permaculture

["Lawrence F. London, Jr." <london@sunsite.unc.edu>]

I found this in my archives tonight & thought it was interesting.
My question is: Would the shade of green of different plant's
leaves matter with regard to filtering sunlight and changing the
ratio of red to far red light thus inhibiting the germination of weed
seeds in the soil beneath them?
--------------------------------------------------------------
 Lawrence F. London, Jr. - InterGarden - Venaura Farm
 mailto:london@sunSITE.unc.edu - mailto:llondon@bellsouth.net
 http://sunSITE.unc.edu/InterGarden
 http://sunSITE.unc.edu/InterGarden/permaculture.html
--------------------------------------------------------------

Date: Sun, 21 Sep 1997 15:39:09 EDT
From: "E. Ann Clark, Associate Professor" <aclark@crop.uoguelph.ca>
To: sanet <sanet-mg@ces.ncsu.edu>
Subject: Re: soil seed bank

Joel:  a fascinating area.  Have a look at a book by Mary 
Allessio Leck, V. Thomas Parker, and Robert L. Simpson, called Soil 
Seed Banks.  1989.  Academic Press.  All kinds of neat stuff relating 
to your questions, including species specific predation.  Chapter by 
Cavers and Benoit includes a table on numbers of viable weed seeds in 
arable soils (range is 250 to 130,000 seeds/meter squared, with most 
in the range of 15-20,000).  

Chapter by Baskin and Baskin (major contributors to this literature) 
discusses one particularly intriguing aspect - that "leaf-filtered 
sunlight inhibits germination of nondormant seeds".  I've looked into 
this and was fascinated to realize that the ratio of red:far-red 
light (which affects the function of phytochrome, a key developmental 
hormone) changes upon filtration through a vegetative canopy.  In 
practical terms, this means that keeping the land covered by living 
plant material REPRESSES germination of weed seeds - leaving the land 
open and bare ENCOURAGES weed seed germination.  Residue doesn't work 
- has to be living stuff, I think.  

Thus, the focus of organic farmers in keeping the soil covered at all 
times is revealed as having yet another beneficial effect - weed 
control.  

This kind of stuff was studied in depth in the UK about 10-20 years 
ago.  H.A. Roberts was a major contributors.  Ann

ACLARK@crop.uoguelph.ca
Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON  N1G 2W1
Phone:  519-824-4120 Ext. 2508
FAX:  519 763-8933