Wouldn't all your flower bulbasaurs actually be ivysaurs since they evolve when they bloom??
Contrary to popular belief the plant is not part of Bulbasaur itself - it is a separate organism that is planted there, as all good trainers should know from the Pokedex: “A strange seed was planted on its back at birth. The plant sprouts and grows with this Pokémon.”
Bulbasaur can’t survive without this plant as it depends on it for photosynthesis. The plant and Bulbasaur have a mutualistic relationship, where Bulbasaur receives photosynthetic energy from the plant and can also utilise it for defensive capabilities such as Spore and Vine Whip; the plant receives a host that protects it and also proliferates it when the Bulbasaur evolves and breeds.
As the plant is a distinct organism, it develops independently and can flower before Bulbasaur evolves. In fact the opposite of your statement is true - Bulbasaur doesn’t evolve when the plant blooms, rather the energy of evolution will force an unbloomed plant to bloom, meaning all Ivysaur will have a bloomed plant.
Bulbasaur and Ivysaur are distinguished by their own physiology, not from that of the plant that is on their back:
The leaf structure is an interface between the host and the plant, and does not truly belong to just one, which is why it changes upon evolution.
