In the early noughties, Caleb Braaten was working in a secondhand record shop in Denver, Colorado, when he came across an album that looked intriguing. The cover of Mother Earth’s Plantasia featured a cartoon of two people cuddling a houseplant, and came with a free horticultural booklet. Best of all, it claimed that its intended audience wasn’t human: you were supposed to play its “warm Earth music” to plants “to aid in their growing”.

“So I put it on and, man, I absolutely immediately fell in love with it,” says Braaten, who now runs Sacred Bones Records. “There’s something about it that is immediately nostalgic. It takes you to this warm place in the past. It’s tickling those same senses as something from your childhood. I think people who didn’t even grow up with that stuff also feel that same warm sensation of … I don’t know. It’s very interesting.”

The more Braaten learned about the album, the stranger its story seemed. It was the work of the late Mort Garson, a musician and easy-listening songwriter who co-wrote Our Day Will Come, the 1963 Ruby and the Romantics single later covered by everyone from James Brown to Amy Winehouse, and an arranger responsible for the shimmer of strings on Glen Campbell’s By the Time I Get to Phoenix. He was also a film and TV composer whose music soundtracked the US broadcast of the Apollo 11 moon landing, and a synthesiser pioneer who probably should be mentioned in the same breath as early electronics heroes Wendy Carlos, Beaver and Krause, and Malcolm Cecil and Robert Margouleff. Yet he seldom is, possibly because of an aversion to putting his own name on his albums: 1967’s The Zodiac came out under the name Cosmic Sounds, 1971’s Black Mass was credited to Lucifer.

Synth pioneer and composer Mort Garson.

For reasons lost in the mists of time, Plantasia wasn’t widely released. It was sold in Mother Earth Plant Boutique in Los Angeles, and had been co-conceived by the shop’s owners, Joel and Lynn Rapp, who wrote the accompanying booklet. Beyond that, says Braaten, it was only available with the purchase of a Simmons mattress from Sears.

During the noughties, however, a cult sprang up around the album, fuelled first by collectors from what Braaten calls “that DJ Shadow deep-digging culture”, then by the internet. YouTube videos of the album attracted views in the millions, commenters hailed it as everything from a precursor to Brian Eno’s ambient music to a prophetic warning about global warming, people made fake Plantasia TV adverts and cover versions of its tracks. Bootleg versions started appearing: the value of an original copy soared to $600. “It became one of those algorithmic YouTube sensations,” says Braaten. “It popped up on recommended-to-watch lists and ended up on work-chill playlists.”

Quite aside from the cosseting loveliness of its warm synth tones and melodies, part of Plantasia’s appeal rests in the way it summons up a lost post-hippy age, a mid-70s moment when the kind of far-out ideas that would previously have been discussed in dope smoke-filled rooms in San Francisco or Notting Hill became mainstream. It was an era in which Erich von Däniken sold millions of paperbacks with his was-God-an-astronaut postulations about the ancient world and the pseudoscientific book The Secret Life of Plants – which suggested plants had emotions, and thus might respond to music – hit bestseller lists.

Perhaps, as Braaten suggests, people respond to Plantasia because there’s a distinct correlation between then and now: there’s certainly plenty of pseudoscientific woo-woo flogged under the banner of “wellness”. Either way, no one was more startled by the album’s rise to cultdom than Garson’s daughter, Day Darmet. “After my dad passed, I took all of his things and I neatly put them away, all the extra music,” she says. “Then friends started telling me there was a whole bunch of Plantasia on YouTube. I thought, you’ve got to be kidding me. I looked at it myself and thought, that’s one bunch of crazy people.”

Her bafflement was compounded by the fact that she didn’t particularly care for the album. “There’s pieces my dad did that are so stunningly beautiful and heartfelt, but this just didn’t do it for me.” Nor did her mother, whose own interest in gardening, rather than The Secret Life of Plants itself, inspired Plantasia. “She thought he had gone off the deep end.”

She batted away a series of inquiries about rereleasing the album until Braaten got in touch, offering to put it out on Sacred Bones – best known as home to singer-songwriters Zola Jesus and Marissa Nadler – as part of a wider retrospective of Garson’s work. Despite her reservations about the album, Day Garson is touched. “It doesn’t move me, but it moved my dad. So whatever moved him must be moving other people. On his epitaph he had me write, “‘The music plays on.’ He was right. The music plays on.”

• Plantasia is available on vinyl and CD from Sacred Bones Records.