In general, annuals and biennials are fairly undemanding when it comes to plant food, especially if they are planted in a nutrient-rich soil. Many successful gardens of these plants get by without any plant food at all, but conscientious gardeners know that a few well-timed applications of plant food will result in healthier, more robust plants and, in most cases, more flowers.
For excellent results, use a water-soluble plant food and feed your plants at the same time you water them. Your plants will also benefit from using a dry plant food. Just be sure to follow the application with a good watering. Proper feeding recommendations can be found on the back of any of these recommended plant food products.
Remember, the need for nitrogen is greater for plants in full sun than for those growing in the shade, so be sure to give plants in full sun a little extra attention.