Many moons ago, I wrote a blog post about haiku, which I am quite proud about - if only because there is no other post at this blog which I took so much effort to write.
Today, I thought I'll add a short addendum to the same since I started reading last week about Kyoshi Takahama (1874-1959) and Sanki Saito (1900 ~ 1962) and came across the term Gendai Haiku, which is used essentially for Modern Haiku in Japanese: (Gendai: 現: the present day, modern times, today). Kiyoko Uda, a president of the Modern Haiku Association, the most avant-garde of Japan's major haiku organizations, put it this way:
“It should be our method that we create haiku which match the times. This is not a new idea and was prevalent in the old days; even Sanki Saito wrote about it before the association existed. Sanki believed: ‘To the difficult question 'what is new?’ I will answer: the new means how the emotions of today's society and people are expressed to fit the times. The haiku must be innovative in any time. So we should begin and continue to express the emotions of the people of this time and generation." - Kiyoko Uda, President, Modern Haiku Association, Tokyo, JAPAN (Gendai Haiku, S.21.10) English Translation: Akiko Takazawa
However, it isn't a new 21st century movement - actually it can be said to start even with Shiki in the 19th century but surprisingly I never heard of the term 'gendai haiku' before even though I might have been writing some and calling them, hesitating to call them haikus, poem-kus! 
In an essay in Frogpond (2012), Paul Miller writes:
The first hint of gendai haiku to reach American shores occurred with Makoto Ueda’s underrated anthology Modern Japanese Haiku published in 1976 in which he translated twenty Japanese poets beginning with and after Masaoka Shiki. However, the volume wasn’t strictly a gendai anthology, and in fact included some poets who were opposed to what would later become known as gendai—such as Shiki’s successor Kyoshi Takahama. The anthology instead was an attempt to give voice to modern poets perhaps unknown of in English. Of the twenty included poets only a handful were born after nineteen hundred. Inspired perhaps by the few poets whose work could be called gendai, several American and Canadian poets tried their hand at gendai haiku; however, these efforts were short lived.
It wasn’t until the 2001 and 2008 anthologies of the Modern Haiku Association (Gendai Haiku Kyokai), which contain hundreds of modern Japanese haiku—translated into English!— that Americans finally took serious notice. It is important to keep in mind that gendai haiku is not wholly representative of all contemporary haiku in Japan, yet the scope of the poems and poets in these anthologies speak to a significant movement. Perhaps equally important for discussion purposes is that the movement speaks to a level of diversity concerning what a haiku can be that is arguably lacking in our country.
In an essay about Gendai Haiku, Peter Yovuwrites:
In his review of The Haiku Universe for the 21st Century, Scott Metz quotes Masaoka Shiki: “Haiku advances . . . only when it departs from the traditional style.” I am not scholar enough to surmise how far Shiki would have been willing to take this departure, but I will guess that he would have been surprised, at the least, to discover the directions that his disciples and those who followed would take. Certainly a departure from realism, as various movements embraced subjectivity, politics, surrealism, feminism, disjunction and other literary techniques rarely encountered before. Some schools promoted the writing of haiku without kigo, a movement many writers in the West have also explored.
So, the term 'Gendai' means more than just Modern or Avant-Garde.
In a post at the Haiku project, this excerpt by Scott Metz elucidates:
“… influenced by changes in culture, society, economics, art, and literature—globalization—many different schools and strands of haiku developed during the 20th century. … Starting with a foundation centered more on realism and experience, 20th century haiku immediately expanded into areas such as politics, subjectivity, the avant-garde, feminism, urbanism, surrealism, the imaginary, symbolism, individuality, and science fiction: in general, free-form and experimental aesthetics. … The rigid limitations and conservatism of traditional techniques (namely 5-7-5 on/syllabets and the necessity of a kigo) were no longer absolutes for Japanese poets.”
I could go on with excerpts from many other online articles but will leave you with this lovely interview with Richard Gilbert. Read the article in its entirety as he is quite the expert on modern haiku, both in Japanese and English.
Gendai haiku” means literally “modern or contemporary haiku,” and loosely refers to expansive ideas of the haiku form arising from the 1920s on, and more particularly to the direct progenitors of the gendai haiku movement.… Literally, the word [gendai] means “contemporary” but just as with “modern art,” something more is implied, in terms of movements, categories, history and personages.… Gendai haiku offer the reader the shape of who we are in the shape of things to come, in resonance with archaic myth, (and) the formal insights of previous ages.… Gendai haiku partake of a tradition and culture in which, unlike that of the historical Judeo-Christian West, nature and culture were not extensively polarized. So in gendai haiku exists an invitation to the present and a future, in congruence with the past. This congruency is also an uprooting, accomplished via expansive and often experimental avant-garde language and techniques. Yet the old is likewise held in the new, in plying the form.
Gendai haiku partake of a tradition and culture in which, unlike that of the historical Judeo-Christian West, nature and culture were not extensively polarized. So in gendai haiku exists an invitation to the present and a future, in congruence with the past. This congruency is also an uprooting, accomplished via expansive and often experimental avant-garde language and techniques. Yet the old is likewise held in the new, in plying the form. The key to haiku, what makes it a brilliant literature, is that haiku cut through time and space as a primary means of birthing and articulating novel realities as environments.
In the 1950s the Beats asked the question, "How do we grow our own culture?" and recently the poet Hoshinaga Fumio commented, "Language is overworked, fatigued." The Beats knew where to start, Hoshinaga knows how. In a world without torture or needless suffering, there would still be, according to Jung, one imperative: to choose to individuate, to encounter the shadow, to grow. So I take your word "imperative" to heart. The great gendai poets know how to begin. At the moment, poets everywhere are searching for the taste of the new; do we hunger for revival even at the expense of survival? What can be learned from the Japanese poets is not the "how" but rather the actuality. How language is unequivocally refreshed. I intuit that we may one day live in a culture which embodies those "energies of the body" inspired by myth; essential poetic navigations which Campbell and others discuss as the roots of human soul. Until that time, gendai haiku is a great reminder, and more, that taste! The taste of an era. And it's brilliant.
Will leave you with a link (no excerpts!) to three more articles:
- A longish essay about
how haiku writers in Japan post World War II started The New Rising Haiku
movement, “a movement to recover the adolescence of haiku. . . . In
order to break the old and feudal tradition of haiku taste and thought, we
hoisted the flag of liberalism and democracy against the exclusionism of the
haiku world and the feudalistic master‑disciple system. That is, to create
gendai haiku as poetry, we advocated the pure poesy of haiku, not the old hobby
- An essay by Richard Gilbert and other modern haiku masters from Japan: A New Haiku Era: Non-season kigo in the Gendai Haiku saijiki
- An interview with Richard Gilbert.
Now… even if in English, to quote Scott Metz again, go… make it new!
 I've written them off and on since 2005 ...often in spring (March-April) or fall (Sep-Oct), coincidentally when something about the weather change spurred me to write them. Here's a few from 2013.