Keeping baskets looking good in summer can be a challenge. Plants in garden beds and even in large planters have a large soil mass to draw moisture and nutrients from. A hanging basket has a large amount of foliage and a small amount of root. The nicer and fuller the basket, the more pronounced this ratio.
Most problems occur shortly after baskets are brought home from the garden center. In June and July the days are the longest, the sun the brightest and the temperature the highest. The plant transpires water through its leaves to keep cool, plus it is growing at its maximum rate; the demand for water and nutrients is at its highest.
We come home to wilted baskets, we dump water on them and it runs out the bottom. We think we have watered the basket well, when in fact the water is running out of the drain hole because the soil in the basket has dried to the point that it repels the water. Dry potting soil does not absorb water easily.
Most basket failures come from water issues, usually too little. I would guess 90% of failures are due to lack of water, the other 10% over watering, insects and disease. How then do we know when to water? The easy answer is when its dry. Pick up a wilted basket and see how light it is. If it is light soak it until its heavy. Do not worry if you put too much water on, it will drain out of the holes in the bottom. From now on lift the basket to see how dry it is. Putting a finger in the soil is not reliable, but weighting it is. Get in the habit of lifting the basket a couple of times a day to judge its moisture. You may need to water twice a day in very warm windy weather, or every second day in cloudy, cool weather. The water should go on the soil not on the leaves. Do not mist hanging baskets, this can lead to disease problems. Plants should be dry and soil should be moist. Do not be fooled into thinking rain will water your plants. Large leaf plants like begonias are like mini umbrellas to shed water outside the pot.
Finally, since we have a large plant in a small pot we should add nutrients on a regular basis. Fertilize at least once a week, more often for wave petunias and calibrachoas. Any good all purpose fertilizer will usually be okay. Slow release fertilizers are an option but not as reliable. Although some use 1/2 ratio of slow release and 1/2 ratio of water soluble fertilizer with good success.