Organic Broadcaster


Cover crop of sorghum-sudangrass smothers, starves Canada thistle

By Dave Campbell, Lily Lake Organic Farm

I would guess that many of you have witnessed more Canada thistle patches in recent years not only in summer-harvested small grain crops, but also in areas that don’t get mowed on a timely basis, such as roadway medians, natural habitat areas, borders of farm ponds and sloughs, and other hard-to-reach areas. If Canada thistle or other perennial weeds such as quack grass have become a problem on your organic farm, there is still time this spring to take proactive steps towards rooting out these weeds from your fields.

It has been my observation that excessive rain typically results in increased spread of Canada thistle where patches are well-established and where heavier soil types, especially clay, are present. We’ve certainly had excessive spring rain in the past few years. You might also have off-farm origins of thistle dispersion, like I do, that can negatively impact your farm. 

My contiguous 224-acre farm borders 11 different landowners. Some properties adjacent to me are owned by absentee landlords and one is a county bike path (former railroad bed). Every summer, I encounter thistledown blowing onto my farm from almost every direction. One summer, I had a “talk” with one of my neighbors. This elderly man didn’t have a clue as to the damage his thistles were causing. He said he grew up in the city and was not aware of the invasiveness of thistles—he really enjoyed watching the thistledown blowing in the breeze.

Canada thistle encroaches on a field of soybean stubble at Lily Lake Organic Farm in Maple Park, Illinois. Photo by Dave Campbell

Sometimes the problem is one of our own doing. During the fall of 1991, I purchased some spelt seed from an individual located in a neighboring state, about 5 hours from home. When I asked him if the seed was weed-free, he assured me it was. I bought enough seed for a 24-acre field. Seed was hard to find that year, so I took a chance on planting this shipment even though I could see it wasn’t “weed-free.” It turned out this spelt seed was contaminated with Canada thistle seed. The entire field was seeded with a sprinkling of Canada thistle seed. Chalk it up to a “buyer beware” experience.

It has long been my opinion that soil fertility issues should be addressed as the first line of defense against weeds, both perennial and annual. However, in my experience, even a soil test that reflects a well-balanced mineral profile with good organic matter levels doesn’t guarantee that you will not have issues with Canada thistle. Calcium levels in my soils are in the ideal range and magnesium levels are close to ideal. I am overall pleased with my soil health, including soil structure, weed control, yields, and grain/forage quality. While I do not have a major thistle issue on my farm right now, I choose to be proactive so that a smaller issue doesn’t get out of hand. 


In 2007-2009, I was involved in the NCR-SARE research project #LNC07-282 with the University of Illinois. Titled “Best Sustainable Management Practices for Perennial Weeds,” the project found 90% control of Canada thistle on plots located in Iowa, Missouri, and Wisconsin, while 95% control was reported on plots in northern Illinois, where the majority of research was conducted. As the title suggested, other perennial weeds, in addition to thistles, were alleviated or totally eradicated by growing sorghum-sudangrass as a cover crop in a fallow year.  

In the first year of the research, I trialed different cover crops to study their effectiveness in rooting out Canada thistle. I found that sorghum-sudangrass was the most effective. I refer to this cover crop practice as a “smother-and-starve” approach. This research project was highlighted twice in MOSES-sponsored field days I hosted in 2010, and again in 2018.

Steps to Control Perennial Weeds

This protocol focuses on an ecological approach of weed management, although some spring tillage is needed as well. Tilling in spring rather than fall forces the plant to use up its energy reserves as it regrows after each tillage pass or each pass with the mower. 

Sorghum-sudangrass grows along the edge of the corn field next to the grass waterway at Dave Cambell’s northern Illinois farm. Campbell plants the cover crop in his smother-and-starve strategy to thwart Canada thistle. Photo by Dave Campbell

Start out in the spring when soils are not too wet by tilling the field or part of the field where you want to use this method. Here in northern Illinois, I plan to sow sorghum-sudangrass around the second week of June, although I have planted as late as July 4 due to wet soils and still have had success. 

