Cultivars and Description

The German iris group has thousands of cultivars from which to choose. Most flower only in spring, but some varieties bloom again in late summer or fall. The blooms can be a single color or multicolored, and range from yellow, to red, to peach, to purple and blue. Generally, the flowers are classified by height into the following groups, from the smallest at 8 inches to the tallest at almost 40 inches: miniature dwarf, standard dwarf, intermediate, miniature tall, border and tall. The swordlike foliage grows from a rhizome in a fan shape, and remains evergreen in warmer climates.

Climate and Culture

German iris grows in a variety of climates from U.S. Department of Agriculture plant hardiness zones 3 through 9. It can withstand drought as well as cold weather. These flowers perform best in full sun, but some of the delicate pink and blue varieties hold their color better with some dappled afternoon shade. German iris prefers a light loam soil with a pH between 6 and 7, but will survive in a range of soil types as long as it has good drainage. Ideally, the rhizome remains hard and healthy with only a little moisture at the surface, while the roots that it sends further underground stay moist. In colder climates, apply a 6-inch layer of organic mulch after the ground freezes to avoid repeated freezing and thawing cycles and heaving of the rhizome. Clear the mulch away in early spring.

Planting and Care

The top of the tan-colored tuberous rhizome of this iris sits just above, just at or just below soil level. If a rhizome sits too far below the soil level, it will rot and die. In nutrient-deficient soils, the rhizomes benefit from a light application of 5-10-10 fertilizer at planting. Space individual rhizomes 8 to 10 inches apart and clumps 18 inches apart. After planting, keep the soil moist but not soaked through flowering. If the flowers grow well, they may not need fertilizer, but if growth flags apply 1/2 cup of 5-10-10 fertilizer per clump after flowering fades in spring, keeping the fertilizer away from the rhizomes. Cut foliage down to 6 to 8 inches above ground in fall, especially if the plant has a foliage disease. When the German irises become too crowded, dig up the tubers, cut them apart and replant them.

Problems

Poor drainage and improper planting cause many of the problems for this plant, including bacterial soft rot and crown rot fungus. Fungal leaf spot diseases can become problems on the foliage, but you can usually avoid them by using drip irrigation or a soaker hose instead of overhead watering. Aphids can transmit the deadly mosaic disease, which causes mottling of the foliage and eventual death. Remove and destroy any infected plants that contract this disease and control the aphid population to reduce the spread. Iris borer is probably the most serious pest of the German iris, tunneling into the rhizome and creating entry points for fungi that cause rot. You can physically remove these pests from the plant, cutting off chewed foliage, and use an insecticide to control the pests over time.