Geraniums are tender (frost-sensitive) perennials often grown as annuals for their heads of bright red, pink, salmon and bi-color flowers. You'll frequently see them referred to as Pelargoniums, the botanical name (true geraniums are hardy perennials and quite different in appearance). In addition to the common type, there are fancy Martha Washington geraniums and trailing ivy geraniums. They are all heavy feeders, needing frequent fertilizing.
Geraniums like a rich soil that is high in nitrogen (the first number on a fertilizer label), but also need phosphorus and potassium (the second and third numbers on the label) for strong stems and flowers. Composted steer manure adds both nitrogen and organic matter (useful for holding moisture) and can be dug into the bed before planting. Add bone meal (1/2 cup per square yard) for phosphorous. Or use a 5-10-5 fertilizer in the amount specified on the package.
A starter solution containing both fertilizer and Vitamin B1 is ideal for getting small plants growing quickly, with the vitamin helping new roots form after transplanting. In addition, if you don't want to fertilize frequently throughout the summer, you can add a pelleted time-release fertilizer. Choose one with three even numbers (also called NPK numbers for nitrogen, phosphorus and potassium), such as 10-10-10.
During The Summer
Without a time-release fertilizer, plants need frequent applications of a balanced (even NPK numbers) liquid formula, applied according to the directions on the package once a month. If your geraniums are in containers, choose a fertilizer with trace elements such as sulfur, calcium, magnesium, iron, zinc and copper.