Alright so a B rank Genjutsu which can be done with no seals at all, and will have the enemy believe that a flower was encircling them before it ill explode, and after the explosion the enemy will actually think he took the damage, so the pain receptors, ans vision ones seem to be controlled by this skill. Best usages would be in the beginning of the battle, so you can end things quickly without really getting hurt. Another one is that is can perfectly be used in places with a lot of vegetation, so that he won't be suspicious of a flower growing in the middle of the water or in the desert for example. Another good usage would be close a very strong noise, since i don't think the genjutsu will control the hearing, so he would be suspicious of the flower petals making no noise.
Strengths to this skill is that, it requires no handseals, so you can almost instantly be able to put your enemy into a genjutsu, another one is that he will actually feel the explosion in the genjutsu and believe that he is wounded. Another strength is that this skill, will be able to fully leave the opponent immobile and he will not worry that he is in genjutsu, if he already had seen and believed the explosion. He will just fall on the ground.
Weaknesses is that I don't believe this genjutsu actually controls the senses of smell and the one of hearing. So if you are not actually able to hear the petals gather, or ultimately,hear them explode you will be feeling very weird in the first place. A second one would be that, if the flower will be growing under you, you will feel a certain vibration in the ground, or slight change of attitude, so if you can't feel that, Genjutsu alert!! And a last one which could be funny is that, if you are allergic to flowers, and you can't or won't sneeze,you will realize that you are in a genjutsu yet again...
I would deceive this genjutsu by relying on the sense of motion as I can't feel anything moving under me or even slightly around me, so how the hell can a flower grow without me feeling it. Another one would be that I can't hear or smell the petals, so if it will be growing around me and I hear, smell or feel nothing I will be able to use Kai-Release.