Susceptible to much fewer pests and diseases than other garden irises, Siberian iris (Iris sibirica) grows easily in flower borders and in massed landscape plantings. Native to eastern Europe, its foliage remains attractive even after the typically blue, blue-violet or purple flowers wane by midsummer. Grow them in USDA Hardiness Zones 3 through 8.
Generally speaking, Siberian irises should receive a minimum of six hours of sunlight daily to promote healthy foliage and robust production of flowers. In high latitudes where winters are colder and summers cooler, full sun exposures favor plants. In contrast, in warm temperate regions where winters are milder and summers hotter and longer in duration, target no more than six to eight hours of sunlight, with shade in the hottest part of the afternoon, from 1 p.m. to 5 p.m. Intense heat and sunlight leads to short-lived flowers, fading petal colors and potentially scalding or browning foliage.
Siberian iris relishes a fertile, moist, organic-rich soil that drains well after rains or irrigation. Ideally, the soil pH should fall in the range of slightly acidic (6.0 to 7.0). Avoid garden sites with alkaline soils, although intensive work to change the soil's pH remains an option. These irises demonstrate tolerance to summertime drought, but watering plants after flowering to ensure 1 inch of water reaches the roots weekly encourages plump roots leading into the winter dormancy.
The use of organic mulch, compost and man-made fertilizers also helps plants grow their best in the garden soil. Balanced fertilizers, such as 10-10-10, should be added in doses per the product label in early spring as new foliage emerges and then again only after the flowering ends in midsummer.
Unlike bearded irises, Siberian irises prosper and flower well for many years before gardeners need to lift, divide and replant the rhizome roots. Once a plant clump becomes massive and a noticeable drop in flower production or foliage size occurs, plan on dividing it. In cool summer regions, divide and replant rhizomes in early spring or, as a second choice, in late summer. In hot summer regions, conduct dividing and replanting only in early fall.
Divide iris clumps into rhizomes supporting two to four fan-like sprays of foliage. Replant these divisions back into the soil at a depth of no more than 1 inch of soil atop the rhizomes. Water them in to compact the soil and settle the soil around the roots. Consider a protective layer of mulch around these newly planted divisions over the winter to prevent the soil heaving that occurs with alternating freezes and thaws. Place the mulch atop the plants only after the soil and air cools in mid- to late autumn. Pull it back in early to mid-spring so sunlight again warms the soil.