That's not quite true. The Bio-Organic Plant Food causes plants to produce more flowers. It doesn't affect how fast flowers are produced. The Fertilizer Bomb is a "combination" plant food that affects all plants growing in the greenhouse when it is used. It makes plants grow larger and produce more flowers (just like the description in the game says).

The effects of applying more than one chemical (or more than one dose of the each chemical) are cumulative (to a point - you can determine how many doses works best for you).

I usually apply the various plant food types immediately after I plant the seeds, but it can be done any time before the plants produce buds.