Hope you have an easy winter up there and it makes it thru with that new growth. That 18" is a good amount of trunk. Would seem a lot for a Glauca but I suppose it's possible. I found here, like trunking palms, trunking yuccas are hard to keep going, but the roots more likely survive and send up new shoots. I've had better luck with trunking palms (Windmills) than trunking yuccas...I assume mainly due to moisture.