Why is tulasī offered to Viṣṇu and Kṛṣṇa, and how?

tulasī is considered Viṣṇu’s most beloved offering — tradition says no naivedya to him is complete without a tulasī leaf.

Tulasī is honoured as Vṛndā, a devotee who became the sacred plant and whom Viṣṇu holds dearest. That is why Vaiṣṇava homes place a tulasī leaf on every food offering to Viṣṇu, Rāma or Kṛṣṇa, and why Śāligrāma and Gopāla are adorned with tulasī garlands. The yearly Tulasī Vivāha after Prabodhinī Ekādaśī celebrates this bond.

How it is commonly done: pluck leaves in the morning after bathing, with a short prayer to the plant; offer them washed, at the feet of the deity and on the bhoga plate. Many families avoid plucking on Ekādaśī, Sundays, Dvādaśī and after sunset, and keep a few leaves picked the day before. Tulasī leaves, like bilva, are said to stay pure for some days.

Once offered, tulasī-prasāda can be eaten. Do not offer tulasī to Gaṇeśa, and families usually avoid it for Śiva too.

Source: Padma Purāṇa tradition; Vaiṣṇava practice · Tulasī Vivāha: after Prabodhinī Ekādaśī

Dharma Q&A — worship at home · Ask a Jyotishi your own question