When I fertilize certain annuals, why is there more foliage growth than flowers? I believe cosmos are one of them.

It could be what you are fertilizing with. Complete fertilizers contain three primary nutrients, listed as three numbers on the fertilizer package, such as 12-12-6. They represent the percent of nitrogen, phosphorus and potash contained in that product. These three numbers appear on all fertilizing products and are always listed in the aforementioned order. A properly balanced fertilizer, in combination with other critical growing factors such as water and sunlight, can promote growth and flowering as well as enhance vigor, general hardiness and disease resistanc

If one of these critical elements is lacking or is extremely out of balance with the other nutrients, the fertilized plant may not perform as expected. For example, if you used a lawn fertilizer with an analysis of 34-4-4 on your cosmos, the overbalance of nitrogen is likely to produce a very bushy, green plant with few flowers. A better combination of nutrients would be an evenly balanced fertilizer such as a 12-12-12 or a 20-20-20. Once the plant is well established, a fertilizer higher in phosphorus (the middle number) will generally enhance and improve the plant’s ability to produce abundant flowers. Consider using a 5-10-5 granular fertilizer or giving the plants a “punch” of water soluble fertilizer (fertilizers intended to be mixed with water and used as a watering solution) such as 10-54-10.