SIBERIAN IRIS CULTURE
reprinted from 'Basic Iris Culture'
Siberian irises are outstanding perennial landscape plants. They are excellent in mixed borders where their grass-like foliage is an asset throughout the growing season. The delicate flowers extend the bloom season and can be used in flower arrangements.
They will grow under a wide range of climatic conditions. Most widely grown are the 28-chromosome group and the tetraploids derived from them. These Siberians are extremely cold hardy. However, they can be challenging to grow in the South due to their need for a cold dormant period to perform well. 40-chromosome varieties of Siberian irises grow best in the Pacific Northwest. Although their culture is in many ways similar to that of the group described above, they require a more constant moisture level and are generally less tolerant of extremes of heat and cold.
Siberian irises grow and bloom well when planted in full sun. However, they will tolerate shade for less than half a day. Bloom will be directly proportional to the amount of sun received. All beardless irises prefer a rich soil with ample organic mater. If the soil is clay, the addition of organic matter will loosen it. If the soil is sandy, the organic matter will help in water and nutrient retention. Depending on what is available, till in an ample amount of well-rotted manure, hay, straw, peat moss, compost, etc. If using hay or straw you may have to add high-nitrogen fertilizer to compensate for the nitrogen tied up in the decomposition process. Ideally the soil should be slightly acid, pH 5.2-6.4. If the soil pH is too high, it can be lowered by the addition of granular ferrous sulfate, aluminum sulfate, or agricultural sulfur.
Siberian irises are best transplanted in the early spring as growth is just beginning, or in the early fall. Some gardeners succeed by transplanting immediately after bloom in areas where summers are not hot and dry. Generally, fall transplanting is best for hot regions and spring transplanting is recommended for areas with cold winters. The roots must be kept moist while out of the ground. If plants must be held, they can be placed in a few inches of water to cover the roots. New transplants must also be kept watered once or twice a week until fully established. Often the leaves of newly transplanted plants will turn quite brown. This does not seem to hurt the plant and new green leaves should eventually appear.
Plant Siberian irises in a naturally moist area or in an area where they can be watered until the new plants become established. Planting in a small depression (1 to 3 inches below the surrounding soil level) will help the plant receive extra water during rains or when watered.
Planting in the cool of the evening is best. If the weather is very hot, it might help to shade the new transplants in some way. Established plantings of 28-chromosome Siberians are very drought tolerant.
Plants should be fertilized only slightly when planted, if at all. (The later they are planted in the fall, the less fertilizer should be used.) Siberian irises are heavy feeders. Alfalfa (pellets or meal) has proven an effective soil amendment or top dressing. A liberal application of a balanced fertilizer, such as 10-10-10, in spring and again just after bloom is beneficial. Foliar feeding with chemical fertilizers like Miracid may help weak plants to grow better.
After planting, mulch at 1 to 3 inches. Oat straw, pine needles, rotted sawdust or wood chips are recommended. The mulch conserves moisture as well as inhibits weeds. Do not mulch with peat moss or grass clippings; both pack down and become impervious to water. Failure to mulch is the most common reason for failure.
Siberian irises may be grown in pots, and it is often necessary to "pot up" plants which are received from other areas of the country. Plants which are grown with irrigation or in high rainfall areas develop a characteristic root structure and will have a difficult time adjusting to dryer garden conditions. Those from dry areas may adapt more readily to higher moisture. Pot in a balanced potting soil, to which has been added some of the natural soil in which the plant will be expected to grow. Pots may be set in an inch or two of water until the plant becomes established, up to a month or two. Siberians grown over long periods in standing water will grow roots that are long and straight, the plants will become dependent upon high moisture, and eventually they will deteriorate. It is best to remove the pots from the water about one day per week and allow the soil to dry slightly.
Siberian irises will often set seed in "bee pods." Unless these are grown for dried arrangements, remove the stalks after bloom is over to save plant energy, to give a neater appearance and to keep unwanted seeds from falling into the garden. These could germinate and grow into plants that are generally less desirable than the original variety. Bloom stalks may be broken off at the top of the rhizomes by pulling them toward the center of the clump, or they may be cut.
The attractive fall golden brown foliage can be cut back after a hard frost, or before new growth begins in the spring. The old foliage makes a natural mulch for plants in very cold areas but may also provide a warm shelter for rodents, which will eat the rhizomes and tender shoots.
Siberian irises should be divided about every three to five years or when the center of the clump begins to die out and quantity of bloom decreases. Dividing a mature clump of Siberians may present problems as the rhizomes often grow very tightly together. Dig the entire clump and remove several nice divisions from the outer part of the clump, replant those, and discard the hard center of the clump. If more divisions are desired, the clump may be broken apart using very strong fingers or in some cases a heavy spading fork, shovel, hatchet or ax can be used. The leaves of new transplants should be trimmed to a length of 6 -10 inches.
The adaptability of Siberian irises and their persistently neat, green foliage throughout the growing season make them an 'asset to any landscape. Planting in a moist sunny area may be all that is required for years of pleasure
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-- Main.RPries - 2011-03-07