If you notice mottling, streaking, curling, or abnormal color patterns on the leaves of your geraniums, they may have been infected with a viral disease carried by insect pests. Once infected, there is no cure; infected plants should be destroyed to prevent the spread of the virus. A regular program of watering and fertilization will keep plants healthy and best able to ward off viral infection.
What diseases affect geraniums and how can I treat them? The first step in combating a plant disease or pest is to identify it. Here are some descriptions of common geranium diseases and their recommended treatments:
Southern root-knot nematodes: These microscopic, soil-dwelling worms can attack geranium plant roots and cause stunted growth, wilting and yellowing. Infested roots will display knots or swellings. These worms are most troublesome in warm-winter climates and sandy soils. Nematode infestation can be prevented with a regular program of watering and fertiliztion in conjunction with the use of compost to introduce beneficial soil organisms. Once infested, a plant cannot be treated; however, the organisms remaining in the soil can be killed through the introduction of beneficial nematodes or by solarization. To solarize your soil, cover it with clear plastic sheeting for three to four weeks. The build-up of solar heat under the sheeting will kill most pests and weed seeds in the top few inches of soil.
Leaf spot: Leaf spot diseases develop during warm, moist weather and cause leaves to display brown or black spots with yellow edges. Eventually, the leaves may drop and the stems may rot. Cut off and destroy infected plant parts as well as seriously affected plants and any soil their roots touch. Since the diseases overwinter on plant debris, if leaf spot occurs, keep the beds clean and replace the mulch. Because the diseases can spread via water, infected tools, or unwashed hands, avoid splashing water on the foliage and keep your hands and tools clean. Spacing plants widely will promote good air circulation.
Botrytis blight: Also known as gray mold, this fungal disease is spread by wind-brone spores and thrives in cool, moist weather. Stems and flowers will rot and develop gray mold, especially during damp, cloudy weather. To prevent its spread, destroy diseased plant parts. To prevent infection, space plants widely to promote good air circulation and allow the soil to dry out between waterings.
Pelagonium rust: This fungal disease causes small, yellow spots on leaf surfaces or powdery, orange spores on the undersides of leaves. Badly infected leaves and plants should be destroyed. Plants with minor infections can be sprayed with sulfur. Watering early in the day, widely spacing plants, and avoiding overhead watering will help prevent infection. What pests commonly attack geraniums? Our zonal geraniums are tolerant of tough conditions; they give new meaning to the term "flower power," blooming non-stop right up until frost. However, they may suffer from an occasional insect infestation. Some common geranium pests include the following:
Geranium aphids: These tiny, soft-bodied, pear-shaped insects suck plant juices and can spread viral diseases. They can be controlled by knocking them off the plants with a strong stream of water, an application of insecticidal soap (pay special attention to treating the undersides of leaves), spraying with horticultural oil, and releasing beneficial insects, such as parasitic wasps, ladybugs, green lacewings, and aphid midges.
Caterpillars: Several species of caterpillars chew geranium leaves. Caterpillars can be handpicked and destroyed or they can be controlled with an application of Bt (Bacillus thuringiensis) which kills caterpillars, but does not harm beneficial insects. Cultivating the soil in winter will kill overwintering pupae. Attracting or releasing beneficial insects such as parasitic wasps and ladybugs will also help to control caterpillars.
Other pests: Common garden pests, such as mealybugs, whiteflies, and spider mites, can easily be controlled by knocking them off the plants using a strong spray of water, watering plants well during dry spells, and attracting or releasing beneficial insects. More severe infestations can be treated by applying insecticidal soap or neem.
What is the best way to overwinter geraniums?
It is almost impossible to overwinter geraniums outdoors, but quite simple to do so indoors. To overwinter entire geraniums or scented geraniums, dig them out of their beds or window boxes and place them in a bright spot inside. Make certain not to overwater your plants during their stay indoors. Some gardeners prefer to grow cuttings of their plants indoors rather than try to overwinter entire plants. To start to root cuttings, remove bottom leaves and place in a glass of plain water on a windowsill. Make sure to protect all overwintering geraniums and new cuttings from freezing temperatures which can occur if your plants are left too close to icy windows.
Why won't my geraniums bloom?
A common reason many flowering plants refuse to bloom is an overdose of nitrogen. Many commercial fertilizers are extremely high in nitrogen. While nitrogen is an essential element for vigorous plant growth, too much nitrogen encourages leafy growth at the expense of blooms. Try supplementing your soil with a balanced organic fertilizer. You might also consider having your soil tested through your local county extention agency. This is a free service which will help provide you with valuable soil improvement suggestions.
What should I do to get rid of the spent blossoms on my geraniums?
The best way to deal with old and withered flowers is to deadhead the plants. Deadheading can be done at any time of year, by cutting or pinching dead buds. You can use any hand tool, pruning shears, or even scissors -- just be sure to avoid new buds and blooms. Deadheading will freshen the appearance of your garden, as well as promote longer-lasting and more abundant blooms.