The plant will be named by palm specialist John Dransfield in the Botanical Journal of the Linnean Society today (17 January).
It has a huge 18m trunk and fan leaves that are 5m across. Its giant terminal inflorescence bears hundreds of tiny flowers. It is monocarpic, meaning that once it fruits the entire tree collapses and dies.
The palm was discovered by a local resident walking with his family in a remote area of north-western Madagascar. It was concealed at the foot of a limestone hill.
Botanist Dransfield, honorary research fellow at the Royal Botanic Gardens, Kew and co-author of The Palms of Madagascar, had never seen the palm before, despite visiting the island many times since the 1980s. He said to HW: "How on earth could I have missed this gigantic palm? Ever since we started work on The Palms of Madagascar in the 1980s, we have made discovery after discovery - new species and new genera - but to me this is probably the most exciting of them all."
He concluded that the life-cycle must be unusually long for this flowering-and-death sequence to have never been detected before.
Plant material was collected from the palm by Dransfield's Malagasy student Mijoro Rakotoarinivo, who climbed the adjacent limestone hill and bent over the tree. Analysis at Kew found it was a new, undescribed genus and species. DNA tests place it in a group of palms that grow across Arabia, Thailand and China but have never been seen on Madagascar before.
Less than 100 plants exist and a village committee has been formed to protect the palm. Fences have been erected to excluded cattle. Local people are working with Kew and the Millennium Seed Bank team to sell seeds to raise income for the villagers and to distribute the seeds to botanic gardens and growers around the world.