Thursday, July 31, 2014

How Deep Should I Plant My Irises?

By Renee Fraser

Mixed beds with irises

Boxes full of irises are arriving in the mail, and iris lovers are unpacking their purchases and trades, planning their planting schemes, or wandering about the yard wondering "where on earth will I find space for all of these new plants? What was I THINKING?" An exciting time of the year.

People who are new to irises are also posting questions in the iris forums and on Facebook, wondering how deep to plant them.  Sometimes irises come with directions from the growers, who may be from a different part of the country, and sometimes they don't. Much discussion has resulted on some of these iris forums, with many of us repeating the advice we heard from our grandmothers:  that irises should be planted with the top of the rhizomes exposed.  I admit to dispensing this very advice myself until last year, when I learned from professional growers that the ideal planting depth does not always leave the top of the rhizome exposed, and that indeed, the rhizome should be covered by as much as two inches of soil, dependent upon the climate.

The American Iris Society website says to plant rhizomes "at or just barely below the surface of the ground.  Irises should be planted so the tops of the rhizomes are visible and the roots are spread out facing downwards in the soil.  However, in exremely hot climates or with very light soils, cover rhizomes with up to one inch of soil."

So the first thing we learn is that iris rhizomes should be protected from the sun and heat in hot climates.  In Cathey's Valley, California, Rick Tasco and Roger Duncan of Superstition Iris Gardens plant their irises with about an inch or so of soil over the rhizome to protect them from the hot California sun. 


Iris rhizome under 1" of soil at Superstition Iris Gardens, California.



Iris rhizomes at the proper depth for hot sunny climates.


The advice to protect the rhizome from the sun is also recommended in areas we would not normally consider too hot and sunny, such as Colorado.  Iris hybridizer and grower Bob Van Liere of Iris4U says "in Colorado it is recommended to plant just below soil level to keep the rhizome from sunburning. We are at 5300 feet here at our garden and most of the Colorado is above 5280 ft. We have customers planting up to about 9200 feet, a short growing season for sure."  So summer sun is something to consider when planting in high altitudes as well as the typically hot climates of places like Arizona and Southern California. 

Bob digging irises at Iris4U in Colorado.  Note the depth of the rhizomes.


Hybridizers who grow irises in very cold conditions also recommend deep planting for iris rhizomes. Chuck Chapman, a hybridizer in Ontario, Canada, experimented with planting depth several years ago, planting rhizomes half exposed, with only the top of the rhizome exposed, and a half inch, one inch, and two inches beneath the soil.  He says they all bloomed and increased, but the best growers, by a small margin, were 1/2" and 1" beneath the soil.  The worst growing rhizomes were half exposed.  Chuck is probably the most northern grower of tall bearded irises, and he notes that there is less frost heave the deeper the rhizomes are planted.  Furthermore, he says that when he contacted other growers about how they planted their irises, all planted them below the surface. 

As a final note, Chuck points out that Bob Van Liere did some research on roots, and the results of his experiments were published in Tall Talk, the periodical of the Tall Bearded Iris Society. He planted rhizomes with and without roots, and those with roots did substantially better than those without, growing "hair roots" off of the trimmed roots. Some people like the longer roots because they hold the iris up and in the ground as it gets established.

I have mixed plantings in my hot Southern California garden, and everything else needs more water than the irises. That translates into problems with rot, especially since my soil is rich and loaded with organic material that stays wet. Hot plus wet equals fungus and bacteria growth.  I lose a significant number of irises some years, particularly in small beds bordered by bricks or stone, which turn into little dutch ovens during the summer.  One trick I use to increase drainage in my mixed beds is to plant them in mounds.  I place the rhizome on a mound of soil at the same level as the surrounding earth. I cover the roots well and the rhizome by just a bit, producing a low mound above the surrounding soil level, to provide better drainage.




Leaves should be cut short enough so that the newly planted rhizome does not tip over, but not too short: about six inches for shipping, and up to ten inches if you are dividing at home. Please excuse the short leaves on this poor little rhizome that was 'accidentally' hacked off my my dear husband's grass edger. (We battle over territory like feudal lords: he tries to maintain his territory for grass, but I slowly annex it inch by inches for flowers.)

Since iris growers won't be able to put food on the table unless they are successful at what they do, I tend to favor their advice, rather than what grandma told me.  So I now plant my iris rhizomes a bit deeper. Whatever you do, though, do not plant them deep like a flower bulb!  They are not bulbs, and if the green leaves are buried they will not thrive and may die.

If you are not sure about how deep you should plant your irises in your particular climate, your local iris society may have some advice. Check here to locate a society near you. And if you have a favorite tip for planting irises, please share it in the comment section below.

And good luck finding spots for those incoming orders!

6 comments:

  1. Thanks for a very interesting and informative article, Renee!

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    1. Talking to growers is always informative and interesting!

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  2. Thanks for a very interesting article. Here in the UK we always plant our irises with the rhizome exposed so I had never really considered planting depths before. At Sissinghurst we are currently lifting and dividing some of our irises. Some of them hadn't been lifted for years and were very congested, it's feels good to sort them out! Helen (gardener)

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    1. Helen, whatever you do at Sissinghurst sure does the trick! We will all look forward to seeing the irises bloom next year after division. Thanks for sharing your method.

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