Japanese haiku have been traditionally composed in 5-7-5 syllables. When poets started writing English haiku in the 1950's, they adopted this 5-7-5 form, thinking it created a similar condition for English-language haiku. This style is what is generally considered "traditional" English haiku.

Over the years, however, most haiku poets in North America have become aware that 17 English syllables convey a great deal more information than 17 Japanese syllables, and have come to write haiku in fewer syllables, most often in three segments that follow a short-long-short pattern without a rigid structure. [...]

THE LENGTH AND FORM OF ENGLISH HAIKU

Today, many bilingual poets and translators in the mainstream North American haiku scene agree that something in the vicinity of 11 English syllables is a suitable approximation of 17 Japanese syllables, in order to convey about the same amount of information as well as the brevity and the fragmented quality found in Japanese haiku. As to the form, some American poets advocate writing in 3-5-3 syllables or 2-3-2 accented beats. [...]

THE FLEXIBILITY OF JAPANESE GRAMMAR

[...] The grammatical units in Japanese are largely independent, and are relatively free to move about within a sentence. [...] Moreover, in Japanese, some of the grammatical particles and the subject of a sentence can be omitted depending on the context, which gives the language further flexibility... Thus there are more places where a Japanese phrase can be divided without disrupting its meaning.

[...] If one wishes to have the brevity and the fragmented quality of Japanese haiku in English haiku, 17 syllables are too long. On the other hand, if a rigid structure is desired, 11 syllables are too short... The majority of contemporary English-haiku poets have let go of the tight forms in favor of brevity to develop the mainstream North American haiku.

THE UNDERLYING STRUCTURES OF THE CLASSIC JAPANESE HAIKU

[...] 5-7-5 segmentation is not a division based on content as we think of it in English. Strictly in terms of content, the classic Japanese haiku are composed of two major parts of varying lengths, such as 5-12, 12-5, 8-9, 9-8, 7-10, and 10-7, in the generally decreasing order of prevalence, with the first two being the most prevalent. Here are some examples from the great masters (with a literal translation by the author):

yuku haru-ya (5) tori naki uo-no me-ni namida (12) - Basho

spring passing -
birds cry, tears in the eyes of fish

neko-no meshi shoubansuru-ya (12) suzume-no-ko (5) - Issa

sampling the cat's food -
a baby sparrow

ware-to kite asobe-ya (9) oya-no nai suzume (8) - Issa

come play with me -
you motherless sparrow

uguisu-no naku-ya (8) chiisaki kuchi akete (9) - Buson

uguisu singing - (uguisu: a nightingale-like bird)
with the small mouth open

A close observation of "free-form" English haiku reveals that they are composed of two major segments. The majority of them are divided after the first or the second line and the rest near the middle, and thus they are in accord with the underlying structures of the classic Japanese haiku... To limit short haiku to those that can be fitted into a rigid three-part structure is to severely limit the type of ideas that can be expressed in this style. [...]

In short, writing within the rigid structure of Japanese haiku is made possible by the remarkable malleability and redundancy of the Japanese language which allows for a multitude of options in expressing a single thought. In languages such as English and its relatives whose grammars are heavily dependent on word order, haiku must and will take a much different form from that in Japanese.

By concerning ourselves too much with the outward form of haiku, we can lose sight of its essence.