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I think I basically agree with most of this Bill, depending on what you
want to consider a disturbance and also whether or not we are talking
about irises thriving or merely persisting.  I don't have personal
knowledge of any 100+ year old clumps of pallida blooming in the woods
<g> - I was thinking more of 100+ year clumps that were not dug,
divided, and planted in 'new' soil, re: your initial post.  The oldest
clumps here ar I. germanica, which has been growing under the eaves of
my unguttered house for 20+ years without being divided.  I do fertilize
them most years, if I remember, and the years that bloom isn't frozen
out, they bloom profusely.  I would say that these irises are thriving
without major disturbance (i.e., roots/soil), and they don't 'come and
go'.  The clumps look pretty much the same from one year to the next,
but I've never tracked individual rhizomes from one year to the next. 
Another interesting winter activity to try <g>

My observation has been exactly what you mention - clumps that persist
without being divided are fairly slow growers that replace older rotted
out 'grandmother' rhizomes in the middle of the clump and slowly expand
the clump.  In my experience, digging and dividing some of these slower
growing rhizomes usually results in no bloom for a year while they get
established (probably while they are sinking roots to China to cope with
the gravel and moisture fluctuations at the surface here).

In any case, our modern TBs have certainly been selected for fast
propagation in highly disturbed conditions, regardless of what the old
pallidas and germanicas do.

As for thriving in mulch, some folks around here (including one
commercial grower) use thick mulch of coarse (sawmill) sawdust right up
to the plant.  As  long as the mulch is coarse enough to allow good
aeration/drainage, has a high carbon/nitrogen ratio (more stuff like
lignin and cellulose than nitrogen), and doesn't get too dry, bearded
irises seem to thrive.

Linda Mann east Tennessee USA zone 7/8

Bill Shear wrote:
> Linda, I'd be very surprised if any of those clumps could be documented as
>               being "100+" years old.  At that age, wouldn't they be many yards in
>               diameter?  
I think most of these naturalized clumps in abandoned yards and
>               gardens carry on because the abandoned garden (perennial grasses and weeds)
>               remains essentially a disturbed situation.  

When succession takes its
>               course and brambles and shrubs invade, followed by trees, the irises are
>               history.  My suspicion is that the clumps are things that come and
>               go--flourish for a few years, almost die out, and then rejuvenate from seed
>               or from a few surviving small rhizomes when some fresh disturbance (fire,
>               trampling by cattle) reduces competition.

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