I suggest using a quack-digger type of field cultivator, preferably on days when it’s sunny, dry, and windy. Figure at least two, maybe three tillage passes will be needed before drilling sorghum-sudangrass. The goal is to stunt the thistle plant multiple times in order to weaken root reserves. Sorghum-sudangrass will grow very rapidly during June and July and will “smother” weeds by depriving them of sunlight. Photosynthesis is not occurring in the weeds because weeds can’t compete with the rapid growth of sorghum-sudangrass.

Another effect sorghum-sudangrass exhibits is a “starvation” effect.  The root system of this plant out-competes invasive weeds for moisture. Even though roots of sorghum-sudangrass are much smaller than that of corn plants, planting at approximately 600,000 seeds/acre generates a massive root system. This vast network of roots will soak up moisture and thereby limit the amount of moisture available to thistle roots, especially during times of dry weather, such as mid-summer into fall. 

Seeding rates for sorghum-sudangrass are recommended at 50#/acre, according to the research. I typically drill 45#/acre and am pleased with that rate, although I would not seed any less than that. Seed catalogs typically suggest planting around 25#/acre when drilled. Keep in mind that this lower seeding rate is suggested for grazing or forage production, not for thistle control. Aim to sow at 1” to 1½” deep, assuming moisture is adequate. 

The next step in the process is to clip the sorghum-sudangrass before the plant fully heads out. Clipping can take place after heading out as well, although less plant regrowth will occur. I typically use my haybine to clip the grass at a height of 12” or a little higher off the ground. Set the windrow gate all the way down to allow for a wider swath of grass to cover the majority of stubble, which will limit sunlight exposure to the few thistle plants still remaining. 

Mowing sorghum-sudangrass increases root mass while forcing roots to penetrate deeper. If you can’t get in to clip your sorghum-sudangrass until well after heading out, then I suggest chopping with a flail mower because the sorghum-sudangrass will have lodged somewhat by this time. Mature sorghum-sudangrass is also hard on rubber haybine rollers. You can come back for a second mowing with a flail chopper later in the fall, or just leave regrowth until the following spring.

Canada thistle is much more prevalent in the northern latitudes of the U.S., due to more hours of daylight during the peak of the growing season. Surprisingly, it doesn’t take much of a difference in daylight hours around the time of the summer solstice to observe a difference in thistle plant growth. Canada thistle has been found in all states of the U.S., but is more commonplace in areas north of 37º latitude, and extends well up into Canada. In Illinois, there’s a recognizable difference in Canada thistle populations near the Wisconsin border versus far southern Illinois.

I have also noticed a very strong allelopathic effect the following year for crops grown after a sorghum-sudangrass cover crop. I have observed a 7-10 day broadleaf weed emergence delay in row crops following a year of sorghum-sudangrass fallow, which is significant. I have been asked a few times if one might grow a short-season cash crop, such as buckwheat after a sorghum-sudangrass cover crop during the same season in order to generate at least a little bit of income. I suggest not doing that. Paul Hoffman, who farms in north-central Illinois, has grazed sheep on sorghum-sudangrass that was drilled to alleviate Canada thistle issues. So far he has had success controlling Canada thistle, in this scenario. For Paul, grazing did not start until mid-to-late August when daylight is rapidly diminishing. The sorghum-sudangrass was not tilled under, and a fair amount of top growth still remained after grazing.

I mow thistle patches, when present, in my small grain fields with a 3-point mower. Immediately after mowing, I hire local kids to clip thistles in sparsely infested areas before the rosettes (buds) open up, in order to greatly minimize the spreading of thistledown. We use heavy-duty clippers made by Corona that last forever, and very rarely need sharpening. (Don’t waste your money on purchasing the cheaper clippers that you will probably toss out the first day of use.) We clip thistle plants around 1 foot or so below the bottom rosette, to prevent more rosettes from forming. We typically go back a second time, about 2 weeks later, to catch thistles missed or that weren’t as mature during the first pass through the field. 

When managing thistle in non-tillage areas, especially in waterways and fence lines, mow before rosettes bloom. Multiple mowings work best by weakening root reserves (carbohydrate starvation.)

Dave Campbell has been growing small grains, row crops, and forages organically in northern Illinois and southern Wisconsin for the past 40 years.  


From the May| June 2020 Issue


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