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This is Lambsquarters or Fat-hen (Chenopodium album). There are cultivated varieties so you might have purchased the seed, but it is also a common adventive weed. It is also an edible and delicious weed, so why not treat it like a crop. The leaves accumulate sodium and so have a naturally salty taste. Cook it like Spinach (Spinacha oleracea). Both are related plants in the Lambsquarter family (Chenopodiaceae)