The birth of the Amla tree
When Bhagwan Brahma first performed dhyaan of Bhagwan Vishnu at the beginning of creation, and Vishnu revealed himself in his four-armed form, Brahma's eyes overflowed with tears of pure ananda. Those tears fell upon the earth. From every drop rose a small tree with fine leaves and sour green fruit — the amla, also called amalaki, or dhatri, 'the mother'. Vishnu himself declared: 'This tree shall carry my direct presence. Its fruit shall be sacred, its shade a place of tapas, and pooja performed beneath it shall be accepted as pooja of my own murti.' Since then no other tree in creation has that status — even the tulsi is worshipped in a pot; the amla is worshipped alive in the earth.
King Chitraratha of Vaidishi
In the ancient city of Vaidishi ruled the Chandravanshi king Chitraratha, whose devotion to Vishnu was so great that his prime ministers were themselves acharyas of the Bhagavata. From boyhood he had kept every Ekadashi, and in his palace garden he had planted, at his mother's command, a single amla sapling brought from the slopes of Mandara. Under his care it grew into a great tree whose branches shaded a whole courtyard, and whose fruit each year was the first offering at the palace temple.
The great vrat in the garden
Phalguna Shukla Ekadashi came, and Chitraratha resolved to keep the Amalaki Ekadashi under his own tree with the whole court. From dawn the tree was decorated with mango-leaf toran, kalash placed at its root, red cloth wrapped upon its trunk. Shodashopachar pooja was performed of the tree itself with Vishnu bhava — full puja of a living being, from padya to arati. The whole court fasted through the day. Through the night lamps burned around the trunk; Vishnu Sahasranama and Amalakya-stotra were chanted without pause. At Dwadashi dawn one thousand Brahmins were fed upon amla-rice and jaggery — the traditional Amalaki bhojan.
The hunter Vidura
That same night a hunter of the lower forests named Vidura, hungry, ill and exiled from his own tribe for a killing done in a rage, wandered into the palace garden looking for food. Seeing the lamps and the courtiers around the tree, he hid in the shadow of a wall. He had nothing to eat, being caught before he could steal, and so involuntarily fasted; he could not sleep for hunger, and so involuntarily kept jagran; and from where he lay he heard the katha and the sahasranama through the whole night. By the mercy of the amla tree alone, the papa of his lifetime dissolved from him without his even knowing.
The next janma
Vidura died within the year and was reborn as Vasuratha, prince of Jayanti — a great king of a later age, famous through many Puranas as an ideal ruler and a supreme bhakta of Vishnu. In his next-birth memory he remembered nothing but a single scene: lamps around a great tree at night, and the sound of the Vishnu Sahasranama drifting over a wall. This is the katha the shastra tells to establish that Amalaki Ekadashi purifies even the one who watches its vrat from the shadows. Since then it has been the practice to keep this Ekadashi under an amla tree wherever possible, or to plant a new one that day — for the planting of a single amla is said to equal the merit of a great yajna.
"Grants purification through the amla, prosperity of the household, and Vishnu-loka at the end of life. Planting a single amla tree on this day equals the merit of a great yajna, and blesses even those who merely witness the pooja from the shadows."