Robert Ripley himself might not have believed it: The easy-care zonal geranium isn't a geranium at all. Its botanical name, Pelargonium x hortorum, places the plant in the Pelargonium genus native to South Africa. A corresponding love for sun, warm weather and relatively dry soil make zonal geranium perennial only in U.S. Department of Agriculture plant hardiness zones 10 through 11. For most gardeners, it's an annual beloved for reliable summer-to-fall color and an undemanding nature.
All zonal geraniums thrive with moderate watering and won't tolerate saturated or bone-dry roots.
Insert a finger into the soil or medium; water thoroughly when the top 2 inches feel dry. Check a pot daily during hot weather; water until liquid runs from its drainage holes. Expect to water less often in the winter.
A plastic pots retains moisture better than a clay one, but any geranium pot must have drainage holes.
Habitually letting a geranium wilt before watering may cause stunted growth and leaf loss.
During the spring-to-fall growing season, feed an in-ground geranium every two weeks with a solution of 2 teaspoons of water-soluble, 20-20-20 fertilizer dissolved in 1 gallon of water.
A potted geranium grows best with a high-phosphorous, micronutrient-enhanced fertilizer such as a 10-15-10 formula. One manufacturer recommends watering twice monthly with 1/4 teaspoon of its liquid, 10-15-10 plant food dissolved in 1 quart of water.
An overfertilized zonal geranium performs poorly. Never give it more than the brand label's suggested amount.
Prune as Needed
Remove dead, diseased or damaged branches from a zonal geranium as they occur. Cut them straight across and between 1/4 and 1/2 inch above a healthy leaf joint. If all the leaves are diseased, remove the entire branch.
A potted geranium benefits from monthly progressive pruning. Cut back one or two of the longest branches to about 7 inches from their bases. When new growth appears on those stubs, prune another one or two branches. Leave at least one or two healthy leaves on the pruned stubs, or they won't recover.
Prune with clean, sharp stem cutters disinfected between cuts in a solution of 1 part household bleach to 9 parts water. Dispose of pruned material in a sealed plastic bag.
Keep new flowers coming by pinching off the old ones at the bases of the stems with your thumb and forefinger.
Watch for Disease
The biggest health threats to a zonal geranium are Botrytis fungal and Xanthomonas bacterial blights. Botrytis covers the plant with fuzzy, brownish-gray mold. Xanthomonas causes water-soaked, brownish spots on the backs of the leaves. As it progresses, the entire plant may wilt.
Watering at the base of the plant early in the day to keep the leaves dry helps prevent blight.
When early Botrytis symptoms surface, cut off the diseased plant parts and dispose of them in the trash. Once mold appears, it's best to dig up and dispose of the entire plant.
A Xanthomonas-infected geranium can't be saved. Remove and dispose of it as quickly as possible.
After pruning or removing an infected geranium, disinfect tools and wash your hands thoroughly before handling other plants.