"Tulasi leaf is very, very dear to Visnu. All Visnu-tattva Deities require profusely Tulasi leaves. Lord Visnu likes garland of Tulasi leaves. Tulasi leaves mixed with sandalwood pulp and placed on the lotus feet of the Lord is the topmost worship. But we must be very careful that Tulasi leaves cannot be placed on the feet of anyone else than Lord Visnu and His different forms. Tulasi leaves cannot be placed even on the lotus feet of Radharani or on the lotus feet of the Spiritual Master. It is entirely reserved for being placed on the lotus feet of Krsna. We can place, however, Tulasi leaves in the hands of Radharani for being placed on the lotus feet of Krsna, as you have seen on the Govinda Album."
Srila Prabhupada Letter to Govinda, 04-07-70, Los Angeles
Lord Krsna's favorite plant is Tulasi. Tulasi garlands are made for Krsna, and His lotus feet are decorated with Tulasi leaves and manjaris (flower buds). The devotees wear Tulasi neck-beads and chant the Hare Krsna maha-mantra on Tulasi beads.
Tulasi plants and their leaves are very important in devotional service, and devotees are encouraged to care for and water the Tulasi tree every day, collecting the leaves for offering to the Lord. Tulasi leaves may be offered with each food preparation.
"The Lord does not care for a single one of fifty-six offerings or thirty-six curries offered without a Tulasi leaf. Lord Shiva, Subdeva, Narada Muni, Brahma, and all others are paying their respects unto thee. Candra Sekara (the author)' who is singing your glories, prays, "Please bestow Prema~bhakti unto me. My humble obeisances unto thee again and again."
Tulasi's leaves should be washed three times before offering, and should be kept in a bowl of fresh water inside a refrigerator. If They are not kept like this, They turn limp and pale. If They are kept in water, but not in a refrigerator, They will turn brown. Be sure to change the water in the bowl every day, and clean the sides of the bowl so no film will grow. (Proof that Mother Ganga is actually pure is that Ganga water will not become murky if kept contained.)
Estimate the required amount of leaves needed per day and try to pick
about the same amount each day. In that way you will always have a supply, and Tulasi will feel the regularity. (Sometimes in the case of manjaris, there will be an extra amount of mercy.) Don' t pick Her leaves sporadically, but aim for
the same time, same amount, every day. How many leaves may be safely picked depends on how healthy and bushy She is. If there are hundreds of leaves on Her, it won't hurt to take a few. If there are only 13 or 14 leaves, don' t take any at all unless they are about to drop off. It depends on the size and health of the plant. Use your
discretion in the matter. Srila Prabhupada said that dried leaves are just as offerable as fresh green ones, and the dried leaves should be sent to centers having little or no Tulasi. Extra dried leaves, manjaris, etc., may be distributed as needed.