This time I bought the Red Bud from the potato seller the second chance I got. Do not talk to the potato girl and you will be able to buy it until the volcano explodes. However buying it on the second opportunity works if you are careful.
DO NOT complete the Ogre quest (Q28) until the plant becomes a Red Lilac or you will not be able to buy fertilizer that causes plant to double its crop.
And Fenlyr gives a relic after completing Q29 that extends plant life, so you might want to equip that immediately.
Here is the schedule I followed (with game hours in parentheses):Red Bud (aka Red Lilac)
Bought Before Ogre Tower, Growth=24 after finding Genevieve in tower;
Growth=31 (
now a Red Sprout) after returning lost girl in Tower (8hr,19min);
Growth=101 after blowing up Volcano (11hr, 38min);
Growth=170 when entering Bunker area (14hr, 35min);
Growth=183 taking roof to Canacopra (15hr, 42min);
Plant becomes
Red Germ after sleeping in “Z’s” bunker (16hrs, 5min);
Growth=59 after defeating EvolBatNest (16hrs, 25min);
Growth=61 just before Q#23 (17hrs, 51min);
Growth=70 when entering Lilith’s camp site (18hrs, 38min);
Growth=104 just before finding Random in Elven Forest (19hrs, 52min);
Growth=126 after finishing Q27 (20hrs, 55min);
Plant becomes
Red Young Lilac after returning to Random in base camp and sleeping (21hrs, 1min);
Red Young Lilac “Nice” comment Growth=10 (21hrs, 21min);
Growth=20 (22hrs, 33min) Received 2 items because I bought fertilizer in Pink Strawberry Fields after Ogre quest (Q28) completed.
Now, if someone with even more time to "waste" wants to calculate whether this is time or event sensitive...

I hope I haven't left anything out, but please feel free to add what you have found. I will report back later when I get farther into the game and note any growth cycle changes. This is such a valuable and curious item.