I found this growing in a hanging basket we thought was empty. Any thoughts?
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It does appear to be Chamaenerion angustifolia, previously known as Epilobium angustifolia. Called fireweed in the US and Canada & rosebay willowherb in the UK. It is called fireweed, because it is one of the first plants/flowers to grow after a forest fire.
It is native to the boreal forest of the Northern Hemisphere. It has a preference for disturbed soil. Loose soil, like the soil in your pot. After it starts to grow it can be very vigorous spreading underground through tough rhizomes. As a garden plant it is best in a contained area, otherwise it tends to show up everywhere. It has beautiful purple flowers that produces strands of cotton/silky like substance attached to the seeds, similar to Aspen (Popolous) tree seeds. This silky material is carried off by the wind. It can travel long distances before landing on the ground. If the soil is disturbed it is likely to germinate.