haikufeels

…haiku relates to Nature and the seasons, senryu relates to human nature. Traditionally, the Japanese have ensured this by insisting that to be a haiku the poem must have a season word (kigo), while a senryu does not. They have always had the same form.

–Cor van den Heuvel,
The Haiku Anthology
Preface to the Second Edition, 1986