going by the leaf I'd say it is a native plant called mugwort. if it is its a weed in your planter.

I have the same plant popping up in my garden, but I don't think it is a mugwort due to your plants leaf shape. Triffid is as good as any name for this weed. Just the same, have somebody standing by you with axe at the ready when you commence to pull it out of the pot, just in case it is a triffid.