In haiku, because of the differences between English and Japanese, in order to achieve the highest quality haiku, there is an allowance for flexibility in the 5-7-5 syllable pattern. It's typically required that the poem be three lines with the middle line longer and totaling no more than 17 syllables.
For example, if a haiku is written in a combination like 4-6-4 or 3-5-4 instead of a 5-7-5 haiku, and it is felt that adding extra syllables or words detracts from the poem, it is better not to change it.
You're lucky that there is no firm rule regarding capitalization and punctuation in English haiku, nor as to whether the haiku comprises a complete sentence.