This was copied by younger self from, I think, Poems on the Underground which my mother owned.

Everything Changes
after Brecht, `Alles wandelt sich'

Everything changes. We plant
trees for those born later
but what's happened has happened
and poisons poured into the seas
cannot be drained out again.

What's happened has happened.
Poisons poured into the seas
cannot be drained out again, but
everything changes. We plant
trees for those born later.

CICELY HERBERT