consistency is all I ask

and how now every nonhuman Dreemurr has said it

In UT, the flowers in question are literally grown from Chara’s grave in both instances of this line. Both Toriel and Asriel are chained to the memory of Chara (and in Toriels case, Asriel to) in some form and their arc is about learning to embrace change/somthing new and Frisk by proxy. In Toriel’s case she is so focused on making sure another childs death doesn’t happen again that she self isolates herself in a tomb of the past. Then attacks Frisk when they try to leave.

In Asriels case it’s arguably worse, Asriel is so obsessed with holding onto Chara he doesn’t even conceive of Frisk as their own person til the end of the game.

In both instances this line is said after they complete their character arcs and learn to let go. Them tending to the flowers on Chara’s grave is a perfect metaphor for them learning to embrace the future while still honoring the past. Chara is dead, there is no bringing them back, but the flowers are still there, Chara’s memory still matters, and both Asriel and Toriel learn to honor Chara while embracing the future.

Chandelier Drive-Thru Tree, Leggett, California, USA