The twenty-one leaves
A common pairing, as given in the Vinayaka Chaturthi vrata books, is: Sumukha with machi patra (Artemisia); Ganadhipa with brihati (Indian nightshade); Umaputra with bilva (bel); Gajanana with durva grass; Harasunu with dattura (thorn apple); Lambodara with badari (jujube); Guhagraja with apamarga; Gajakarna with tulasi; Ekadanta with chuta (mango); Vikata with karavira (oleander); Bhinnadanta with vishnukranta; Vatu with dadimi (pomegranate); Sarveshvara with devadaru (deodar); Phalachandra with maruvaka (marjoram); Heramba with sindhuvara (five-leaved chaste tree); Shurpakarna with jaji (jasmine); Suragraja with gandaki; Ibhavaktra with shami; Vinayaka with ashvattha (peepal); Suravandya with arjuna; and Kapila with arka (calotropis).
Lists differ slightly between regions and books. Tulasi, not offered to Ganesha on other days, is included on this one day in this tradition.
How it is offered
Collect the leaves the day before, wash them and arrange them in order. After the sixteen offerings, offer the leaves one by one, saying for each "Om [name]aya namah, [leaf] patram pujayami", for example "Om Sumukhaya namah, machi patram pujayami". Then offer all the leaves together, saying "Om Shri Ganeshaya namah, ekavimshati patrani pujayami". Where some leaves cannot be found, durva or akshata is offered in their place.
After the festival the leaves are immersed with the image, returning the plants to the water.