UPDATED AUGUST 4, 2026:
Upcoming on Thursday August 6 is the Feast of the Transfiguration of Our Lord. The liturgical color is white. (It should be remembered here than when the liturgical color is white, the rules permit the priest to wear gold or silver, Therefore, if you are arranging church flowers you should check with the sacristan to find out which vestment the priest will use and which altar frontal will be selected.)
A flower associated with this feast is the field blossom commonly known as the goldenrod. Its official name is solidago and that name is used by florists.
There are some interesting aspects to the association of this flower with the feast.
As Mary Gardener John Stokes notes in his essay Wayside Marian Shrines, in some parts of Germany goldenrod is known as "Heavenly Radiance" (Himmel-Brand). When it is bloom in the fields, one can "see the goldenrod as infused and radiant with God's aura, which we would follow back to its source in the divine radiance." For theological reasons, it should be borne in mind that Stokes is speaking metaphorically. He is not saying we can actually see the divine radiance in the plant.
A review of the scriptures presenting the Transfiguration make clear the connection of the flower to the feast:
"And it came to pass about eight days after these words, that he took
Peter, and James, and John, and went up into a mountain to pray. And whilst he prayed, the shape of his countenance was altered, and his raiment became white and glittering." (Luke 9:28-29;)
"And after six days Jesus taketh with him Peter and James and John, and
leadeth them up into an high mountain apart by themselves, and was
transfigured before them. And his garments became shining and exceeding white as snow, so as no fuller upon earth can make white." (Mark 9:1-2.)
"And after six days Jesus taketh unto him Peter and James, and John his brother, and bringeth them up into a high mountain apart: And he was transfigured before them. And his face did shine as the sun: and his garments became white as snow." (Matthew 17:1-2,)
While it is true that the scriptures refer to white rather than gold, I think the idea is that there is an association with gold because Jesus face "did shine like the sun".
It seems then that the best flowers for the day are a combination of white and golden blossoms. As for how to do a good arrangement utilizing goldenrod, sources I have read suggest that the arranger use white roses or calla or other sculptural bloom at the base, then place goldenrod feathering upwards. Trim away the lower leaves of the goldenrod to help with the radiant effect.