permaculture

[Paul Cereghino <paul.cereghino@comcast.net>]

Jennifer,

In short, I'd say, depth, moisture, paper-layer and biological activity 
interact.

I'd differentiate between 'wood chip' and bark mulch.  They can have 
very different properties.  I perfer the waste stream from the arborist 
industry which is chipped branches including wood, bark and leaves and 
is free as opposed to bark mulch which is relatively biologically inert 
(the chemical compisition and structure of bark is designed to resist 
fungi and decay and shed water) and provides secondary income to the 
industrial logging industry.  I have been more satisfied with the more 
biologically active mulches at providing weed control.

As a little bit of a sidebar...
I tend to associate wood chip mulch with long term woody plantings.  It 
seems like woody mulch is much more resistant to self-sowing and plants 
adapted meadow-like environments.  I tend to mulch deep and once, 
knowing that a foot (30cm) of wood chip will become 6" (15cm) of wood 
chip in a year and 3" (7cm) in two.  And focussing this kind of effort 
toward mulching on areas where I want to strongly control composition of 
ground vegetation and shift toward woody plantings and forest 
understory, or might not have the planting stock to install right away.  
I have been reserving patches of a straw topped cardboard over 
manure/rockpowder heet mulch where I want more fast growing 
self-designing herbaceous communities.. this too may be psychological...

With a DEEP woody mulch, the perennial weeds that survive are often easy 
to extract, with most of their reserves spent (easy to harvest dandelion 
root!).  In a shallow woody mulch then the extra weed control becomes 
critical.  I tend to focus the paper/cardboard assist at root crowns of 
woody plantings where mulch is thin is where .  Planting above grade 
then helps with avoiding buried root crowns for those species that are 
sensitive.  I find tap rooted stuff (dandelion, hairy-cat's-ear seems to 
pop up again, not necessarily a bad thing) but my rhizomatous grasses, 
less so or in thin spots only.  I try to stack the odds in my favor and 
improve soil by layering manure and rock powder beneath.  Moistening the 
ground before applying is critical so that weeds are actively growning 
and more vulnerable to starvation.

Whoops rambling on again... just been thinking about this stuff lately...

Paul Cereghino