Geraniums make attractive additions to the landscape when grown in containers at entrances, on decks and patios, or in window boxes. Choose pots at least 8 to 12 inches in diameter, large enough to hold enough substrate for a good root system, and to contain enough water to prevent wilting during hot, dry weather. Plants may need to be repotted into larger containers later in the season if they have grown large and begin to wilt between waterings. Water as often as needed to prevent wilting, because the leaves of wilted plants turn yellow and drop off, resulting in unattractive, leggy plants. Select containers with adequate drainage holes, and do not allow pots to sit in saucers of water. Instead, place the pot on a layer of gravel in the saucer. Avoid overhead watering which wets leaves and flowers, which can encourage development of diseases. Do not use heavy clay garden soils which may not provide sufficient aeration. A mix of equal parts of garden soil, peat moss and perlite makes a good growing medium. Leaf mold, composted pine bark or composted yard waste can be substituted for peat moss. Mix in ¹/2 teaspoon of a complete garden fertilizer plus one teaspoon of dolomitic limestone per volume of each 8-inch pot. Commercial potting mixes are ready to use, light in weight, convenient for gardeners and support good growth of geraniums.
Source: North Carolina Cooperative Extension Service 
Alice B. Russell Extension Specialist