"Easy - it's 17 syllables (5-7-5), everyone knows that!" Well, in Japanese, that is perfectly correct. Haiku is one of their principal literary forms, even more so than - say - the sonnet is in English. Many writers of haiku in English therefore believe this discipline translates directly, and that a true haiku in English must also be 17 syllables, arranged in 3 lines, 5-7-5.
But consider this (one of the earliest Japanese haiku) :
fu-ru i-ke ya ~ ka-wa-zu to-bi-ko-mu ~ mi-zu no o-to
It doesn't need a knowledge of Japanese to realise that those "syllables" (they are actually called "onji") are of almost equal length. Therefore the metre of a Japanese haiku flows naturally.
Now look at this, in English :
strength is pitiful ~ if not married to wisdom ~ a thought for today
This is the 5-7-5 syllables of a Japanese haiku, but it has a different feel. Disregarding the content for the moment, the sound and flow is not the same. 'Strength' takes as long to say as the three syllables of 'pitiful', and likewise 'thought' is another weighty syllable. Unless uniformly short syllables are chosen (which would be artificial), it is impossible to re-create the metrical sound, flow and feel of a Japanese haiku in English.
On its own, this does not matter too much. English can certainly "adopt" the metrical discipline from another cultural form, and use it to create its own versions. But then there is the issue of content. Returning to the Japanese haiku above, its direct translation into English is :
old pond - frogs jumped in - sound of water
Which is a syllable count of 2-3-4. In fact, Japanese "onji" say less than English syllables, and therefore the Japanese haiku - 5-7-5 - nearly always translates into English as something rather smaller. The haiku is truly succinct, far more so than the 5-7-5 micro-poems they've become in English.
What do we conclude from all this? First, it is not possible to convey the Japanese metre of a haiku in English, unless almost by accident. Second, a true haiku uses fewer images, and more succinctly, than its 5-7-5 English equivalent. There is absolutely no reason why English should not attempt the discipline of writing micro-poetry that incidentally employs the 5-7-5 count of Japanese haiku, but whatever you end up with, they are not real haiku.
Writing English haiku should be perfectly possible, but to capture the spirit of the Japanese form, fewer syllables and fewer images should be used. What is the ideal? Very difficult to say - each poem should be taken on its own merits; however, anything from the 2-3-4 of the frog poem above, to perhaps 4-6-4.
A lot more could be said about the haiku : the difference between haiku and senryu; the need for a 'seasonal' reference; the 'twist' or 'contrast' of two ideas, images, or concepts. But for now, I wanted to address the "hot potato" of the syllable count, a source of controversy and disagreement.