Sure it's only a tiny seam in the concrete, but field milkweed (Asclepias syriacus) begins as a tiny seed that develops a tenacious, running root. Once it has laced its roots throughout the soil behind the wall it becomes a bank of buried starch energy. It'll take many killings or pullings to make it sprout its last. A plant that can fill this niche and be so healthy amid heat and fumes is a tough one indeed. No wonder milkweed is so tough to control in gardens where it isn't wanted.

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