by John Coble
Siberian irises are among the easiest of all types of iris to raise and bloom in the temperate climatic zones. Their graceful stems, blooms foliage and neat habit of growth make them the most adaptable irises for the perennial border and for landscaping. Their handsome foliage is attractive all year, even after the first frost when it turns rusty red-brown.
Our garden varieties of Siberian irises come to us through hybridizers, from two species, Iris siberica and Iris sanguinia, which are primarily found in Central Europe and Asia. There they evolved in the rich soils of grassy meadows; very moist in spring from flooding streams and mountain snow melts, moistened by summer rain on a deep prairie-type soil and provided with a natural mulch of old foliage and dead grasses which kept them from drying out.
Your Siberian irises will perform best for you if you can provide a rich soil with some composted organic matter if necessary that will retain some moisture during dry periods and will also help you attain mildly acidic pH 6.5-7.0, which they prefer. Most garden soils though, are satisfactory “as is” for good Siberian performance.
This depends on your own soil type and its natural fertility. The plants will respond better with an early spring application of a fertilizer higher in nitrogen, followed by an additional feeding of a balanced fertilizer just at the end of the bloom season. This is when they are maturing new increase fans and rhizomes.
Sun and Moisture for Siberian Irises
Siberian irises are very adaptable and hardy plants. They like lots of moisture in the spring and can survive dry periods in the late summer months but will be healthier plants and develop into specimen clumps faster if kept moist all summer. Try to plant them with other perennials that you normally irrigate during dry periods in July and August. They love full sun (especially in the northern areas) but will grow in light shade.
A mulch of organic matter will benefit the plants in summer by conserving soil moisture and keeping the soil cooler. The mulch is also very beneficial in keeping down most weeds. Siberians are very hardy, but a mulch applied after the ground is frozen helps prevent the heaving and thawing which is responsible for the loss of many perennials over winter.
Siberians are more resistant to disease than most other garden irises. They are, however, not immune to the iris borer. If this is a pest in your area, a systemic insecticide spray is necessary for control. Two applications of Cygon 2E are recommended in the spring. The first, after the first few days of 20 degree Celsius weather, when the fans are three to four inches tall and the second spray when the fans are six to eight inches tall.
Transplanting and Dividing of Siberian Irises
Opinions differ on this subject, and your local climate and gardening practices will influence your preferred time for transplanting, as well as other cultural suggestions made here. A recommended time for digging and dividing older clumps is right after bloom.
At this time new root growth is still active. However keeping transplants moist for the following 6-8 weeks is most important for successful establishment and hot dry weather may decrease survival. So this approach may work best in the cooler, wetter parts of the country. If watering is not always possible at this time, you may prefer early September transplanting when fall rains are somewhat more predictable and the sun’s rays less intense.
Others have found that early spring transplanting as new growth is evident can also be effective. The late spring transplants, if properly cared for will produce more vigorous plants for the next year. Two to four fan divisions are recommended for transplanting and the roots must be kept moist whilst the plants are out of the ground.
Plant the rhizomes one inch deep (slightly deeper in sandy soils) Siberian clumps can grow undisturbed for several years, dividing being necessary when either the clumps become crowded or when vigor declines and blooms get smaller.
Siberian irises normally bloom at the end of the tall bearded season, 24-34 inches tall, with foliage that continues to grow after bloom until 36-40 inches tall.
Some varieties are available that bloom around 10-12 inches tall with neat foliage clumps around 14-16 inches tall by summer. Hybridizers have produced a palette of colours from purples and blues to white, pink lavenders and creamy yellows.
Bitones and special colour patterns of sharp contrast offer great diversity when choosing Siberian irises for special plantings and landscaping. Some varieties have silvery blue foliage, some a fresh green, that is most attractive all summer in perennial borders.
Iris flowers are generally identified by a structure of three upright “standards” and three lower segments known as the “falls”. Siberian irises are of the beardless category.
You may also wish to visit the website of the Society for Siberian Irises (SSI) which is an organized section of the American Iris Society that exists for the culture, enjoyment and improvement of Siberian irises.