5,7,5 is... debated. It's how haiku is taught in school. But English language writers have been deviating from it for quite a while. This is from poetry foundation (an American site):
'Despite its formal history, the haiku’s composition has expanded somewhat over time. This is due in part to the differences between the Japanese language and Western languages. In its original Japanese form, the haiku is often divided into 17 mora (a Japanese unit of syllable weight) and arranged in a single vertical line. However, in English there is no exact equivalent to the mora unit. As a result, in English and other languages, haikus are most frequently adapted into three lines of verse, usually unrhymed, composed of five, seven, and five syllables, adding up to seventeen syllables total. However, many American poets, such as Jack Kerouac, began to gradually depart from this traditional syllable and line count, in favor of depicting images as succinctly as possible'.
Some poets feel that having to stick to 5,7,5 can detract from capturing the essence of a moment, which is also what haiku is about. In my opinion some writers take paring the haiku down too far and end up conveying...well...not very much at all. Like this:
The moment
She opens her eyes
Harvest moon
I just invented that but I see quite a lot like this. What does it even mean?
However this one I posted:
Late January sun
A hawk tethered to the stump
Dreaming of mountains
I think is much better like this:
Yesterday's snow
The tethered hawk
Dreams of mountains
This is from another American site
Some Characteristics of Haiku:
An English-language haiku sometimes contains of 17 total syllables
English-language format is sometimes composed of 3 lines of 5-7-5 (syllables)
2 simple subjects are often placed in juxtaposition
These 2 subjects are often separated by punctuation
A keen or unusual observation is made by comparing the two subjects
Haiku often contains a seasonal reference
Poems are traditionally about nature or the natural world