The plant, which has the heaviest corm ever recorded (153.9kgs) has produced seven leaves in the 12 years that it has been in the care of RBGE.
Senior horticulturist Sadie Barber who was gifted the corm in 2003 by Hortus Botanicus in Leiden, Netherlands, said: "We are thrilled to finally see, and smell, this incredible curiosity of the plant world after 12 years of careful cultivation by the Horticulture team at RBGE. The spadix has so far reached an impressive height of 255cm and may even grow a few centimetres more in the next days.
"It really is one of the most extraordinary flowering plants we have ever seen, and great to think that something that grows naturally so far away can be enjoyed by visitors to the Garden here in Edinburgh.''
She added: "The full bloom only lasts one or two days so visitors should hurry to catch the spectacle while they can."
Explaining the scientific significance behind the plant, tropical botanist Mark Hughes said: "The titan arum grows only on the Indonesian island of Sumatra, and its flowering here in RBGE symbolises our research and conservation efforts in that country. We have another 440 species of plants in our glasshouses from Indonesia, many of them incredibly rare and endangered but also fascinating and beautiful. In the past five years our scientists have described over 50 new species from Indonesian forests, showing how much remains to be discovered and protected for the future.''
The titan arum is a giant among plants, with a massive flowering structure that rises some three metres above the ground. A single leaf can reach a height of six metres and a spread of five metres and looks like a small tree rather than the herbaceous plant that it actually is. Its flowering is rare and unpredictable. The flower emits the pungent smell to attract pollinating insects such as carrion beetles and flies.
Successfully bringing the titan arum to the point of flowering has involved replicating the conditions it would experience in the rainforests of Sumatra. The Lowland Tropics House provides the required high humidity and temperatures. During the day the temperatures are between 21-25 degrees centigrade, and by night above a minimum of 19 degrees centigrade with about 80 per cent humidity. The 1,000 litre pot is watered with a high potash liquid fertiliser (tomato food) and care is taken to avoid waterlogged conditions as this could cause the corm to rot. Often an Amorphophallus titanum will die after flowering, but with careful cultivation a plant can continue to produce more leaves or flowers in subsequent years.
In 2010 staff at the garden had to borrow scales from Edinburgh Zoo to weigh the corm which at 153.9kgs, smashed the existing world record of 117kgs, held by Bonn Botanic Gardens, Germany, by 36.9kgs. At the time of the weigh in it had grown from the size of an orange to measuring 952mm wide and 426mm high with a circumference of 280cm.
More work is needed to establish the conservation status of titan arum in the wild. It is currently only known from the Bukit Barisan range of mountains in West Sumatra and is classified as Vulnerable (V) on the 1997 International Union for Conservation of Nature (IUCN) Red List of Threatened Plants.
Sumatra is part of RBGE's ongoing research in Southeast Asia on diverse tropical plant families including the Zingiberaceae (gingers), Begoniaceae (begonias), Gesneriaceae and tropical trees in the Sapotaceae and Malvaceae.