K i guess I dont understand your zone thingy. I googled and zones 9 and 10 seem to be hotter dryer climates.
Dianella tasmannica grows naturally here in the mountains, under a tall forest canopy. Rainfall is +1000 mm (40 inch?) ( thats quite high for Oz) and sometimes snow cover, frost is not that much of a problem because of the canopy cover. Temps for its natural range are around minus 5 (23F) to ?30 (80F) degrees c. The plant is usually in or near a drainage line.
There are the zones with correlating temp ranges. It seems to speak to minimum temps and not high temps. That plant of yours might do well in the NW US or the coastal southern areas. I'm guessing it's a rare addition to our horticultural banks and people aren't sure where to cast it.
