The Blessing of Herbs on the Feast of the Assumption? Is there any confirmed historical evidence as to the origins to support the tradition that the Blessing of Herbs and Fruit is associated with the Feast of the Assumption?
It's currently celebrated as a holiday in Bavaria and Saarsland in Germany

There are special traditions about trees and plants on the Feast of the Assumption of Mary. The first ripe walnuts or hazelnuts are called Mary's nuts (Mariennuesse) and are given to children. People may go out into the fields and meadows to collect herbs with medicinal and culinary properties.


https://www.timeanddate.com/holidays/germany/assumption

The Vatican registry acknowledges the tradition as a legitimate Christianization of a pagan tradition.

In the Germanic countries, the custom of blessing herbs is associated with 15 August. This custom, received into the Rituale Romanum(200), represents a clear example of the genuine evangelization of pre-Christian rites and beliefs: one must turn to God, through whose word &quot;the earth produced vegetation: plants bearing seeds in their several kinds, and trees bearing fruit with their seed inside in their several kinds&quot;(Gen 1, 12) in order to obtain what was formerly obtained by magic rites; to stem the damages deriving from poisonous herbs, and benefit from the efficacy of curative herbs.


http://www.vatican.va/roman_curia/congregations/ccdds/documents/rc_con_ccdds_doc_20020513_vers-direttorio_en.html

And it appears to be cataloged in some German almanac of superstitions (or maybe pious belief, I don't speak German much) in 1534

“an unser frawen himmelfart da tregt alle welt obs / büschel allerley kreuter / in die kirchen zu weihen / für alle sucht und plag uberlegt / bewert. Mit disen kreutern geschicht seer vil zauberey“

https://www.brauchtum.de/de/sommer/mariae-himmelfahrt.html
Which seems to mean that everyone would come to Churches on Our Lady of the Assumption's day and bring their herbs to ward of plagues and consecrate the churches.