Phormiums

Latin Name:
Phormium tenax and Phormium cookianum

Divide plants in the spring. You can start new plants indoors by potting larger rhizome pieces and allowing them to grow a bit, before locating outdoors. Phormium can also be started from seed, although it needs a warm temperature of over 60 degrees F., to germinate.

Problems & Pests: Mealy bugs can infest plants and are hard to eradicate from inside the long leaves. Many times it’s easiest to just dispose of the infested plant.

I have split Phormiums a few times. Most recently last month I took two out of two very large pots by the front door and they are now very happy in 7" and 9" pots waiting to either be planted or sold.
New Zealand flax was a fight, as it was huge, but this two I have dug up split and re planted around the garden.
Ideal world, when it's not so warm and the plant is resting.
But as long as you give it plenty of water to settle in you shouldnt have any major problems. A couple of sharp spades to slice through the roots, or a sharp knife. You may loose a few leaves as they tend to grow at angles to eeach other. Pul these off or cut them down.
Good luck.