Following sloka indicates the spiritual significance of Thulasi.
Yanmoole Sarva Teerthani Yanmadhye Sarva Devata
Yadagre Sarva Vedascha Tulasee Tvaam Namamyaham
That means all sacred Theerthas reside in the roots, all the Deities reside in the middle, and all the Vedas occupy the top portion, the tip of the Thulasi plant and I am offering my obeisance and salutations to such sacred plant.
It is further said that the Trinal Lords Brahma resides in the apex of Thulasi, Vishnu in the middle and Shiva in its roots and Asta Dighpalakas in its branches. It is believed that Goddess Gayathri, Lakshmi, Saraswathi, and Sachi Devi always reside in Thulasi’s flowering springs.
Thulasi is compulsory and a must in the worship of Lord Vishnu who gets very much pleased when He is worshipped with Thulasi.
In Vaishnava sampradaya Thulasi is extensively used and no religious ceremony or ritual is complete without Thulasi.
Worship of Lord Maha Vishnu is incomplete without adding Thulasi leaves. We find Lord Vishnu always adorned with Thulasi garland.
Thulasi is a must in any religious ceremony including performing sacred rites to fore fathers. Dried Thulasi stems are also used while performing Homas.
As per Brahma Vaivartha Purana it is said that as soon as one gets up from the bed one should have the darshan of Thulasi plant which is equivalent to having darshan of all the sacred Theerthas in three worlds and also equivalent to having darshan of Lord Sri Lakshmi Narayana.
One will get absolved from sins with the darshan and pradakshina of Thulasi plant.
We find people worshipping Thulasi with the following sloka as soon as they get up from the bed.
NamasThulasi Kalyani Namoh VishnuPriye Subhe
Namo MokshaPradhe Devi Namah Sampathpradayine
Even growing, touching, watering, transplanting and fostering of Thulasi plant is said to be highly auspicious and meritorious. Lord Sri Maha Vishnu always resides where there is Thulasi, Lotus and Salagrama.
It is believed that one who worships Lord Vishnu with Thulasi will not take rebirth and will attain salvation. Worshiping Lord Vishnu with Thulasi leaves helps one in getting cured of incurable diseases.
Sacred water (Theertha) given in temples by the priests is called Thulasi theertham. It is a tradition and belief that we find people giving Thulasi Theertham to the dying person in the last stages with a belief that the soul would go to Heaven. We also find people adding Thulasi branches to the funeral pyre with a belief to provide sanctity and spirituality to the departed soul.
One who dies in the vicinity of Thulasi will not go to Naraka Loka. It is said that even Yama Doothas (messengers of Lord Yama) will not dare to enter the house where Thulasi is present and worshiped. Even performing sacred rites to forefathers in the shade of Thulasi plant is considered to be very sacred and meritorious.
One who lites a lamp near Thulasi regularly during Sandhya kala (Sunrise & Sunset) will have Vaikunta Prapthi. Lighting lamps with dried stems of Thulasi dipped in oil/ghee is highly meritorious.
It is said that worshiping Thulasi is equivalent to taking bath in sacred river Ganges and one will get Ganga Snana Phala. Taking bath in Ganges River, having darshan of Narmada River and worshiping Thulasi are all equal in merits.
In olden days and even to day in traditional Hindu families we find women praying and circumambulating the Thulasi plant, pouring water daily in the morning and evening for the welfare and well being of her husband and family.
While Thulasi is used in the worship of both Lord Vishnu as well as Lord Shiva, one exception we find is that it is not prescribed in the worship of Lord Ganapathi except on the day of Vinayaka Chaturthi.
Thulasi is a sattvik plant and it is said that by using Thulasi one will improve his Sattvik nature. We find people wearing Thulasi Mala a chain made out of Thulasi beads around their neck while performing Japa and also during divine worship as a protection from getting negative thoughts and evil influences.
While Thulasi as such can be given as charity, adding it to any other charity will enhance the merits of such charity.
Kartheeka Sukla Dwadasi also known as Uttana Dwadasi is celebrated as a festival of Goddess Thulasi when Thulasi plant is symbolically married to a Salagrama Shila (Lord Vishnu) called Thulasi Damodara Vivaha with all grandeur. On this day while worshipping we find people keeping a branch of Amla tree (Amalakki) besides the Thulasi plant.
This day also marks the end of Chaturmasam the day on which Lord Sri Maha Vishnu gets up from His Yoganidra. This day is also referred to as Brunda (Vana) Dwadasi in the name of Divine Thulasi. If that day is missed out it is generally celebrated on the Full Moon day, Kartheeka Pournami. Good day