Tall and, well, willowy is this choice and beautiful pure white form of rosebay willowherb. Lovely lime green leaves and long-lasting spires of virginal white throughout the summer. Needs a bit of space (5 ft x 3 ft) but so worth it. Sun or semi-shade. Any reasonable soil. To 5 ft. 2 litre pot.
Last few available
Needs space as mentioned above but you can control it every year or two if needed. Just pull up the runners you don't want.
Deadhead as required. Cut back to the ground in winter.
Chamerion or Epilobium has a very wide distribution but its key native habitats tend to be rocky slopes, screes and anywhere with stony and often disturbed soil. That's why they tended to pop up on sites that had been subject to bombing or fire in the world wars, earning it the name fireweed.
Epilobium syn Chamerion Angustifolium Album is a tall hardy perennial with willowy leaves and spires of white flowers in summer suitable for sun or semi-shade.