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Thistle Control and Exotic Legumes

One thing you need to remember if you want to eliminate Canada thistles from
your planting long-term is that you have to remove all the available root
space.  We have found that here in central Iowa a good diverse matrix of
species (about 35 species) will do it without the use of herbicides. 
Species like Solidago rigida and Helianthus grosseratus are very important
members of that matrix.  The thistles do not disappear immediately, but
about the fourth year they shrink to 4 or 5 inches in height and they likely
winter kill due to lack of food reserves in the root systems.  It does not
appear that soil type makes a difference, although they definitely persist
longer in very wet soils. We also pull thistles when weeding and if you do
it in the bloom stage, it will reduce a clone by 95 percent.   

In summary competition is the key, you have to out compete them in the root
zone and because plants communities want to diversity, you want it to happen
with positive prairie plants not weedy perennials.  

In regard to the Sericea, thank goodness it is not in our area, but we do
have other exotic legumes and the worst one is Red Clover.  Unlike the
prairie plants, it stays green well after mid-October when all but the cool
season grasses are dormate.  During the past 4 or 5 years I have sprayed it
in early November with a mixture of Round-up and have begun to eliminate all
the adult plants, we then burn in the spring and get a new crop of
seedlings, but since they do not flower the first year, we repeat the
procedure to eliminate the new plants.  It does take time, but will
eventually bring red clover under control.  This procedure may work for
Sericea, however, however,I do not know if it goes dormate in late fall. 
This procedure will also work on Canada thistles, and smooth brome, but may
remove cool season natives such as Canada wild-rye, Virginia wild-rye and
June Grass.

One thing you need to keep in mind is that immature successional stages are
very vulnerable to fall sprayings with Round-up.  Many immature plants do
not go dormate in late fall, as a result they are very vulnerable. 
Generally your planting should be at least 3 years old before this will
work, check for green natives before you spray it.  

Every plant has an Achilles heal, you just need to find it. 

Carl Kurtz

Central Iowa

 
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