Container growing is successful if the conditions PCI like can be created. For areas with very cold winters this offers the advantage that the containers can be brought into a cool greenhouse for the winter. For nurseries, container growing lets pots of irises be displayed when plants are in full flower, then swapped out with new pots.
All PCI grow well in pots for one to two years from seed; around year three, they may begin to die. This may be due to several factors: breakdown of potting soil, poor aeration, water logging and hot soil, which leads to over-heated roots. PCI do not like any of these conditions, and when they are unhappy, they die.
Matilija Nursery, Santa Rosa Valley, CA (http://www.matilijanursery.com) is very successful with container grown PCI. The nursery uses five gallon rose pots; keeps pots in midday shade; and adds perlite to the soil mix. The nursery waters its containers fall through spring, every week until flowering stops, then waters every other week through the summer. They fertilize regularly, because these plants do not have access to the nutrients that would be present in garden soil.
Matilija Nursery also brings containers with PCI buds into bright shade indoors during flowering. These plants are kept cool, watered weekly, and returned outside when flowering is past. This success means that cool, bright shade in a greenhouse should also be successful.
To grow PCI in containers, follow Matilijaís example!
Coarse mixtures are important for good drainage in containers. Use fine gravel, coarse sand or perlite to improve drainage of the soil mix. We recommend a 1:1 ratio of soil mix to coarse additive.
Generally, large pots or planters (five gallon or larger) work well if pots are in shade for the warm part of the day, and not too cold in winter. Avoid small, heat-retaining pots, such as black plastic pots. Wood and light colored plastics are better than concrete or heavy ceramic pots in warm climates.
This should be enough room for one plant for four to five years. Plan on repotting when plants reach the edge of their container. Mulch the surface to help the roots stay temperate.
Water weekly until flowering ends, then water every other week throughout the summer.
Use fertilizer; half the dose recommended on the label is good.
PCI can probably take light frosts in containers. If prolonged cold weather is expected, move containers to a cold frame or cool greenhouse until temperatures are back above freezing.
Ryan Grisso recommends wine barrels for those who are not able to easily bend over; he gardens in the San Francisco Bay Area, CA, and uses wine barrels to grow both seedlings and display plants.
Grisso, Ryan. 2005. Hybridizerís corner, SPCNI Almanac, Spring 2005, p 15.
Tamberg, Tomas. 1988. Part 5: Overcoming obstacles, SPCNI Almanac, Spring 1988, pp 21-22.