Christian folklore, traditions and legends concerning plants? With the present situation of the coronavirus affecting many of us, and many sensing the need to be able to get outside and do something, if one’s local restrictions permit it, I would like be able to go out “in the wild” and be able to identify various plants that are either related to Christian folklore and/or Christian symbolisms of some sort.

What got my interest in this subject was when I was doing research on Christian traditions with foodstuffs from around the world. 

For example, some time ago I came across an interesting tidbit (at least for me) about Ostrich ferns also known as the fiddlehead fern: 


  Fiddleheads or fiddlehead greens are the furled fronds of a young fern, harvested for use as a vegetable.
  
  The fiddlehead resembles the curled ornamentation (called a scroll) on the end of a stringed instrument, such as a violin. **It is also called a crozier, after the curved staff used by bishops, which has its origins in the shepherd's crook **. - Fiddlehead fern (Wikipedia)


Another more common example can be found in the plant known as St. John’s Wort:


  The common name "St John's wort" may refer to any species of the genus Hypericum. Therefore, Hypericum perforatum is sometimes called "common St John's wort" or "perforate St John's wort" to differentiate it.
  
  St John's wort is named as such because it commonly flowers, blossoms and is harvested at the time of the summer solstice in late June, around St John's Feast Day on 24 June. The herb would be hung on house and stall doors on St John's Feast day to ward off evil spirits and to safeguard against harm and sickness to man and live-stock. The genus name Hypericum is possibly derived from the Greek words hyper and eikon, in reference to the tradition of hanging plants over religious icons in the home during St John's Day.


Thus my question is as follows: Does there exist a good resource material source book that deals with such plants involving Christian folklore, legends and traditions? Symbolism is likewise included. 

If no book truly is out there, I would then prefer an online source of information. For what it is worth, I am not limiting this question to any specific Christian tradition. Any sources in other languages or non English countries are also welcome.
Christian folklore, traditions and legends concerning plants?

This post is a community wiki answer, so that others may add their own appropriate or interesting (local) Christian plant lore if so desired. 

There certainly no one source that will cover everything about Christian plant folklore, traditions, legends and symbolism.

One of the best sources, I found, can be seen in the book Plant Lore, Legends and Lyrics (Myths, Traditions, Superstitions  and Folklore of the Plant Kingdom) by Richard Folkard Jr. Chapter 5 (page 40) deals with The Plants of the Christian Church. This chapter deals with plant on a biblical basis, as well as in relation to the lives of saints and Christian festivals and feast days.

For plants in the bible the Catholic Encyclopedia has an excellent article published as such: Plants in the Bible.

Some modern named plants are not readily known. For example I remember being asked out to dinner with a family that ate only cheese and fruits at lunch. In the course of the meal we ate some passionfruit. No one knew the origin of why this fruit was called thus and it was decided that we I’d not know which Passion, it was to stimulate? We were all wet behind the ears on this topic.

If fact the passionfruit can be found in Mexico, Central and South America. The symbolism of the  passion flower is as follows:


  The "Passion" in "passion flower" refers to the passion of Jesus in Christian theology. In the 15th and 16th centuries, Spanish Christian missionaries adopted the unique physical structures of this plant, particularly the numbers of its various flower parts, as symbols of the last days of Jesus and especially his crucifixion:
  
  
  Blue passion flower (P. caerulea) showing most elements of the Christian symbolism.
  The pointed tips of the leaves were taken to represent the Holy Lance.
  The tendrils represent the whips used in the flagellation of Christ.
  The ten petals and sepals represent the ten faithful apostles (excluding St. Peter the denier and Judas Iscariot the betrayer)
  The flower's radial filaments, which can number more than a hundred and vary from flower to flower, represent the crown of thorns.
  The chalice-shaped ovary with its receptacle represents a hammer or the Holy Grail.
  The 3 stigmas represent the 3 nails and the 5 anthers below them the 5 wounds (four by the nails and one by the lance).
  The blue and white colors of many species' flowers represent Heaven and Purity.
  In addition, the flower keeps open three days, symbolising the three years' ministry.
  The flower has been given names related to this symbolism throughout Europe since that time. In Spain, it is known as espina de Cristo ("thorn of Christ'"). Older Germanic names include Christus-Krone ("Christ's crown"), Christus-Strauss ("Christ's bouquet"), Dorn-Krone ("crown of thorns"), Jesus-Lijden ("Jesus' passion"), Marter ("passion") or Muttergottes-Stern ("Mother of God's star").
  
  
  


Bouquets of basil leaf are blessed on the Feast of the Exsultation of the Cross (September 14) and sprinkled with Holy Water, after a legend that this herb grew at the foot of the Our Lord’s Cross on Good Friday. This is equally an Orthodox pius tradition and pius belief.


  Some account for the connection between the herb basil and the Cross as follows:
  
  The herb, basil has long been associated with today’s feast of the Exaltation of the Holy Cross. The word “basil” is related to basileios, from the Greek word for king.
  
  According to the liturgical legend, the Empress Saint Helena found the location of the True Cross by digging for it under a colony of basil. Basil plants were reputed to have sprung up at the foot of the Cross where fell the Precious Blood of Christ and the tears of the Mother of Sorrows.
  
  A sprig of basil was said to have been found growing from the wood of the True Cross. On the Feast of the Exaltation of the Holy Cross it is customary in the East to rest the Holy Cross on a bed of basil before presenting it to the veneration of the faithful.
  
  Also, from the practice in some areas of strewing branches of basil before church communion rails, it came to be known as Holy Communion Plant. The blessed basil leaf can be arranged in a bouquet at the foot of the crucifix; the dried leaves can also be used by the faithful as a sacramental. - Blessing of Basil on the feast of the Exaltation of the Holy Cross
  
  


Birch boughs and (St. John Wort, as well as other plants and flowers were used to decorate the the Church doors in England on St. John’s Eve (June 23) in the 16th century.
Source


  The Baptist's day, midsummer day, was a general holiday, when everyone did indeed rejoice, a day full of games and sports and dancing. On the eve of the feast everyone's door "decorated with birch leaves, St. John's wort and white lilies and such-like, garnished upon with garlands of beautiful flowers, had also lamps of glass, with oil burning in them all the night. . . - Catholic Activity: St. John the Baptist's Day