The geranium, Pelargonium xhortorum, is widely grown in flower beds, containers, and hanging baskets. A wide variety of flower and foliage colors are available. From the true orange flower of 'Orange Appeal' to the fluorescent shade of 'Orbit Violet' there is surely a color to coordinate with any landscape plan. Flowers are packed densely on umbels rising above the plant foliage. Flowers may be single, double, or semi-double. In past years, most varieties were grown vegetatively from cuttings. Today, many varieties are available from seed. Plants grow 12 to 20 inches tall. Some of the named series, such as 'Elite' and 'Orbit', are known especially for compact growth which is desirable for containers and bedding displays.
Geraniums grow best in full sun. They like moist, well- drained soils and prefer a cool root zone. To keep flowers coming continuously throughout the summer, regular deadheading is necessary. Remove the spent flowerheads as they begin to deteriorate. This will prevent seeds from forming and force the plant into producing additional blooms. It also improves plant appearance as well as reducing the chance for disease.
Like all plants, geraniums have their share of disease and insect problems. One disease being seen this season is bacterial leaf spot. This disease is caused by the bacterium Xanthomonas pelargonii and is especially prevalent in warm, wet weather where plants are grown in crowded conditions. Disease symptoms include small (pinhead size), circular or irregular, brown, sunken spots on older or lower leaves. Large numbers of spots will occur on a single leaf, these will coalesce killing a large portion of the leaf which will then drop off. As the disease moves through the plant, the lower leaves wilt and yellow. In severe cases, the stem will possess black stem cankers killing the upper portion of the stem. Leaves infected with bacterial leaf spot should be removed as soon as it is noticed. Severly infected plants should be removed. There is no chemical cure for bacterial leaf spot. Make sure and destroy infected plants and plant parts this fall.
Another common disease of geranium is a fungal disease known as botrytis leaf spot or botrytis blossom blight. It is caused by Botrytis cinerea. Botrytis is favored under cool, moist conditions or where plants are watered frequently. Leaves develop zonate, brown leaf lesions which develop a grayish brown mass of fungal spores. The lower leaves will yellow and rot. Flowers may also become infected. They show discolored petals which wilt and fall. Remove affected leaves and flowers. Fungicide sprays, when environmental conditions are favorable, will help reduce levels of this disease. Chlorothalonil (Daconil 2787) is widely available for homeowner use and will help to control botrytis. Rust, root rots, stem rot, and leaf spots are other diseases known to infect geraniums.
Insects that frequently attack geraniums include aphids, cabbage loopers, and fall cankerworms. The four-lined plant bug, scale, and slugs can also cause damage. Properly identify the insect pest and control with the recommended insecticide.
Geraniums are one of the most popular annual plants grown by gardeners today. Proper selection, location, and care will keep them blooming and healthy all season long.
This article originally appeared in the July 29, 1994 issue, p. 124.