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Bearded irises usually bloom reliably for three to four years but produce
fewer and fewer blossoms in succeeding years. This is because their rhizomes increase each
season until they eventually become overcrowded and starved for nutrients. Dividing these
clumps and replanting individual rhizomes in freshly prepared soil gives the plants a new
lease on life allowing them to re-gain their blooming strength. The best time to divide
irises in our area is in mid to late summer, so the new plants will have plenty of time to
become established before freezing weather arrives.
1. Dig the Clump
If the soil is dry, water the bed thoroughly a day or so before digging. When you have
more than one variety scheduled for division, it is wise label each clump to avoid
mix-ups. Use a pitch fork or spading fork to loosen the soil around and under the clump,
taking care not to cut into the rhizomes growing near the edge. Lift the entire clump out
of the ground and shake or wash away any soil clinging to the rhizomes and roots.
|2. Make Divisions
The clump will consist of older, spongy rhizomes with lighter-colored young ones
growing from their sides. Cut the young rhizomes away from the older segments with a sharp
knife. Discard the older pieces and any parts that are undersize or diseased. To reduce
moisture loss, trim the leaves to about a third of their original height. Each division
should consist of a vigorous, firm rhizome and a fan of healthy leaves.
|3. Check for Borers
In our area, bearded
irises may be infested with borers, pinkish-colored larvae (or grubs) with brown heads.
These creatures tunnel into and devour rhizome tissues, leaving a wound open to infection
by bacterial soft rot. Extract and kill any borers you find, and cut away all damaged
tissue. To help prevent infection, soak the rhizomes for about half an hour in a 10
percent solution of household bleach, followed by a dusting of powdered sulfur. Then lay
the trimmed plants in a shady place for several hours to allow the cut ends to dry and
|4. Replant the divisions
Always plant bearded irises in a sunny location with good drainage. First check for
any borer pupae in the
soil if you are replanting in the same location. Destroy them, or they will emerge in fall
as moths and lay eggs on or near the new plantings. If possible, remove some of the old
soil and add twice as much compost, or compost plus fresh soil. Also give the plants a
light application of 5-10-10 fertilizer. Then, space the divisions 12 to 18 inches apart.
Arrange them in a triangle with the fans of two of the divisions pointing outward, and the
fan of the third pointed into the space between. To plant, dig a shallow hole for each
division, forming a low mound in the center on which to set the rhizome. Drape the roots
down each side of the mound, and firm the soil around them. Its important to cover a
freshly planted rhizome only lightly with soil. Those that are planted too deep are much
more susceptible to borers and rot.
Water the young plants at once to settle the soil around the roots. If the weather is
hot and sunny, temporarily shade them by placing a shingle on their south side. Irrigate
every 10 days or so throughout the rest of the summer.