We live in South Central Texas - 100 miles north of Houston. I have a twleve foot Norfolk Pine that lives in a hugh pot outside. What temperature do I need to be concerned about moving the plant to some type of shelter?
Lots of older Norfolk Island Pines in ground around here. They suffer when it dips below freezing, but we've had frosts plenty of times with no noticeable damage. Last winter killed the ends of some branches closer to the ground, maybe up to ten feet, but over that everything stayed green and thriving. With decent pruning, they'd look okay. Not my favorite tree because it requires attention to keep the rigid geometry. There's one near me that is twin trunked and about 1/3 dead branches on both. Kinda makes the trailer park it's in look past its prime.
Yep, they'll take a degree or two (even 3) below zero when they're in the ground. But in a pot (even a big one), it is more exposed to root damage by cold. Sorry, but Houston is too cold in winter to keep it outdoors permanently. Even in Brownsville, it wouldn't be easy.