First Happenings: Adrian Henri in the ’60s and ’70s, ICA

If you bought a Beatles album in the Sixties, chances are you also bought The Mersey Sound, that best-selling collection of poems by the Liverpool poets Brian Patten, Roger McGough and Adrian Henri. It was launched at the Cavern Club in 1967 to musical accompaniment. Their poems felt new, accessible and exciting. "Love is feeling cold in the back of vans," wrote Henri, "Love is a fanclub with only two fans / Love is walking holding paintstained hands / Love is /."

But though he was best known as a poet, Henri was primarily a painter, as well as a collage-maker and performance artist. He taught at Liverpool Art College, having studied at Newcastle under Richard Hamilton, who remained an influence, as did Kurt Schwitters and Allan Kaprow, the American Happenings inventor. Kaprow’s How to Make a Happening, recorded in 1966, lays out, rather bossily, 11 rules for how to do it right (steer clear of art and culture, keep boundaries blurry, don’t rehearse, give up the idea of putting on a show, use real places and people and don’t rely too much on imagination).

This small exhibition at the ICA is like a time-capsule – those confusing, tumultuous times captured under glass. It’s slightly dizzying, peering in at the artefacts 50 or so years on. Happenings or events (he preferred the latter term, partly because Liverpool shops called sales "events", as in Furniture Event and Discount Event), said Henri, consisted of "what you couldn’t stick to a canvas – people, obviously, smells, music, perishable objects, places." He saw events as a direct transition from collage and started setting them up in 1962. City, put on in Liverpool that year, was probably the first such in England. The handwritten plans for it are here. Equipment was to include two tape recorders and a staple gun with music from a portable radio tuned to Radio Luxembourg (your station of the stars) and tapes with sound effects of the city. "Some painting, collage etc to be done during performance." The cast was to include a poet, a painter, a photographer, a stage manager, a man in mask (in audience), and an electrician.

Large, bearded, black-spectacled, here he is, in black and white TV footage declaiming his ironic Batpoem to music from the Liverpool Scene, with his band: "Help us smash the Vietcong / Batman / Help us show them that they’re wrong / Batman / Help us spread democracy / Get them high on LSD / Make them just like you and me / Batman /." There’s his Batmask (main picture), along with a Batcomposition collage and a catalogue for a comic exhibition at the ICA in 1970 that featured Henri along with Jules Feiffer, Stan Lee and others. And an extraordinary painting, the Sgt Pepper-like Entry of Christ into Liverpool in 1964, is an homage to James Ensor’s Entry of Christ into Brussels in 1889, featuring Henri’s heroes and friends: Pere Ubu, William Burroughs, Charlie Parker, Charles Mingus, George Melly, and Patten and McGough among others.

The many posters provide a guided tour of an era: Love Night – lots of pink hearts – at the Everyman Theatre in Liverpool; the Amazing Adventures of the Liverpool Scene, with an album produced by John Peel, who called him one of the great non-singers of our time (the Liverpool Scene were on the bill at Pop Proms at the Royal Albert Hall In 1969, along with Led Zeppelin and Blodwyn Pig); an Apollinaire night at the ICA in 1968, also featuring Michael Kustow, then director of the ICA and who often put on Henri’s work. Letters, singles, badges, photos vie for attention. In one, Henri wraps Yoko Ono in bandages for her Fog Piece at the Bluecoat in Liverpool in 1967.

There’s a fascinating letter on thin airmail paper from Allan Kaprow addressed to Dear Mr Henri: "I think your idea of working with the local environment directly is important. The hard job is to gently sidestep the arty crowd and keep your big toe deep in the ground. It’s hard because arty types seem to be supportive…but at bottom all they want is good taste and this is equated with fashion. The job… is to ride in and above the world all at once; the communication systems no longer permit anyone to be alone." And this was in 1966! "And don’t think you have to recreate America," Kaprow goes on. "It’s not good for you or Liverpool."

Henri remained firmly grounded in Liverpool. Not for him the fashionable London scene embraced by McGough and Patten. And Liverpool was where he met Catherine Marcangeli, the French curator of this exhibition and surely the world’s greatest Henri expert.  Love Is… could have been written for her. She met him in 1986 when she was 19, travelling in England before university, and they were together, off and on and long-distance, for 15 years, while she forged a brilliant academic career in France and New York. She was 35 years younger than him and when he had a heart-bypass in 1999 followed by a series of strokes, she commuted from her job as a senior lecturer in art history at Paris-Diderot University to look after him and encourage him to keep working. She’s the executor of his estate (he died in 2000) and has catalogued his Total Art archive as well as editing collections of his poetry and curating exhibitions of his work.

• First Happenings: Adrian Henri in the ‘60s and ‘70s at the ICA until 15 March
• Adrian Henri: Performance, Environments and Happenings at the ICA cinema, 28 February. A panel explores Henri's work in poetry, music and art