How To Fertilize And Water Annual Flowers And Bedding Plants
Annual bedding plants, such as begonias, petunias and pansies, are those which are useful to provide seasonal splashes of vibrant color throughout the landscape, given they're fed and cared for properly. They're called "annuals" because their lifespan is for only for one year, or one season. 

Annuals are most often planted outdoors in prepared flower beds or containers (container gardens). Annuals are popular because they provide gardeners a way to change up the look of the landscape from one season to another.

In areas that experience cold winter with freezing temperatures, there are two basic seasons for annual flowers: warm season and cool season. 

During the spring and summer, there are literally hundreds of warm season annual plant types to choose from. Among some of the more popular types are begonia, geranium, impatiens, million bells, petunia and salvia, to name a few. When planted and cared for properly, most warm season annuals will bloom from planting time in spring until the first frost in fall or early winter ends their cycle. 
Once the warm season annuals have completed their cycle it's time to replace them with cool season annuals such as the ever-popular Pansies and their dwarf cousins: Violas.

Why Fertilize Annual Plants?
The two primary reasons for fertilizing annuals are to encourage growth, and create healthy, vigorous, attractive plants that will produce an abundance of flowers. But be careful! There is often a temptation to over-fertilize in the hopes of producing more blooms, faster. If you force an annual plant beyond its natural growth rate by over-fertilizing, you might end up with mostly foliage and no blooms. Too, over-fertilization can predispose the plant to insect or disease infestation, and reduce tolerance to drought or temperature extremes.

Evaluate Soil Conditions
There's usually not a need to perform a soil test before planting annual flowers, however, professional seasonal color installers might use a soil testing kit to test for nutrient deficiencies or pH as their livelihood depends on guaranteeing spectacular flower beds for their customers. If a new annual flower bed is prepared properly, or an existing one refurbished between the seasonal rotations, fertilization methods are quite simple.

Fertilizing Instructions

Before fertilizing annual bedding plants, you need to make sure that the soil beds you intend to plant have been properly prepared. Without a suitable planting bed, you will never get satisfactory performance from your annual bedding plants. Unlike many hardy ornamental shrubs, trees and other plants, which grow well in the ground (native soil), most annual bedding plants require and prefer a much lighter soil and sharp drainage. Constantly soggy or wet soil can and will often lead to root rot and other harmful plant diseases. To provide optimum growing conditions, it is best to plant annual flowers and bedding plants in a "raised" or "mounded" flower bed.

Types of flower fertilizer

There are two basic types of fertilizer for use on annual flowers in garden beds or in containers:

Granular Fertilizers - These are useful in both garden beds and container gardens. I use granular slow release flower food in all my annual flower garden beds. You can fertilize at time of planting with a slow-release, granular or capsule-type flower food that will last for the entire season. I mix some in with the soil during planting time, and then broadcast some on top before placing the mulch. Some granular fertilizers will slowly release over the entire season and others for only 4 to 6 weeks. Always follow application instructions on product label. 

Water Soluble / Liquid Fertilizer - If you like feeding your plants on a more regular basis, you can use a water soluble fertilizer to liquid feed every week or two, or on an as-needed basis. I liquid feed all my container flower gardens. Why? Because the more frequent watering of plants in container gardens leaches granular fertilizers from the soil through the drainage hole more quickly. Too, I spray both the foliage of plants and water the soil, which seems to keep the foliage greener. That being said, there are timed release fertilizers that are more resistant to leaching. When using fertilizers, always follow application instructions on product label.

How To Water Annual Flowers

There are many, many types of annuals, so, without going into details about each type, here's some general guidelines and tips for watering.

Many annual plants, such as impatiens and other leafy types, like a consistently moist or damp soil, but not one that is constantly soggy or wet, which can lead to harmful plant diseases and often death of the plant. Others, such as begonia and portulaca, like the soil to dry somewhat between waterings. 

I often use observation to determine whether my annual plants need water. If the leaves of a plant are wilted, I check the soil moisture. If during the heat of summer wilting occurs while soil is still moist, it is a sign that there might be root damage from too much water previously. The plants I have growing in pots wilt more often than those growing in garden beds, which is common. If plants are not wilted chances are there is still sufficient moisture in the soil. 

If your annual plants growing in pots are wilting too much and require more frequent watering than you can handle, consider moving them to a location where they'll get some shade or filtered sun during the hottest part of the day. Or arrange some taller plants around them to provide some shade. 

Too much direct sun on some types of annual plants growing in pots can cause root temperatures to rise to a level where metabolism slows dramatically and water movement to the rest of the plant is seriously impeded. Growing plants in light colored containers can help keep soil temperatures down. Plants growing in cooler ground temperatures usually don't have this problem.

In general, whether you're watering annual plants in garden beds or containers, if you keep an eye on your plants and use the finger-test to check the soil to a depth of at least a couple inches each time before you water, you'll soon develop a feel for when water is needed. If the soil is dry, provide water. If moist, leave the plant alone...even if the leaves are wilting. 

 Err on the side of watering less, not too much, as it's much easier to kill plants by overwatering than by underwatering.

Other Helpful Tips

  • Be careful not to apply fertilizer too heavily. Doing so may cause the plant tissue to burn, or even result in plant death. Read product labels carefully and follow directions to avoid toxicity problems.

  • If you're worried about over-fertilizing your plants, you might consider easing your mind by using a "goof proof" natural or organic fertilizer. Organic fertilizers are usually made with natural ingredients such as composted manures or other organic matter, and as a result are much less-likely to burn your plants.
  • Avoid using fertilizers that have a high nitrogen (N) content. Nitrogen is the first of the three numbers on any package of fertilizer. Too much nitrogen can cause too much foliage growth, and too little flower production. Many annual bedding plants appreciate more phosphorus (P), which is the middle of the three numbers.

  • As a general rule, the slower the plants habit of growth, the less fertilizer it needs.

  • Be careful not to over-water. Most annual plants do not like a constantly soggy soil. Check soil moisture before watering. If the top inch of the soil in container gardens is dry provide water. If the top 2 inches of soil is dry in a garden bed provide water.

  • A 1-inch layer of shredded wood mulch or bark chips around the plants in garden beds or containers can help to conserve moisture in the soil.