No all flowers are alike, as any gardener knows, so you will need to properly identify the type of flowers in your garden before you can determine the best time to fertilizer your flowers. Flowering times vary from flower to flower, and this will heavily determine when you fertilizer your flowers. There are perennials and annuals, not to mention all the flowers that bloom at different times, such as spring, summer, and some even in the fall. Because annuals only bloom once, you cannot wait until the fall to fertilize them as you would with most other plants because by then it doesn't matter. The plant would have bloomed and withered already and no amount of fertilizer can coax it to come back to life after it has already died. Therefore, fertilization should be done while the annual is being planted and as you are preparing the hole to plant it in. Be careful not to use any chemicals that may harm or 'burn' the roots of the plant. This will inhibit the plant from developing strong and healthy roots and may cause it to fail entirely.
Perennials should be treated in the same way, however, because they don't die within a year, they will require another treatment sometime after their initial planting. Therefore, it is recommended that you fertilize 6 to 8 weeks after the initial treatment in the spring and then again each spring afterwards. You should continue this process for the duration of the plant's life.
For plants that flower in the spring, you will want to fertilize them as soon as new growth emerges. Do not apply fertilizers to flowers that bloom in the summer at this same time. These plants will have to be fertilized when new growth begins to emerge in the early summer. It is important that you don't fertilize at improper times, not because it will cause severe damage but because it will be wasted time and effort.