Answer: To quote the New York Botanical Garden Illustrated Encyclopedia of Horticulture on the barberry: "all except the lowest growing form barriers unbroachable with impunity." I think it fills the bill! Barberries come in many wonderful forms, leaf shape and color, are hardy all around, and all are adorned with thorns. Look for barberry at local nurseries this fall. Also, consider Rugosa rose and hollies. You could make quite a nice, diverse fence with those three!
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