Growing species Iris in planters when your garden and climate don't work

Kathleen Sayce, June 14, 2020

In 2017 I moved two irises, Iris attica and I. hartwegii australis, from the open garden to planters where I could control rainfall and drainage. I wrote about this in Fall 2017 for World of Irises. 

Iris hartwegii australis, photo by Richard Richards

Iris attica in flower

I used styrofoam planters and a planting mix with ample pumice, to ensure good drainage. Both planters went alongside the east side of my house under the roofline, to reduce winter rainfall. On average, this lowers the winter rainfall by half, from 86 inches (cm) to 43 inches (cm) per year. Both planters are otherwise open to rain, snow, hail, and in sun half the day.

Iris hartwegii australis after planting, early spring 2017

After planting, the surfaces  were covered with granite chicken grit (5 mm) to provide surface stability in heavy rain. The planters were placed in a group near a hose bib so that summer watering is easy to accomplish. We have bird baths nearby that we clean and refill every day in summer; watering the planters is easy. At least once a week I soak both planters, mimicking summer rainfall. 

Iris hartwegii australis, spring 2020

Iris hartwegii australis has not yet flowered, but it has put out several new fans, 6 fans when planted, 27 fans today. Compare the original planting to the planter this spring, from early spring 2017 to late spring 2020 in the accompanying images. 

Fans are larger, healthier, their color is excellent, and there are more fans each year. Meanwhile, the remaining plants in the open garden have vanished.

Four growing seasons later, it is clear that Iris attica thrived with the move, though seed set has been low. Despite its small size, this species fills its planter every two to three years, after which I take it out, prune it back and replant. Two years ago, I sent more than twenty rhizome pieces to a regional iris group for their summer sale. The plant left in the garden disappeared. 

Iris attica flowering in 2018, this is a happy plant!

For both species, I concluded that winter wet and summer drought in a fine sandy soil did not let them survive or thrive. Use of compost, mulch and fertilizer was not enough for either species. 

Conditions in the open garden were simply not close enough to their native habitats. 

 By keeping both species in planters, using a potting mix with compost and pumice, placing these planters under the eaves and near a hose bib, and with occasional foliar sprays, both have done better.  

I am still working on promoting flowering for I. hartwegii australis. I would like to know about your fertilizer regimes in your gardens for winter dry, summer rainfall species, to help me decide how much more I should be doing. 

Every spring I think, this might be the year when IHA finally flowers! Meanwhile, flowers or not, I have been very pleased that this touchy Pacifica iris is still living in (or next too) my garden. 

Overcoming Climate—An experiment with Iris attica and Iris hartwegii australis

Kathleen Sayce, May 2017

Gardening on the coast in the Pacific Northwest, I grow many hybrids and several species of iris in the Pacifica (Californicae) group of beardless iris. The vigor of tall bearded iris is daunting—miss a year to divide and transplant, and my garden is overrun. I had to use explosives to clear areas (Joke! It just felt like black powder was more effective than a spade). 

Iris attica flowers, after seven years, three in the ground, four in a planter.

In 2010, the North American Rock Garden Society’s western study weekend was in Medford, Oregon. Gardens on the tour included Baldassare Mineo’s rock garden, Siskiyou Rare Plant Nursery, and a dozen private rock gardens in the area. A tiny bearded iris, Iris attica, came home with me. I planted it in the sunny end of a flower bed; it flowered the next year, and then began a slow decline. Four years ago, I removed it from the garden and planted it in a styrofoam box. 

In this new container, deep and well drained, tucked under an east-facing eave, Iris attica flourished and flowered. Which was when I discovered that squirrels, chipmunks and/or voles were eating the flower buds as they began to emerge and show color. [These varmints also ate my rainlilies.] But the plant was now sturdy and healthy; despite no flowers, Iris attica thrived in its new home. 

A working method to grow Iris attica in the Pacific Northwest:  a planter, a dry outside location (under eaves), very porous planting mix, and protection from animals during flowering. 

Last year we rebuilt the cold frame and added a band of heavy 1/2 inch wire mesh about a foot high all around the edge. This spring, when I saw buds emerging on Iris attica, I put the box in the cold frame; the mesh band was already open for spring. The result you can see, sans nibbling, is a planter packed with flowers. When it is done flowering, the planter will go back outside again. 

So, you are wondering, what is the link to Pacifica iris? 

I also have Iris hartwegii australis in my garden. This was grown from wild-collected seed, collected many decades before it was a listed species, then grown in a garden, and seeds from that plant passed to me. It flowered once and has been declining ever since. It’s native to mountains in southern California, which means the long wet winters here are probably wetter than it likes. It might quite like a box under the eaves. 

Iris hartwegii australis, ready to go into its new planter. Note the extensive root system--many more roots than on hybrid PCI plants. 

Richard Richards told me this about its native habitat:  

        “I. h. a. grows in its native range in decomposed granite with superb drainage.  It gets maybe 15 inches of water, occasionally in the form of snow, from November to April.  In the summer it gets a thunder shower about once a month.  There are often two or three inches of plant litter, mostly pine needles, above the young shoots in the late winter, and it grows up through this litter.” 

This spring, I dug up a sprawling clump of Iris hartwegii australis and tucked it in a styrofoam planter with a highly porous mix of coarse pumice and potting soil. I added some fresh compost and biochar for more soil carbon. I dressed the top of the planting mixture with granite gravel (chicken scratch), as its home mountains are geologically old granites of the Transverse Ranges in southern California.  Tucked along the eaves, the rainfall should be cut to under 40 inches, more like its home. This planter is close to a hose bib, and gets half days of sun (when we have sunshine). Summer soaking to mimic thunderstorms in the mountains is easy. 

Granite chips added on top, to help keep the soil mix in the planter in heavy rain, and in this case, to remind I. hartwegii australis of its natal home in the Transverse Ranges, southern California. 

My hypothesis is that native plants in these boxes do better than in thin-walled, dark-colored plastic pots, because the planting mix is deep and cool. I also grow Erythronium, Lilium and other bulbs in these boxes, where they can live for several years without transplanting. PCI seedlings do very well, though their roots will push through the styrofoam if left in there too long. In the winter, the well-insulated walls also protect the roots from freezing—just as in the ground. 

I’ll report back in a year or two on how this iris likes the planter. Or sooner, if it goes toes up!