How to Kill Thistles

How to kill thistles. Occasionally I get asked how to deal with pernicious weeds. This post is about thistles. We mostly have Canadian Thistles in our fields but also run across an occasional Bull Thistle.

If I have a thistle patch I tend to plant one of our higher value crops there. It may seem like an odd thing to do, but if there is a valuable crop in the area I tend to weed that area well and this will reduce the weed pressure.

The season before I plant a valuable crop I let the thistles grow to bloom. Thistles exhaust their root reserves at the same time their blooms are the brightest. If you mow them at the height of blossom it is lethal to the the plant.

Canada thistles thrive in low calcium and very low manganese soils. Bringing the levels of these nutrients to nominal levels will help suppress thistle proclivity. This soil balancing can be increased by adding compost to the soil.

Though it takes several seasons this approach really works. I have some places where thistles were very thick and they are almost completely gone three years later.