CULT: Organic Matter

Amy Rupp wrote:
> 
> I've heard that organic material and bearded iris are anathema.
> Certainly topdressing with compost has been charged with rotting an
> entire patch (Ginny's).
> 
> Is it merely putting organic material near or over the top of the
> rhizome that is bad, or is it in the soil as well?  I have yucky orange
> "sandy loam" (read:  clay with a little sand) and I planted my iris in
> this without any additions.  Tilling this soil is precarious because of
> the tendency to compact worse and turn clay into concrete.  We typically
> do raised beds here but that's a pretty extreme solution.  With a raised
> bed, the *upper* soil will stay wet longer if the upper layer is not
> deep enough, because the clay sublayer will not accept the water.  For
> roses we recommend 2' (yes, TWO FEET) of raised material.
> 
> So.  To use or not to use compost?  What about foliar feeding the
> foliage (currently just past most bloom) with fish emulsion and seaweed,
> tonic of choice?  Is now the time for more alfalfa meal?

Hi Amy,

Well I will take a crack at answering this one because I have heavy clay
soil as well, and hope that others will jump in if I botch it somewhere
along the line. But also be aware I do not have the same summer sun here
as you do in Texas.

Organic matter in the soil would be great for the irises - caveat - as
long as it doesn't lower the PH too much (or if you compensate for the
PH with other amendments), and as long as you don't add too much
nitrogen. 

Too much nitrogen in the soil and/or covering the rhizomes with anything
that is going to hold moisture would probably not be good. Warm
temperatures and moisture/humidity surrounding the rzs are generally
blamed as the causitive factors for rot as well as supplying too much
nitrogen. The rzs like to "take the sun" so be sure the backs are
exposed.

Clay is generally very rich, but doesn't give up its nutrients as well.
Organic matter will help get that out of the clay, improve dranage, and
help keep oxygen available to the roots (also important). My raised beds
are formed by placing 2 X 12 boards on edge as a retaining barrier. I
then dug out 6" of clay inside the boards, and backfilled with my
planting matter. Excess water that collects in the bottom 6" will leach
out under the edge of the boards, but I try not to over water and have
that happen. The natural "wicking" action will draw water into the top
12" from the lower layer.

What is too much nitrogen, you ask?? Well, hmmmm, ahhhh, cough, ahemmm.
Difficult to say, and each experience is a little different. 10-10-10
fertilizer is about the best. I would add only composted organic matter
to the soil to aviod the "over nitrogen" risk.

As to foliar feeding post bloom, my local sage (Gigi Hall - 20+ years of
iris growing experience) says 1/2 strength feeding now is pretty good. I
wasn't aware that fish emulsion and seaweed tonics were very good for
foliar feeding, but then I don't use them much. 

Well I hope that helps, I hope someone will jump on my case if I have
offered any bad advice.

BTW, how did you get your Sunset Zone number?

John                     | "There be dragons here"
                         |  Annotation used by ancient cartographers
                         |  to indicate the edge of the known world.

John Jones, jijones@ix.netcom.com
Fremont CA, USDA zone 8/9 (coastal, bay) 
Max high 95F/35C, Min Low 28F/-2C average 10 days each
Heavy clay base for my raised beds.
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