I have long thought that patches of Canada thistle are there because the soil, probably compacted, needs their tap roots. Now I hear Klaas Martins say that there seems to be a built-in control in the process. Evidently the symbiosis between root and soil life requires that the tap root growth be in anaerobic conditions. So that is why after persisting and thickening several seasons, the thistle patch fades and disappears. Those roots have worked themselves out of a job as they made the compacted layers of the soil more permeable and thus more aerobic.
It is easier to observe this in a pasture than under crop conditions with the continuous tillage and/or chemical interventions.
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