Your new PCI plants grew well for a couple of years. Now it’s year four, or five, and you noticed the central part of the plant is looking bare as the new fans grow out around the outside. The plants are crowded; and flowering is tapering off. Perhaps you would like to try them in a new area with light and soil more to their liking, or move them around so that flower colors are more harmonious.
What to do? It’s time to divide, refresh the soil, and perhaps rearrange the iris bed.
First, PCI can be successfully divided and moved only when roots are actively growing. Check the roots by gently pushing aside the soil to look at the growing front of the green fans of leaves. If you see abundant new roots, it’s time to dig. This is simple to determine: active roots are white. Roots grow in late winter to early summer, and mid fall to early winter. PCI are dormant at other times of the year, and should be left alone.
PCI plants grow in clusters of fans, with thin small rhizomes connecting each cluster to the rest. Carefully dig away around clusters, keeping new roots intact. Chop or cut through the connecting older rhizomes, and lift the clusters to transplant with as many fresh roots, and some old roots, as possible. Trim the old rhizomes back, and cut off old, dead fans. Cut off old leaves, and if shipping to another garden, cut the healthy green leaves back to four or five inches long.
Replant immediately in acidic soil, well drained, with a part day of sunlight; morning light is preferable. Mix compost into the soil if sandy or clayey; spread mulch after planting to keep the roots temperate. Water in with a dilute fertilizer mixed into the water. Prepare a new label, and put it in next to the plant.
Fill in the excavated area of the old plant with a mix of compost and soil. Take the time to cut out and remove dead fans and rhizomes. Pull off or cut back dead leaves and tidy up the remaining parts of the old plant. Top dress around PCI with compost and mulch, and give plants a watering with a dilute fertilizer mix. Renew the original label if it’s fading or partially unreadable. Now your PCI is ready for another four or five years of growth.
When root growth occurs during the year is a function of your microclimate. For example, PCI can typically be transplanted in the Pacific Northwest from September to November, and in very mild years, into December. The spring transplanting season begins in March and continues through June.
Northern California is similar, the transplanting season starts in October or November, continues through December, and starts again in February.
PCI growth in Southern California starts later in the fall, may go almost completely through the winter in mild areas, and ends in the spring in April or early May when growth stops for the summer after flowering ends.
In higher elevation gardens, the growing season, and transplanting of PCIs, may start later each spring, continue later into early summer, and start sooner each fall, ending sooner in mid to late fall.
When dividing, gardeners suggest leaving one half to one third of the parent plant undisturbed and in place, removing half to two thirds to plant elsewhere. This way, if all transplants die, the remaining portion of the parent plant will continue to grow.
Weed out any other roots, such as grasses or perennials, or woody shrub roots, while PCI plants are out of the ground.
Bowers, Bonnie. 1984. Suicide prevention among the natives, SPCNI Almanac, Spring 1984, pp 6-7.
Lenz, Lee. 1978. Propagation of Iris innominata, SPCNI Almanac, Fall 1978, p 4.
Weiler, John. 1987. Success in transplanting Pacificas in the Central Valley, SPCNI Almanac, Spring 1987, pp 6-8.