The hardiness of S. guaranitica clones varies a great deal. Some are pretty tender - others regularly survive colder winters. The "best" clones (Van Remsen, Purple Majesty, Argentine Skies, Sapphire Blue, Blue Ensign, etc.) do not in my experience set seed regularly, nor do the seeds seem to be particularly viable. One is probably better off starting with a plant from one of the superior varieties. Here is a link to an older article from us on this species:
http://www.fbts.com/everything...
Have fun!
