We have strips of bull thistle in our pasture, along the areas where our flood control district spread dredging spoils last year. We actually have two kinds of invasive thistle that I hate- bull thistle and Canada thistle. Both are class C noxious weeds here, meaning we aren’t required to control them, but we are encouraged to do so. Canada thistle is supposed to be the more difficult one to manage, but thankfully I think our population of it is starting to dwindle. I suspect the way we are rotational grazing the grass, and that the sheep eat and trample it, is making it difficult for that weed to thrive in our pastures.
Canada thistle (left) has shiny, ruffley, light green leaves and is a fairly petite plant. Bull thistle is a big boy, often growing taller than me, and has dark green, giant, hairy, spiny leaves. The bull thistle is a much more annoying plant; I find that it’s spines sting more, and because it grows so tall, walking past the plants is painful. They sting me right through my jeans. Canada thistle, on the other hand, tends to only get boot high here and isn’t nearly as prickly.
We have come to observe over the years that it’s best to get grass seed down on disturbed soil right away. Grass on its own doesn’t “volunteer” well, so bare spots will populate with weeds for years before grass might naturally want to come in. Whereas if we get the spot thickly seeded with a good variety of grass, the grass has the advantage from the get-go. We didn’t do this in the pastures last year, so lo and behold, now we have strips of thistles to deal with.
Last year we did quite a bit of mowing because the sheep couldn’t keep up with the grass. This year we have less grass and more sheep, so mowing won’t be necessary at least until July, I think. The bull thistles are starting to form flower heads, so I figured I’d better do something quick, to prevent them from spreading seed. Though the sheep do actually nibble on them (they appear to do it carefully, but seem to enjoy eating them just as much as anything else), they can’t manage to eat such big plants in their entirety.
Bull thistle is a biennial, meaning each plant lives two years. So supposedly if you can manage to keep one set of plants from going to seed, the population should dwindle in three years. I decided to machete these plants down. I’m hoping this will stunt them pretty good, and if we mow once later in the summer, that should devastate them adequately.
Once the sheep have grazed a section, the thistle are easy to see and get to, since the grass is neatly eaten and trampled all around the thistle plants. So I’m following behind them with machete in hand to clean up rectangles they just finished. It would be much harder to do with the thistle mixed into tall grass, since grass doesn’t machete very well.
Hitting the thistles with the machete is easy and satisfying; they are tall and have hollow, woody stems which slice neatly with idle swings of the blade. I’m hoping that they’ve put a lot of energy into those soon-to-blossom flower heads, so cutting them off at the ground will give them a serious setback. The only thing that worries me is the warning on the King County noxious weed website that says that leaving cut flowers on the ground might still enable them to finish seeding. I hope not!