Preparing Beardless Iris Beds at Dry Creek Garden

by Jeff Bennett

Overview of iris plantings at Dry Creek Garden 

Image (C) 2023 Maxar Technologies | Google Earth

Dry Creek Garden is located in Union City, California and is part of the East Bay Regional Park District. Iris plantings were established for the 2019 American Iris Society National Convention and are maintained by park gardener Jeff Bennett.

In 2021, I decided I wanted to improve our Louisiana iris beds at Dry Creek Garden so that they would hold more moisture and require less physical watering. At the same time, I wanted to try my hand at growing Japanese irises. California is known as a great location for growing bearded irises and of course Pacific Coast native irises and their hybrids, but not so much for the beardless classes (including Louisiana, Siberian, Japanese, and Pseudata hybrids). In this blog post, I will walk you through processes I used to prepare beds in this extremely drought-affected area of the country.

Before preparing beds for new irises, be sure to orders are placed well ahead of planting time to have the greatest choice of varieties. I do this in January and February, when the growers and hybridizers usually update availability and open up their websites for ordering. The irises you order won’t be ship until September, so you have plenty of time to plan and prepare the beds. 

For the new Louisiana iris bed, I dug 10 to 12-inches deep and created a  trench 5 x 30 feet - with the occasional help of Mount Diablo Iris Society or local community volunteers and sometimes other park staff. (Sorry, to readers in every other country in the world. I never quite learned the metric system.) We mounded up all the clay-like, native soil in the pathway next to the bed so it could be returned to the trench later. 

Excavated trench with a mound of removed soil and concrete edging piled to right.
All photos by Jeff Bennett

Next, we lined this trench with weed block to prevent bindweed from coming through very easily. On top of the weed block, we added a layer of 6-mil plastic (is mil metric?). The plastic will prevent the water in the bed from soaking all the way through to China. Once the bed was lined with plastic, we started refilling it with soil in the adjacent mound. We added about four inches of straight compost and then top this with about four inches of the native clay, trying to mix this together as we went. This was followed with another two to three inches of compost and another two to three inches of soil, again mixing it up. Finally, we added a final layer of two to three inches of compost on top. To finish the edges, we used recycled cement from my home patio (which had been removed and hauled here the year before). The cement edging helped conceal plastic from view and gave the bed a finished appearance. Since this area had already housed bearded irises before, I returned the same drip lines onto the top of the bed. The half-inch drip lines have emitters every foot and we planted a Louisiana iris every three feet on each side of the bed using the emitters as a guide. 

Completed and planted Louisiana iris bed.

When I receive any iris orders, I always pot up rhizomes to give them a good start (see my post "Get That Order Planted" from last summer). The bed was completed in January 2022 and potted Louisianas were transplanted from their pots directly into the new bed then. Since they like water and we weren't getting ANY rain, I used a hose to give the bed its first flood soak to settle them in. Now it was their turn to do their work!

Louisiana irises thriving in new bed.

After planting the Louisianas into the new ground bed—alphabetically by name of course—I turned my attention to getting the Japanese irises transplanted into their new water-holding containers. Unlike Louisiana irises, Japanese irises CANNOT dry out. Their roots must remain moist at all times. For them, I decided to use large metal water troughs, typically used for livestock. I got the watering troughs at a local hardware store, but they can also be purchased from farm supply stores. Livestock-watering troughs are galvanized metal and have no drain holes anywhere.

New livestock-watering troughs for Japanese irises.

I first put about three inches of wood chips on the bottom to make a water reservoir below the soil. I then bought a rhododendron soil mixture to provide the acidic pH Japanese irises prefer. I layered this soil with compost to the top of the tubs, about three inches each of rhododendron mix and compost, mixing these together while filling the troughs. Japanese irises like their roots to be wet but the rhizomes need to be above the water table to grow well. To create this effect, I measured down from the top of the tubs five inches and drilled ¼-inch holes in the metal to allow excess water from rain or irrigation to escape. I drilled eight holes in each container, two on each side of the oval-ended tubs. They were now ready to plant. I have nine tubs -  seven for Japanese, one for Pseudatas/Species-X, and the last one for Siberians. For a quicker visual effect, in each tub, I planted seven to eight varieties. Since the irises were growing in pots, I just transplanted them alphabetically to the tubs (rootball and all). It was now February 16, 2022.

Filling livestock troughs with layers of rhododendron soil and compost.

Newly planted beardless irises in livestock troughs.

Everything started growing immediately as it was the season to do so. By early April, the Louisianas were showing good growth and the first flower opened on April 27, less than 100 days after the plant was transplanted into the ground.

Louisiana iris 'Eyes Wide Open' (Heather Pryor, 2016) blooming approximately

100 days after transplanting into the new trenched bed.

The Japanese irises were looking great by early May and the first Japanese bloomed on May 19, just in time for our scheduled Japanese iris judges' training on May 20. 

Japanese iris 'Devi Banri' (Toyokazu Ichie 1989) blooming in May 2022 after being transplanted

into livestock troughs in February.

None of the Siberians bloomed during their first year in the tub, but they put on lots of increase and we are looking forward to them blooming this year. I reiterate that we are growing them the same way as the Japanese irises with wettish roots and rhizomes above a water reservoir. 

This year we created another bed like the Louisiana bed, for the Japanese irises. These will be planted very soon, now that the California storms have passed and we can dig in the soil again. I am adding peat moss to this bed for acidity along with all the compost. We're hoping to grow Japanese irises successfully in our specially-prepared ground beds in this otherwise Mediterranean climate (no natural summer rainfall)!

We had also begun preparing a Japanese iris bed in the same fashion in the heart of the garden down near the creek last fall. Just as we were about to add peat moss to this bed before planting, the atmospheric river of storms hit California starting on December 31, 2022. THANKFULLY, we had not planted the bed yet by transplanting the Japanese irises from their pots into the ground as this area ended up getting flooded and under three feet of water, devastating this part of the garden. Flooding deposited 20-24 inches of silt there and strong winds toppled six mature trees nearby. We are now cleaning up the garden from this storm. In about six months, we hope to re-open the damaged area to the public (~Fall 2023). The damage was bad! Thankfully, the iris area on the hill was not harmed by the flooding. HORRAY!