Monday, January 18, 2021

Dishing the Iris Dirt

By Bryce Williamson

No, The World of Iris blog has not joined The National Enquire in dishing iris dirt on people, but rather this post is about real dirt and the problems iris growers face when growing irises in the same garden for many years.

I first planted irises in this yard in 1977 and the bloom the next year was amazing and just what I would have expected from soil that had never had irises grown there; however, as the years went by, the quality of growth and bloom declined and this seems to be a common story among iris growers.

Looking back, many of us have tried different things to get good growth and bloom. Some were more successful than others; other methods no longer are allowed by environmental regulations. I know of no one that really understands why modern bearded irises deplete the soil.

The most obvious thing is to fertilize more—Region 14 hybridizer Vern Wood wrote in an article for The Bulletin of Region 14 that he applied fertilizer heavily so that it looked like a light snowfall.

In the good old days, it was possible to fumigate soils and this seemed to reset the soil for a period of time.

Large growers like Schreiner’s rotate their fields, but that is not really possible in home gardens. I have tried letting areas of the yard go fallow, but that does not seem to really work.

I have even shifted the main planting of named irises to the front yard and that helps for a time.

Bringing in new soil helps too, but again it only helps for a time. 

Over the last 8 years, I have been on a different program. 

Once the area to be used is cleared, we apply 15-15-15. Some will question the numbers of the fertilizer, but that was what irises growers in this valley were using when I first joined the iris society and I have not had a reason to change. Once the fertilizer is down, I water heavily and I want moisture down 4 or more inches into the soil.

Then I buy chicken compost. It is more expensive than steer compost, but steer compost can contain unwanted seeds. The chicken compost may be a bit hot upon arrival, but that quickly is resolved over a few days or a week. That chicken compost is moved into the area to be renovated and covered to a depth of 3-4 inches. The amount of compost is determined by the area to be covered. In the good old days, if I bought enough compost, the delivery charge was waived, but that perk has done the way of the dodo... Ah, the good old days.

Then we bring in the largest rototiller we can get into the yard; when I moved here, I could have a tractor and tiller brought in and that was wonderful because it would cut deeper into the soil, but these days the infilling of what was once a semi-rural area has sadly removed that option. The area is then ready to be tilled.

I insist that the area tilled must be cut in at least two directions. All of these preparations I like to have done between the end of bloom season and the start of shipping season when my purchases start to arrive.

The plants grow well, bloom freely, and there is the added bonus of the soil being very friable. Although it is early November as I write this and I am having problems figuring out how to come up with images, I will dig a rhizome or two in the morning so that you can see the quality of plant this process produces.

A sample of an iris grown in revitalized soil.



  1. great info! do you think disease builds up in the soil, so it's important to rotate/clean/ or other strategy?

    1. I believe in rotation when possible. With no commercial needs here, I have room to rotate the irises.

  2. Thank you, Bryce! My soil is exhausted too, and this year I tried amending with steer manure and peat moss. Next year I will try your secret recipe.

  3. Thank you/ Great information that everybody can benefit from. How long do you let the compost lay in the soil before you plant new iris. And when you say the compost is brought into the area to be covered, are you covering it with dirt once laid, or just till it in?

    1. Since I am dependent on my good help, the compost usually sits here for a few days before it is moved. I like the area to be covered and then tilled for at least 3-4 weeks before I replant. Sometimes the compost is a bit hot

  4. Excellent article. Organics are important. Our soil here contains some clay. The compost improves aeration dramatically--very important here in the east where rainfall is most plentiful and soil is easily compacted.

  5. .... not only are organics important, but replenishment of micronutrients are important too. Bearded iris are heavy users of magnesium, selenium, and boron. Normally those micros are present in barely adequate amounts in most soils at a neutral to slighly alkaline ph. Iris clumps can rapidly deplete the soil's store of those nutrients locally, and they'll need to be replaced. I use Azomite for micros with my fertilizer program as well as a combination slow/fast release nitrogen formula in my all purpose fertilizer mix. Then its double dug to at least 12" A ridiculous amount of work but then i've been able to use the same bed(s) for growing iris 10 years +


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