Proper watering is one of the surest ways to ensure your outdoor plants are healthy and growing well. Too little water and the plants wither and die. Over-watering is less common, but the results are often the same as too little water. More water is not necessarily better. It may cause fungus to grow on the soil or on the plants, and overly wet soil may suffocate the roots so they can't properly pull nutrients from the soil.
Check the soil and the leaves, especially the underside of the leaves, for signs of fungus. Powdery mildew is common among over-watered plants and appears as a blueish-white powder.
Examine the lower leaves on the plant and those close to the stem on bushy plants. If the leaves are yellowing or showing discolored spots, they are likely over-watered.
Stick a trowel 5 to 6 inches into the soil, then remove it. If the soil clumps and sticks to the trowel, it is likely too wet.
Allow the top ½ to 1 inch of the soil dry out between waterings. Most plants require 1 to 2 inches of water a week in a single, deep watering as opposed to frequent, shallow watering.
Work organic matter into the soil around the plants to aid drainage so the soil doesn't become waterlogged during rain storms. Use 2 to 3 inches of mature compost for any soil type, or coarsely shredded bark for hard, clay soils. Lay it around each plant and work in with a hand cultivator tool, taking care not to damage any plant roots.