Answer: Your zip code places you in zone 6A, which depending on microclimate could actually be as cold as zone 5. Mimosas are considered reliably hardy through zone 7 and into zone 6. For this reason I am not sure I would recommend planting a mimosa on the basis of winter hardiness alone. This particular mimosa "Summer Chocolate" is not listed in the Monrovia online plant descriptions. In a quick search I did not find any mention of its being sterile. Here is a typical description of it:
Good luck with your clients.
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