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Re: CULT:toxic Canadian thistles

  • Subject: Re: [iris-talk] CULT:toxic Canadian thistles
  • From: laurief <laurief@paulbunyan.net>
  • Date: Sun, 30 Sep 01 08:10:24 -0600

Hi Dorothy,

>Our 1.2 acres in a city are also overrun with Canadian thistles and horses 
>tail and goldenrod.

Ah yes, I refer to this combo on my farm as "The Clash of the Titans" - 
nearly comparable to the time (when I was just starting to garden and 
absolutely didn't know I shouldn't be accepting any and all plant "gifts" 
from friends) I planted Japanese knotweed, aegopodium variegata, and mint 
next to each other in my new perennial border.  YIKES!  But I digress.  I 
do have all three of the above weeds in abundance here, but the only one 
to have caused a problem with the irises so far is the thistle.

>I never thought about the thistles 
>being toxic; just thought they crowded and shaded the irises.

I think there must be more to it than that.  I never allow the thistles 
to get tall enough to shade the irises, and I doubt they suck up any more 
nutrients or crowd any more closely than the dandelions, plantain, 
clover, grasses, and campion do when they grow right into the middle of a 
clump.  No, there must be something else in those Canada thistles that 
inhibit growth in neighboring plants.  Soon after they encroach on an 
iris, the iris's top growth all dies off, and the rzs remain absolutely 
stagnant until the thistle is removed.


zone 3b northern MN - clay soil

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