Your silver plant in question is an Artemisia. Silver Mound is also in this family. I think that yours is Artemisia absinthium. I have grown it. It does spread. It is a great filler for cut flower arrangements. You can also dry it and use it for dried cut floral pieces. It is easily contained if you put a ring around it that is also buried about 6-8 inches under the soil or just dig a ring around it each spring.
I also agree with the vote for ragweed.
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~ Patti ~
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