The Weekly Weeder: How to kill Star Thistle

          Star thistle is starting to bloom all over our town.  When I lived in Grants Pass in the ‘80s, I knew it only by reputation as a noxious weed.  It was way out in the county on neglected farms, of which there were few.  When I came back in 1999, it had taken over a field at Parkside School.   Now it is on all over vacant lots and neglected portions of business properties around town.  More and more of it grows on residential properties, particularly outside of fences and along streets. 

Cut but still flowering and gathering litter

          Star thistle is a gray-green plant with a distinctive growth habit, about a foot to 18 inches tall that has no spines except on the outside of its flower buds, but those are wicked.  When the yellow, ½ inch puffy flowers go to seed, the very sharp spines fold down around the stems and are no longer dangerous, but as long as they are blooming, they are spiny.  Mowing shortens the plant, but it continues to bloom and make seed under the mower blades if cut above the bottom branches.  The seeds do not fly on the wind, but stick to fur, clothing and mowers with their fluff and thereby spread through the winter.

Growing along a sidewalk on Agnes and N Street, railroad property

          It has very tough stems and roots and does not pull easily, even from wet ground.  It loves dry ground but also germinates and grows in watered ground, despite rumors to the contrary.  But it can easily be killed by cutting under the crown from which the initial rosette of leaves grow, and later, under the branches of flowers after the leaves die off as it flowers.  It will not branch again on the bare stem below the branches. 

          There are biological controls.  The most effective are probably goats, which eat it even when it is showing its spines.  There are also two weevils and two flies which attack the developing seeds, but they can achieve only about 50% control, and using other methods like cutting can interfere with the breeding of the control insects.

          Mowing and cutting works only if one cuts below the branching of the stems after the plant has started to bolt, or below the rosette of leaves before that.  But a sharp pair of scissors and sharp eyes are probably more efficient than using a weed whacker to beat them into submission, considering the toughness of the stems below the branches.

          Glyphosate spray (Roundup, etcetera) kills the plant but fertilizes the ground for broadleaf plants like star thistle and other weeds.  Replanting the ground afterwards with a broadleaf ground cover like clover can suck up that fertilizer and eventually stop star thistle seeds from germinating by covering the soil and out-competing it, particularly if weeded as well.

Tilling before the plants bloom and retilling after each rain can get rid of star thistle after several seasons.  If you don’t want these or other windblown seeds to invade the tilled ground, replant to a strong competitor like tall fescue grass and/or clover in the fall.

                               

Rycke Brown, Natural Gardener          541-955-9040        rycke@gardener.com