Look you bunch of pedants. Brummies make two syllables where one is written. Japanese make the punctuation count as a syllable. Haiku is more an intention than a form. It is just oh I can't be bothered the rest is a quote from a web-site explaining stuff like this:
'In Japanese haiku have seventeen onji, or syllables, in groups of five, seven and five. The onji are less varied than our syllables ('Hello' is two, but takes less time to say than 'thrust', 'whinge' or 'mourn'), though there is experimental evidence to show that the seven-syllable lines are said a little quicker and are exactly equivalent in time-value to the five syllable lines. All three lines are therefore, in one sense, of equal length, though the middle one is of greater density.
'The line-lengths of five and seven onji are deeply rooted rhythmic units in Japanese, and have the highly memorable qualities of an English rhymed couplet. Slogans, advertising headlines, proverbs, witty sayings and all forms of traditional poetry are composed in these rhythmic units.
Translators have adopted different policies when searching for an English equivalent: some tried using rhyme (this did not catch on and is now a dead-end); some used a fuller four-line form which looked more like a native English quatrain, notably Noboyuki Yuasa who translated the Penguin Classics version of Basho's The Narrow Road to the Deep North (this did not catch on either). There are two styles which have survived: a small group of translators and writers reproduce the Japanese syllabic pattern exactly in English; and a much larger group keep the translations as minimal as possible, on the grounds that the striking features of haiku are shortness and spareness.
'Two syllables, sometimes one, in a Japanese haiku are often spent on a kireji, or 'cutting word,' rendered in English by a punctuation mark. Fifteen or sixteen syllables remain. Some Japanese words may be shorter than ours, but on the whole they will be slightly less dense and the information in seventeen syllables can be translated in eight to twelve syllables, so there is a case for keeping English language haiku a little shorter than Japanese ones.'
Go to the site yourself to read the rest. I'm going to do something nicer.