How to Pluck Tulasi Leaves for Offering to Lord Krishna

The Vayu Purana says that one should be clean, preferably fully bathed and wearing clean clothes, before touching or plucking Tulasi. With your right hand, carefully pick the tulasi leaves or soft tulasi-manjaris, along with their stems-one at a time-while repeatedly chanting the tulasi-cayana-mantra:

tulasy amṛta-janmāsi 

sadā tvaṁ keśava-priyā

keśavārthaṁ cinomi tvāṁ

 varadā bhava śobhane

O Tulasī of effulgent beauty, you were born from nectar, during the churning of the milk ocean. You are always very dear to Lord Keśava. Now, in order to worship Lord Keśava, I am collecting your leaves and mañjarīs. Please bestow your benediction on me that my worship will obtain success.

Take special care to avoid causing Śrīmatī Tulasī-devī any pain. If when picking Tulasi leaves one breaks the branches, Visnu feels pain in His heart. Pluck her leaves with your right hand while holding the branch in your left, taking care not to break it. Do not use scissors or a knife to cut the leaves from the plant. When finished, recite the ksama-prarthana-mantra, begging her pardon:

cayanodbhava-duḥkhaṁ ca

 yad hṛdi tava vartate

tat kṣamasva jagan-mātaḥ

vṛndā-devī namo ’stu te

O Tulasī-devī, I offer my respectful obeisances unto you. Kindly forgive me if I have caused you pain by picking your leaves and manjaris, O mother of the universe.

Do not pluck Tulasī leaves in the morning before daylight or in the evening after dark, nor at any time on Dvadasi-tithi, the day following Ekadasi. Even if the tulasi leaves have become dry because they were cut on the previous day or in the morning, they may still be used in Deity worship.

Avoid Plucking Tulasi Leaves on Dvadasi

Tulasi should not be plucked on dvadasi. The Visnu Dharmottara says, Vaisnavas should never pick Tulasi on dvadasi tithi.

samkranty adau ni siddho'pi

tulasyavacayah smrtau

param sri visnu bhaktastu 

dvadasyam eva nesyate

It is generally forbidden to pluck Tulasi on Sankranti, (Purnima, Amavasya, Dvadashi, Sunday) etc., according to the strict rules of the Smritis, but the most important rule (according to Shrila Sanatana Goswami) which the Vaishnava devotees should certainly follow is to not pluck Tulasi leaves on Dvadashi.(From Hari Bhakthi Vilasa 7.353)

nac chindat tulasim vipra

dvādaśyaṁ vaiṣṇavaḥ kvacit

Oh brahmanas, a Vaisnava never picks Tulasi leaves on Dvadashi, (the day after Ekadashi).

Why should we avoid plucking Tulasi leaves on Dvadasi? Srimati Tulsi Maharani, the pure devotee of the Lord, keeps fasting on the whole Ekadashi and takes rest on Dwadashi. So the devotee in order to not to disturb or hurt her, doesn't pluck leaves on that day. Then what to do to get the Tulsi leaves for offering on Dwadashi and other purposes? Follow advance savings. Keep some advance Tulsi leaves for Dwadashi for offering purposes. Or, one may pick the leaves fallen on the ground around the pot and then after thoroughly washing it, can use it for offering. 

Mantra when offering Tulasī leaves to Lord Kṛṣṇa

etat tulasī patram klīṁ kṛṣṇāya namaḥ

From: http://www.prabhupada-iskcon.de/tulasi/tulasi-mantras.html