I don't know what it is, but in that space between shed and fence it is probably getting less light than usual and growing tall and sparse compared to its normal habit. A close up of the leaf may help, is it a three part leaf?
Here is knotweed.
Can you get any closer to the stem against the shed? It does resemble a Persicaria/Polygonum, but I don't think Japanese. It looks like it has a wrapping ligule at the node like Polygonum, but a closeup will show more detail to confirm ID.
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