Before observing Lord Vishnu or Shri Krishna, we often offer them Tulsi leaf. Tulsi is basically a holy plant which grows to a height of 2-3 feet. This plant has vertically branching foot-stalks which bears small flowers. Most probably, its root name is tuḷas, and Tulsi is its Sanskrit version. Normally black tulsi is said to be Krishna Tulsi. But did you ever thought about why we do it?
Here check out why do we offer tulsi to Lord Krishna

We are told in a simple language that ‘a specific substance offered to a specific deity is the favourite of that Deities. For example, Shri Gaṇapati likes red flowers, Shiva bel (Aegle marmelos tree) leaves, and Shri Krishna Tulsi etc. Then a story to explain why that Deity likes that specific substance is narrated. In reality, superior Deities like Shiva, Shrivishṇu, Shri Gaṇapati do not have any likes or dislikes. The reason for offering specific substances to specific Deities is as given ahead.

One of the objectives of worship is to charge the Idol with Chaitanya to assists in our spiritual progress. So as to generate that Chaitanya, the substance offered to that Idol has more ability to attract the pavitraks of that Deity, from as far as the Maharlok, when compared with other substances. Red flowers have the ability to attract the pavitraks of Shri Gaṇapati, bel of Shiva, Tulsi of Shri Krushna etc. Hence, Tulsi leaves are offered to Shri Krushna.

A Tulsi is planted in tulsi-vrundavan (A small rectangular structure in which the Tulsi [Holy basil plant] is grown) in front of the house. Tulsi is even wedded to Shri Krushṇa.