It is said that, we should not touch or give water to Tulsi or Peepal tree on Sunday. What is the reason behind it and what happens if someone does it?
This faith is profound in many hindu followers. It is belief that Tulsi is Lord Vishnu's beloved. so, most followers especially Vaishnavas strictly follow rituals while doing prayer (archana) of Lord Vishnu on daily basis.
NA CHINDYAT TULASIM VIPRA DVADASYAM VAISNAVAH KVACIT || vishnudharmottara purana.
Oh brahmanas, a Vaisnava never picks Tulasi leaves on Dvadasi, (the day after Ekadasi).
For some specifics days in Garuda Purana
BHANUVARAM VINA DURVAM TULASIM DVADASIM VINA JIVITASYA AVINASAYA NA VICINVITA DHARMA VIT || Garuda Purana
A scripturally learned person, if he does not want to decrease his duration of life, he should not pick Durva grass on Sundays or Tulasi leaves on the Dvadasi day.
From Padma Purana
DVADASYAM TULASI PATRAM DHATRI PATRAS CA KARTIKE LUNATI SA NARO GACCHET NIRAYAN ATI GARHITAN || from PADMA PURANA conversation between Lord Krsna and Satyabhama
Any person who picks Tulasi leaves on Dvadasi or Amalaki leaves inKartika month will fall into hell.
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The Ashwatha ( Ficus Religiosa Moraceae /Hindi Peepul /Telugu Raavi) tree spreads far and wide with auxiliary flexible extensions ( don't know botanical name) becoming trunks with a trunk diameter growing up to 3 metres. It provides cool shade for foot travellers between villages in hot summer.
Suitable even for long term human habitation under its shade, this Bodhi was peaceful enough for the Buddha to penance and attain Enlightenment.
Ficus religiosa is used in traditional medicine for about 50 types of disorders including asthma, diabetes, diarrhea, epilepsy, gastric problems, inflammatory disorders, infectious and sexual disorders.It is one of standard trees to be inside Hindu temples. I for one like the rustle noise of the leaves make with weak currents of wind. On ETV YouTube Kandadai Ramanujacharyulu has given a proper reply:
He mentions that even if you see Thulasi on the way while passing by veneration is felt and shown.
I have no references but in this background It is plain commonsense to guess that it should not be cut down. It need not perhaps have a reference in Vedas for something so widespread in use and so beneficial. It was given a position of respect.. not to be even touched... however it is impractable that such a friendly tree is to be kept untouchable and and so my guess is that quite arbitrarily a day of the week dedicated to the Sun ( sunshine and water are alone adequate for it to develop to its immense size, e.g. as the one in Adayar) and a day has been presribed to drive home its importance and man's ecological responsibility.
The same is logic can be applied for the Thulasi. It is a powerful anti-oxidant, there are stories true or maybe later even woven up for adoption ( it succeeded!) , is given a place of worship (short regular worship duty is assigned to women and given a central importance in Sri Krishna Thulabharam drama) as it freshens up the air and so on. Nowadays even "Pancha Thulasi" is commercially marketed. I have used it in its diluted form as freshening eyedrops.