Since I wanted to know I re-read Rick & Roger's posts on Facebook about Superstition's iris culture practices, which are very unique. Carl gave the basics...here is more detail, with thanks to Roger Duncan who answered my questions with detailed information.
They transplant all their irises in September/early October into newly prepared beds.
The bed preparation begins with breaking the old bed down (after moving the irises to the new bed) via rototilling, then planting a cover crop in late October/early November. The cover crop mix is about 40% oats, 40% wheat, 10% vetch, and 10% other grains (barley, rye, etc) that becomes the 'green manure' when cut down and tilled in.
They plant the cover crop in late October or early November and irrigate it with sprinklers. They found that if they waited for the rainy season to start there was too much competition with the local migrating birds (redwing blackbirds and meadowlarks) who like to eat the seeds. The cover crop germinates in a couple of weeks and the grains grow slowly over the winter, then in late winter, the vetch germinates and twines up the grain stalks. Early next Spring when the soil is dry enough for the big tractor to get into the field, they get it cut with a flail mower and tilled in. At that time, the cover crop can be anywhere from 3 to 4 feet high. That is usually mid to late March or early April but has been as late as May in wet seasons. Makes no difference if the grains have formed seed heads because the solarizing will kill the seeds.
After the cover crop has been tilled in, they occasionally water the area to help it break down in the soil, then after a month or so they form the beds which are slightly raised with drainage ditches on both sides and down the middle of each 2-bed set-up so the rain flows away from the beds without causing damage. Then in late May or early June they put the plastic down to solarize. The plastic is actually greenhouse film with UV-inhibitors built in and is somewhat costly. Regular construction grade plastic would break down in a few weeks under the intense unrelenting blazing wall-to-wall every day California sun and 100-degree plus summer temps. In our climate Roger says solarizing gives the full effect in only 2 weeks if DONE RIGHT, but they leave the plastic on through the busy summer order filling season and then uncover the beds when they are ready to transplant in September and early October.
And so goes the cycle...transplant irises, till, plant cover crop , till, solarize, uncover, plant irises. In this way they use half of the ground for growing the current year's iris crop while the other half of the ground is being rejuvenated by the cover crop and solarizing for the following year.
BTW, only the commercial beds get solarized, the seedling beds get the cover crop, but no solarizing. They say it isn’t fair to properly evaluate seedlings grown under totally perfect conditions, thus those beds don’t get solarized. They started this cover cropping/solarization method about 20 years or so ago and they originally followed a University of California study, but have added some minor tweaks along the way. Not organic, but much better for the environment using mainly the sun and water and hardly any chemicals.
Warning label: this process works wonderfully in our Central California climate, but may not work as well or even at all in other climates.
