Flowers recall important events in the plan of salvation that occurred in gardens.
"The three pillars of eternity, the three events, preeminent and transcendent above all others, are the creation, the fall, and the atonement. These three are the foundations upon which all things rest. Without any one of them all things would lose their purpose and meaning, and the plans and designs of Deity would come to naught." -Bruce R. McConkie
All the earth was a garden when it was created.
The fall occurred in the Garden of Eden.
The atonement took place in the Garden of Gethsemane and at the Garden tomb. (The first witness of the resurrected Lord, Jesus Christ, mistook Him for a gardener.)
Flowers are particularly prominent in the middle of the celestial room.
Flowers are beautiful and don't need to symbolize anything. But if they can help us remember the "pillars of eternity," the creation, the fall and the atonement then they are even more beautiful and more fitting for the temple.
Elder McConkie gave a powerful final witness of Jesus Christ in general conference in 1985. This was the conclusion of his message:
As we read, ponder, and pray, there will come into our minds a view of the three gardens of God—the Garden of Eden, the Garden of Gethsemane, and the Garden of the Empty Tomb where Jesus appeared to Mary Magdalene.
In Eden we will see all things created in a paradisiacal state—without death, without procreation, without probationary experiences.
We will come to know that such a creation, now unknown to man, was the only way to provide for the Fall.
We will then see Adam and Eve, the first man and the first woman, step down from their state of immortal and paradisiacal glory to become the first mortal flesh on earth.
Mortality, including as it does procreation and death, will enter the world. And because of transgression a probationary estate of trial and testing will begin.
Then in Gethsemane we will see the Son of God ransom man from the temporal and spiritual death that came to us because of the Fall.
And finally, before an empty tomb, we will come to know that Christ our Lord has burst the bands of death and stands forever triumphant over the grave.
Thus, Creation is father to the Fall; and by the Fall came mortality and death; and by Christ came immortality and eternal life.
If there had been no Fall of Adam, by which cometh death, there could have been no Atonement of Christ, by which cometh life.
And now, as pertaining to this perfect Atonement, wrought by the shedding of the blood of God—I testify that it took place in Gethsemane and at Golgotha, and as pertaining to Jesus Christ, I testify that He is the Son of the Living God and was crucified for the sins of the world. He is our Lord, our God, and our King. This I know of myself independent of any other person.
I am one of His witnesses, and in a coming day I shall feel the nail marks in His hands and in His feet and shall wet His feet with my tears.
But I shall not know any better then than I know now that He is God’s Almighty Son, that He is our Savior and Redeemer, and that salvation comes in and through His atoning blood and in no other way.
God grant that all of us may walk in the light as God our Father is in the light so that, according to the promises, the blood of Jesus Christ His Son will cleanse us from all sin.