Most annuals do not require high levels of fertilizer, and will do well with just the initial fertilization during soil preparation. However, any check in growth caused by insufficient nutrients can reduce the quality of the plant and its bloom, so its usually better to make an additional application during the growing season. About 6-8 weeks after planting, apply about one-quarter to one-half the recommended bed preparation rate of fertilizer to the planting bed. Whenever a dry fertilizer is used, follow its application with a good watering to remove fertilizer from the foliage. Liquid fertilizers can also be used. It is suggested that they be applied to damp (not dry) soil.
Deep, infrequent watering is much preferred over frequent, light watering. The former encourages a deep root system. The amount and frequency will depend on natural rainfall and the type of annuals grown. The foliage should be kept dry to help prevent foliar diseases. When this is not possible, water early enough in the day so the foliage dries off before nightfall.
After annuals are planted, it is suggested that a 2-3 inch layer of organic mulch be applied around the plants. Not only is it attractive, but it also helps conserve soil moisture, retard weed growth and keeps soils cool. Some good mulches to consider are compost, shredded leaves, dry grass clippings, bark chips, hulls, and pine needles. In the fall, the mulch can be incorporated into the soil adding organic matter and helping to improve soil structure.
This is not the most fun activity, but it is essential to keep the weeds out and preventing competition for space and nutrients in the garden. Weeds should be removed as soon as you see them with shallow cultivation. When mulch is used and the canopy of the flowerbed grows closed, weeds should not be a major problem.
Many annuals require little additional care to keep them attractive and blooming all summer. Some flowers fall off cleanly and do not need to be manually removed. Others require "deadheading." An annual lives in order to produce seed. All of its energies are directed toward this task. If you "deadhead" – pick the spent flowers before they start to set seed – the plant will produce more flowers in an effort to ultimately produce seeds. This practice keeps annuals in the flowering stage longer and usually results in a greater number of blooms. Annuals such as marigold, zinnias, salvia, geranium, cosmos and other spike or single stem flowers benefit from this practice.
In order to control the growth of some annuals, pinching or the removal of the growing tip is suggested. This will encourage more compact growth and a neater habit. The tops of some plants, such as petunia and impatiens, may be cut back 6-8 inches in mid to late summer after the first flush of flowers has subsided to promote a second flowering period in the fall. A good time to do this is right after the Fourth of July holiday. After cutting, fertilize and water well to encourage regrowth.