There are two optimal times of year to start seeds. One is in late summer when seeds are fresh. The second time is late winter to early spring when moderate temperatures return to your garden.
PCI seeds need a period of cool damp conditions to germinate. This happens naturally in the late summer and early fall on the West Coast. In other regions, late winter or early spring sowing is more successful, as the seedlings can avoid extreme cold during their first few months. In hotter climates, fall sowing results in larger plants before the heat stress of summer comes.
If you have a cool (60 F or cooler), frost-free cold frame or greenhouse, you may sow seeds throughout the winter. Cold stratifying for 20 days, by holding moistened seeds in a refrigerator, seems to promote rapid even germination.
Sow seeds in well-drained, mildly acidic potting soil, or in a seedbed that is well drained, mildly acidic, and partially sunny. Put the seeds on the surface and cover with coarse sand 1/4 to 1/2 inch deep. Water well.
Label the pot or row with your crossing code, if this is a new cross, or with the species name or hybrid name, and the date. Update your sowing notebook or computer file with this information. Do not rely on your memory to recall which seeds were planted where, and when.
Fresh PCI seeds typically germinate in a few weeks following cold stratifying. Older seeds may take longer. As seedlings emerge, water with dilute perennial fertilizer to encourage growth.
Particularly with old seed, presoaking or scarification may help seeds take up moisture.
Scarification: Considerable testing has been done on scarification, which is nicking the seed coat to promote water uptake. There is anecdotal evidence of its success: A European member wrote to say that his seed order in its envelope was smashed en route, to the extent that the seed packets and envelope were stained. He was astonished to find that these crushed seeds germinated the following spring.
Cold Stratifying: Lewis Lawyer concluded that holding seeds at 40 F for 20 days seemed to promote the most rapid and even germination, not longer or shorter periods, or colder conditions. So if you are inclined to try cold stratifying, limit the chill to 20 days in the refrigerator, making sure seeds have enough moisture. Or plant directly outside in the fall on the West Coast, and let nature do the chilling for you.
Optimal Temperatures for Germination: Seeds appear to germinate most readily at temperatures between 40 and 60 F. Higher temperatures inhibit germination––in several experiments, starting PCI seeds at 70 F resulted in no germinations; colder temperatures (below 40 F) reduced germinations. There do not appear to be germination inhibitors. Gene Loop, Lewis Lawyer and others researched this carefully in the 1990s, and concluded that PCI do not have germination inhibitors.
Two species definitely prefer 50 F and cooler temperatures: Iris innominata and I. tenax. These are also the most cold-tolerant PCI species.
Iris munzii germinates as well at 60 F as it does at 40 or 50 F, so this species may tolerate slightly warmer temperatures.
In the tests done in the 1990s, no PCI species germinated at 70 F or higher temperatures.
Providing enough moisture to seeds can be done in several ways:
Regardless of the method used, by the next spring the seeds and or new seedlings should be outside, in rain, sunlight and normal temperatures.
Lawyer, Adele. 1996. Seed: Harvesting and planting, SPCNI Almanac, Fall 1996, pp 10-13.
Lawyer, Lewis. 1995. More on Seed Germination, SPCNI Almanac, Fall 1995, pp 10-14.
Loop, Eugene. 1994. On the germination of PCI seeds, SPCNI Almanac, Fall 1994, pp 5-7.