Growing Siberian Iris

'Caesar's Brother'

Siberian irises are favored by many gardeners  over the more popular bearded irises for their ease of maintenance, longevity, and disease and pest resistance. Their flowers are smaller than bearded iris blooms, but their grass-like foliage looks attractive all season long. Siberians are also more cold and heat tolerant.

Siberian irises are mostly hybrids of two southwest Asian species, I. sibirica and I. sanguinea (USDA hardiness zones 3-9). The narrow, grass-like foliage forms a vase-shaped clump to 2 feet high. Autumn foliage turns rusty brown in late fall. Flowering stems rise above the foliage to 3 feet tall in May-June.

Siberians perform well in most garden soils, but are at their best in humus-rich, mildly acidic soil. In spring feed with 10-10-10 or an equivalent granular fertilizer. In early summer apply a water soluble fertilizer such as Miracle-Gro™ or Jack’s™.

In Northern states (zones 3-6), Siberians want full day sun (or 6-hours minimum). In Southern areas, shelter from the mid-afternoon sun. Ample spring moisture is key, along with adequate rainfall during the summer. These iris clumps tolerate moderate summer dry periods of a week or two.

Mulch iris in spring to conserve soil moisture and prevent weed infestation. Voles and mice may munch on rhizomes. Remove spent bloom stalks, and brown and withered leaves to keep planting looking attractive. In late fall cut back all leaves 1-2 inches above the ground.

Siberian irises are generally pest-free with very little susceptibility to the two main problems of bearded iris, namely iris borer and bacterial soft rot. Plants tolerate deer and rabbits.

Divide big healthy clumps into 2-4 fans in either late summer or fall, depending on where you live. In northern zones spring or late summer are the best time to plant or divide. In southern climes, early fall is a better time. Clumps can grow undisturbed for several years, unless they become crowded, plant vigor declines, or bloom size declines.

Over 200 varieties are available, principally from online iris nurseries.

Soil Test Before Planting

Tilled soil ready for planting

Soil testing is your best low cost insurance for enjoying a beautiful garden or lawn through this year and years ahead. In mid-winter, before the rush to spring garden begins, take soil sample(s) to the local Extension office or mail directly to your State University soil lab. 

The local Extension office may charge you a very modest fee, usually $10-12 per sample in most states. Many local garden centers and farm cooperatives may also offer this service.

Within a few weeks a soil analysis report will arrive in the mail. It tells you the current nutrition level and pH of the soil. Also, how much chemical fertilizer, limestone, manure and other organic components to add. For organic farmers and gardeners, inform the soil lab staff that you are growing organically.

This accurate diagnosis will save money. Your garden soil may be rich enough to not need any fertilizer, or too high in certain nutrients, lime or sulfur. The soil pH may be too acidic (below pH 7.0) or alkaline (above pH 7.0). Many garden plants as azaleas, rhododendrons, dwarf conifers and blueberries prefer an acidic pH around 5.0. A soil high in alkalinity (pH above 7.0) could mean that some minor nutrients like iron or manganese may be unavailable to some plants.

How to collect a sample: Test your garden or lawn soil at least once every 4-5 years. You may want to separately test lawn, vegetable, fruit, and flower garden areas. If you specialize in growing roses, test this garden spot separately. For each garden section, randomly collect (dig) 4-5  spots to a depth of 5-6 inches into a bucket. From the bucket mix the sample and collect enough soil to fill a large baby food jar. The soil should be aired dry for several days before mailing it to the soil lab. Label the sample "roses", "vegetables", or "flowers".