After much experience with this Yucca we are not sure why it is associated with the Yucca Rigida. Dark blue leaves which are rigid, but our mature varieties have never grown much of a trunk. Possibly just a Yucca Recurvifolia x Yucca Filamentosa. Nonetheless a good looking Yucca hardy around 0 degrees F. Young specimens have single head but after a couple years, and blooming they grow multiple heads.