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Cut off faded Japanese iris flowers to maintain an attractive look and stimulate new ones. Once the blooming period is over, cut off the old stems. Do not, however, cut the leaves back severely after flowering. The plants need their leaves for the rest of the season to store up energy for next year.
Watering Japanese Iris
Japanese irises require lots of moisture, about an inch a week. If they are in a regular garden bed, run a drip system when rainfall is sparse to keep the soil from drying out. Mulch the plants well. Near a pond where the water table is just below the soil surface, they can manage on their own. For information on products see the file on Choosing Watering Equipment
Fertilizing Japanese Iris
Japanese irises are heavy feeders and should be fertilized twice during the season. At planting time or every spring sprinkle a general purpose fertilizer formulated for acid-loving plants on the soil near the irises. Do not allow it to fall on the stems or leaves. Repeat this later in the summer just before they begin to bloom. As with other bulb plants, do not use manure.
Mulching and Weed Control
A 2 to 3 inch layer of chopped leaves, shredded bark or other attractive organic material spread on the soil over the iris rhizomes protects them. In the summer it discourages weeds and helps the soil retain moisture. In the winter it insulates the iris roots against the typical alternate freezing and thawing of winter. This minimizes soil heaving and disturbance of the shallowly planted rhizomes.
Propagating Japanese Iris
To acquire more Japanese iris plants and to keep existing ones blooming at their best, divide the broad clumps every 3 or 5 years. Do this either in the spring or early fall. Carefully dig up the knarled clumps of rhizomes and wash them off with the hose. Slice through their densely matted roots with a sharp axe or large knife, to make smaller chunks composed of 4 to 6 rhizomes each. Discard sections of older, weakened rhizomes from the center of the original clump. Replant the most vigorous ones as directed above. When dividing in the fall, trim back iris foliage for easier handling. Newly divided clumps of iris may not bloom the following season. With good soil conditions and an annual application of fertilizer, most clumps will bloom within a year or two.