Friday, August 27, 2010

#*&@$^+ Yellow Starthistle

We just finished a contract grazing job for the Placer Land Trust in the Bowman area of Auburn.  We had 210 ewes on site for 3 weeks - they grazed mostly yellow starthistle, dead annual grasses and clover, along with some coyote bush and blackberry.  At this time of year, when the starthistle is maturing, its spiny seed heads must be high in energy - the sheep love to eat them!  They probably consumed 90-95 percent of the starthistle seed heads at the site.

Yellow starthistle is an introduced annual weed (it came here from central Asia) that in some ways acts like a perennial plant.  It has adapted effectively to out-compete the other annual plants in our rangeland ecosystems - its deep taproot (as deep or deeper than some perennial grasses) can get to soil moisture that other plants can't reach.  It also matures later than most of our annual grasses, allowing it to take advantage of late season moisture (like we had this year).  More than most annual plants, it is physiologically adapted to produce seed every year.  We saw an amazing example of this on the land trust project.
Herd impact - sheep will trample starthistle as well as graze it.

As we moved the ewes to their last paddock on Monday evening, we walked through the paddock we grazed at the beginning of the project.  We had moved the sheep off of this paddock 14 days earlier.  During that time, we'd had warm temperatures (85-95 degrees at this site) and no precipitation.  In fact, the site had received no precipitation for at least 90 days.  Based on the difficulty we had in putting temporary fence posts in the ground, I would say the soil was completely dry.  However, on Monday evening, we noticed new growth on many of the starthistle plants we had impacted two weeks earlier.  These plants were finding enough energy and moisture to produce seeds - far fewer than they would have produced without grazing impacts, but seeds nonetheless.
Grazed starthistle pushing new growth.

Next year, we'll try to impact the site earlier in the year - we plan to bring ewes and lambs to the project site in April.  If we can graze the starthistle before it flowers, we should decrease seed production and allow competing plants to thrive.  Based on this week's observations, however, I would expect the starthistle to produce some seed despite our best efforts.  #*&!$^+ starthistle!

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