Kate and The Terrible, Horrible, No Good, Very Bad Weed: Canada Thistle

By Kate Samra

As a Campus Farm intern at Matthaei-Nichols and an advocate for sustainable farming, I have a keen interest in modern methods to organically control aggressive and invasive weeds on food producing land.

My research project this summer involves exploring several different large-scale weed-control techniques on a specific weed-prone plot of the Campus Farm. I aim to determine which of these methods best inhibits new growth of weeds unique to this section of the farm, the most notable and abundant of these weeds being Canada thistle, or Cirsium arvense. Canada thistle is an aggressively spreading perennial weed that has deep, creeping roots which make it difficult and time-consuming to remove. While Canada thistle is the main problem species in this section, there are several other weeds we aim to eliminate such as bindweed, creeping purslane, and pigweed, which have been treated and will be observed as well.

Flower of Canada thistle (Cirsium arvense).

Ideally, I would like to work towards finding a less strenuous and more natural way of removing these weeds without destroying the soil, microbe population, or nutrient base that is vital to successful plant growth on the farm. Though it may be easier to use herbicides to kill off these weeds, the Campus Farm strives to model organic agricultural practices that are safe to humans and that conserve the plant and insect biodiversity of the area.  

I chose to divide the thistle plot into six different 12 x 30-foot sections, five with a different weed treatment on each, and one control plot. The first plot was covered in black food-grade plastic and the second was covered in clear food-grade plastic. Each of these plastics was purchased from Johnny’s Seeds and both “bake” the weeds so that they are unable to grow or germinate. The next plot was sprayed with horticultural grade vinegar. At 20% concentration this vinegar is 15% stronger than household vinegar and is often used in gardens to kill weeds. The fourth plot was covered in heavy contractor’s paper and mulch and the fifth was planted with a cover crop of buckwheat to attempt to compete with the weeds and reduce the area the thistles are able to spread.

The experimental plot on the Campus Farm with different sections of weed control methods being tested.

I hope to get a full month of cover and growth and then collect my results, so I will not remove the treatments and take measurements until August 1st. While the results of my research are not yet complete, there are some visible differences between the quantities of thistle and other weeds emerging in each plot.

Horticultural Grade Vinegar (20%) used for weed control

Because Campus Farm student management changes every semester, it’s important to pass along information and new discoveries of what works best in different areas of the farm. I hope that I am able to conclude which of the organic weed-control techniques I’ve explored this summer, or combination thereof, is most successful in eliminating Canada thistle and other invasive weeds. By doing this, next year’s management may have an easier time deciding how to approach the weeds in this area. In addition, while weed control is dependent on soil type, crop history, types of weeds, planting patterns, etc., it’s possible that a method that may work best in this experimental plot could work well in other sections of the farm too.

Kate Samra, from Marshall, Michigan, is a sophomore studying plant biology and minoring in sustainable food systems. Her interests include permaculture, urban farming, agro-ecology, and food justice. She is working as a sustainable food intern this summer, a position funded by the Kitchen Favorites plant sale.