These Yucca's have been planted all over the city I live in along parkstrips and other common areas. I can't figure out for the life of me what kind they are or where to buy some. The only Yucca's offered at the Nurserys around here are Yucca Filamentosa. We are at about 5k feet and my best guess is that we are in a zone 5. So I'm thinking these must be very cold hardy yuccas.

I was wondering if it could be a Yucca Glauca but that's a complete shot in the dark. They have long, narrow, stiff leaves and get pretty big.