cool, I trimmed them back. The stems had become really thick in places, too... like between an eighth-inch and a quarter-inch. Because there was so much of the stuff already growing, I just tossed the trimmings instead of replanting. And of course I'm getting branching at the trim points, and will probably need to trim again next week. It's crazy.
My tank is a 30-long, with 3-year old Amazonia under gravel, PAR value of ~50 at the substrate, and I dose 3mL of NilocG Liquid Macros and Micros every other day. I re-scaped this aquarium last month, and wanted the whole back of the tank to be one wall of a single plant. I considered Italian Val, but chose this. The other stem plants I've tried in the past -- Mayaca fluviatilis and Proserpinaca palustris -- failed fairly quickly.