Monday, September 12, 2011

Studying a Flower - the Plumeless Thistle

Here is the problem with a novice naturalist walking through a restored prairie and seeing pretty flowers - I assume they all should be there!  It turns out that the pretty pinkish purple flowers I saw on a walk I did way back in July (how summer flew by!) were actually Plumeless Thistle, an invasive weed, and it was everywhere, at least near the walking path I was on.
One invasive species on another - a Japanese beetle on a Plumeless Thistle bud.  From Prairie Flowers in July. 
While walking through the prairie on bright July day, I wanted to observe the prairie species mix to see if I would find any patterns.  The main pattern I found was centered around water availability.  The highlands where there was no standing water found home for yellow coneflower, wild carrot, thistle, some milkweed, and turf grass gone to seed.  The lower areas where the creek ran through hosted cattails, grass, a spiky purple plant that looks like salvia, and some strange broadleaf species that seemed like it would be more at home on the forest floor.  Near the paths in higher elevations, I was taken by a pretty purple flower that I found and thought I could learn a little more about it. 

Studying the thistle, by Amy.
The Springbrook Prairie that I visit has paths that are paved on the ground with mowed areas to each side, so this disturbed area is ripe for an opportunistic invader such as the thistle to propagate. The next time I visit the prairie, I will try to see if the species is prevalent even at the inaccessible areas that do not have walking paths, but I would guess not as much.  The thistle invader is a problem for native grasses because it shades them and competes for resources.  A little bit of natural history - thistle seeds are wind dispersed, much like a dandelion.  This type of thistle is prevalent in Northern Illinois but not at all in Southern Illinois, so it seems to be highly susceptible to heat variation because the differences between these two areas in terms of temperature is typically only 10 degrees Fahrenheit.    

It may be invasive, but it is beautiful.  From Prairie Flowers in July

The thistle has a defense strategy that involved adapting its leaves to create thorns - everywhere.  There are thorns all around the bud, thorns on the stem, and even thorns on the end of each leaf lobe.  And they are sharp, so of course no animal wants to eat them and no human wants to pull them.  So they survive long enough to set seed, and the seed dispersion mechanism is strikingly similar to the dandelion.  In fact, except for the thorns, size, and color differences, the two plants are very similar - likely because they belong to the same Asteraceaefamily.  Seeds are tiny and dispersed with the help of thin threads that fan out to form a thick fuzz, ripe for catching the wind.  Seed germination is said to be 95%, each plant is capable of producing 10,000 seeds, and they can persist in the soil for 10-20 years.  But the thistle does not do well in established, intact ecosystems.  So perhaps the prairie can survive it at the fringes and be better able to combat it in the less accessible areas.  In case you're curious, late spring burns with routine cutting and pulling throughout the growing season works best to control the thistle in restored prairie ecosystems. 

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