Japanese Iris Hardiness
Japanese irises are comfortable as far north as the Great Lakes and central New York, surviving into New England in the milder coastal regions (zone 4). They can take winter temperatures as low as – 20°F.
Locating And Planting Japanese Iris
Japanese irises respond enthusiastically to proper planting conditions and care, growing larger and stronger if their needs are met. Plant them where they can get full sun or, at least, morning sun. While many mistakenly think they are bog plants and must be planted only in or beside water, Japanese irises also do well in regular garden beds as long as they are watered adequately so that the soil never totally dries out. These irises prefer a soil that is acidic (pH 5.0 to 6.5), that is rich and heavy with organic matter and that drains well in winter.
Plant in either the spring or the fall, but the best time is shortly after a plant has bloomed in the fall. Loosen the soil 1 foot deep when preparing the bed for planting. Then plant two or more rhizomes together in a shallow indentation in the soil about 2 inches deep. In light soil, they can go three inches deep. Unlike the rhizomes of bearded iris, those of beardless, Japanese iris are smaller knobs, joined into tightly webbed clumps by lots of small, fleshy roots. Cover them with soil, water well, and mulch (see below). Space plants about 15 to 18 inches apart. They will fill in rapidly and become crowded in 3 to 5 years.