All living organisms need food of some type, and flowering plants are no exception. Different types of flowering plants have different needs for food, commonly called fertilizer. The type of food and the frequency your plants need it are linked to the condition of the soil they grow in.
Soil can be acidic, alkaline or neutral. Many flowering plants need slightly acidic soil, between 5.5 and 7.0. A pH of 6.3 is considered perfect for most flowers, according to the Soil Nutrient Analysis Laboratory at the University of Connecticut. Soil test kits are available at garden centers and can help you determine your soil's pH, which can differ from one location in your yard to another.
If the soil is sandy and tests below 5.5 (acidic), applying hydrated lime can raise the pH. Clay soil needs more lime, and peaty soil needs even more. If soil is alkaline, applying sulfur can lower the pH. Generally, sandy soils need about 1 ounce of sulfur for very square yard of surface area. Three times as much sulfur is best for other types of soil. Any change in soil pH will not take place immediately, but over a period of six to nine months.
Nitrogen, Phosphorus and Potassium
Fertilizers typically contain three main nutrients: nitrogen, phosphorus and potassium, which are referred to as the N-P-K ratio. A fertilizer with a low N, or nitrogen, percentage will encourage plants to flower--for example, an N-P-K ratio of 5-10-5 would be appropriate. This ranking usually applies to chemical fertilizers that are commercially available. Worm castings have lower N-P-K numbers of approximately 3.2-1.1-1.5, and organic compost comes in at 4-4-4, so natural plant foods require larger quantities and more frequent application.
Fertilizer helps annual flowers when added to their soil at planting time, usually spring. Any balanced plant food is suitable--for example, one having an N-P-K ratio of 13-13-13. Annuals also benefit from an application of the same fertilizer two months after planting and one final time in late summer.
Fertilizing Perennials and Ornamental Grasses
Adding fertilizer at planting time is also helpful for perennial flowers and ornamental grasses. A plant food with an N-P-K ratio of 13-13-13 is appropriate at that time and also six to eight weeks after planting. Established plantings of perennial flowers and ornamental grasses benefit from fertilizer in spring and again six to eight weeks later.
Fertilizing Bulbs, Roses and Wildflowers
Spring flowering bulbs will flower more when they receive fertilizer in the fall and when they pop up above the ground in spring. Five tablespoons of liquid fertilizer with an N-P-K ratio of 10-10-10 is recommended.
Roses need fertilizer in May, June and July. A liquid fertilizer with an N-P-K ratio of 10-10-10 helps them bloom.
Wildflowers need very little, if any, fertilizer because they are "designed" to survive outside of areas that humans tend, and most have low nutrient needs. Adding compost or another natural fertilizer to the soil at planting time is often all they need.