Haiku and Senryu are Japanese poems, both of which have 17 syllables, of three lines with 5-7-5 syllabication, but the similarity ends there. Haiku are aesthetic nature poems. Haiku require a kigo or nature word (theme). Haiku have a kireji or cutting word that shifts the focus of the haiku. In 17 syllables, the haiku shifts focus. The core difference between haiku and senryu lie in the poems' differing theme or purpose. Senryu (poems written by the Japanese poet of that name) are usually cynical or darkly humorous. Senryu poke fun at the human condition. Senryu can be spoof, parody or satire poems. Senryu have no specific kireji or kigo, but as dark humor, they do often have a shift or perhaps a twist would be a better word. Senryu generally have a 'sting' in the tail. I think it behooves the would-be writer of haiku to understand how haiku and senryu differ.
In my haiku, Popsicle Purple (which isn't the best example of nature, but perhaps season), see if you can find my shift in tone:
Popsicle purple
drips sticky sweet on new dress
yahoo for summer
Yahoo is my cutting word, cutting across the negative tone of soiled clothing and shifting to a positive view that this means summer has arrived. This poem has a bit of senryu styling as it does poke gentle fun at parents for giving a child a popsicle when she is wearing new clothes. And it pokes fun at a child's predisposition to spill, especially on new clothes. Lastly, Popsicle Purple juxtaposes a child's delight in summer, signaled by the arrival of popsicles.
Here are other senryu.
migrating fowl find
crashing on ice-covered pond
winter remains still
and another
he scales a tree to
commune with nature
disturbing a nest of eggs
(I both can identify with and feel sorry for the poor person described here, as well as the nest of eggs. it would be like me to do something completely wrong for the right reasons. The commentary is that when you would "commune with nature" best to check to make sure you are not endangering it in so doing._
The best way to describe the difference between haiku and senryu is that they represent nature vs. nurture. Nature in the Haiku is merely reflective whereas in senryu there is a bit of commentary.

