MARGARET SMITH, PHD, FORAGE AGRONOMIST, ALBERT LEA SEED STEVEN J. CARLSON, PHD, RESEARCH AGRONOMIST
Canada thistle (Cirsium arvense L.) is manageable in both organic and conventional farming systems, but the approach is much different. The challenge is certainly greater on organic farms. Organic growers should think in terms of management rather than control as it is difficult to completely eradicate this introduction from Eurasia. Organic management methods include:
- Smother/cover crops
- Biological control agents
Understanding the life cycle and growth of this weed helps in understanding these approaches to management.
Canada thistle is difficult to control once established because of its extensive root system, which can grow more than ten feet horizontally per year and up to eight to ten feet deep. Canada thistle roots and shoots produce allelopathic chemicals that can inhibit both seed germination and plant growth of crop species. These chemicals also inhibit growth of beneficial soil microbes such as mycorrhizal fungal species which assist crop plants in harvesting moisture and nutrients from the soil. ‘
Canada thistle does suffer from competition for light, above ground, and for root space and resources, below ground. One reason the weed is so aggressive is that our annual cropping systems don’t provide enough competition, either below OR above ground.
LIFE-CYCLE AND REPRODUCTION
Classified as a noxious weed in most states, Canada Thistle is a perennial that can reproduce from seed, but the plant’s vegetative reproduction poses the larger threat to cropping systems. Most Canada thistle populations start in a field from seed, though root segments may be also transported on tillage equipment. Once a population is established, spread within the affected field is largely by underground spread of roots and buds.
Plants begin to flower in late spring and reach the bud stage in late May to mid June. Left to mature seeds, a plant can produce 1,200 seeds or more, which can remain viable in the soil for more than 20 years. Plants within a patch are either male or female (dioecious) and must be pollinated by insects between patches. Seeds spread by animals, clothing, farm equipment, contaminated crop seed, or by wind.
VEGETATIVE REPRODUCTION: ADVENTITIOUS ROOT BUDS
Plants that rise from adventitious root buds are very aggressive and can compete in most cropping systems, whereas plants from seed are less competitive and generally take eight to ten weeks to establish a perennial plant system.
Left undisturbed, new, viable shoots from Canada thistle develop primarily in the spring from adventitious root buds (shoot-forming buds originating on roots). Thousands of buds can form per square foot of soil in a well-developed root system, but only a small percentage will elongate to form shoots. The remainder stay dormant due to hormonal suppression by the plant, but new shoots may form from parent shoots when mowed.
Weed scientists, Dr. Matt Leibman, Iowa State University and Dr. Adam Davis, University of Illinois, note that “weed management in organic farming systems requires the concerted use of multiple physical, biological and cultural tactics.” Working with Canada thistle is no exception; a variety or combination of control methods will result in the best suppression.
Historically, Canada thistle was controlled by tilling every three weeks during the growing season.
Tillage breaks up the root system and kills emerged shoots. Deep-tillage passes break up established roots. Then, once buds have emerged, shallow tillage passes kill the new shoots. Cutting the root system into short segments breaks apical dominance, which stimulates root buds to grow and depletes important carbohydrate reserves in the root system. Generally, nutrients flow from the roots to the shoots until third or fourth leaf stage of growth, at which time nutrients produced by the shoots begin to replenish the root reserves.
Some organic growers use this strategy following the harvest of a small grain. Jon Findlay from Cara, Michigan does weekly tillage for six weeks following small grain harvest. He alternates deep with shallower tillage by chiseling at 6 to 8 inches deep followed in one week by an offset disking 3 to 4 inches deep. After this series of tillage passes, he plows and follows with an oat-forage pea fall cover crop. Jon uses this strategy only once every six years in his rotation and reports excellent thistle suppression, with corn yields increased 35 bu/A compared to corn following a heavy thistle infestation.
On Paul Hoffman’s farm near Earlville, Ill. where thistle infestations are moderate, he uses shallow tillage every 2 to 3 weeks in July and August following small grain harvest. He follows this with a cover crop mix of oats, 4010 forage peas, a brassica, buckwheat and sunflower. The following spring, he works this cover crop residue with vertical tillage. This stimulates a flush of buckwheat, self-seeded from the previous year, which he works down at height of 4 to 6 inches before planting corn.
2. SMOTHER/COVER CROPS
One of the best smother crops for Canada thistle is a warm-season summer annual: sudangrass or a sorghum x sudangrass hybrid. Dave Campbell farms near Maple Park in northern Illinois and has used this strategy for years after participating in an Illinois study to evaluate the smothering effects of sudangrass and sudangrass with cowpeas (70:30 ratio). In the research, both of these plantings reduced Canada thistle populations by 96%. In this same study, neither buckwheat nor summer fallow adequately suppressed the thistle.
Dave tills three to four times in the spring with his quack digger, then plants Viking 100 sorghum x sudangrass hybrid in mid June. Dave drills a heavy seeding rate of 50 lbs/A and is considering going up to 60 lbs. He wants a very thick stand to completely shade the Canada thistle. He flail chops this in August and doesn’t removed the forage, then allows the sorghum x sudangrass to regrow as the weather allows. In the late fall, after frost has killed the sorghum x sudangrass, he chisel plows the field. He avoids early fall tillage to avoid stimulating any new shoot growth from the few remaining Canada thistle plants. Dave shared that this method is not as effective on his heavier soils and in years of high rainfall.
Where thistle populations are heavy on Paul Hoffman’s farm, he uses a similar strategy to Dave’s, but Instead of mowing the sorghum x sudangrass, he strip grazes it just at or before heading with his sheep flock. He has been excited about this practice, which has reduced his Canada thistle population from 60-65 shoots/yd² to about 1/yd², a 98.5% reduction rate!
Repeated mowing near the soil surface will deplete Canada thistle root reserves. Mowing weekly following the bud stage is most effective, but it results in no marketable crop after mid-June. A hay crop combines the strategies of competition and defoliation.
Alfalfa and forage grasses, particularly orchardgrass and tall fescue, compete well with Canada thistle. When raising hay, to provide the most competition, maintain optimal fertility levels for a healthy, vigorously growing crop. The competition from established forage roots and the rapid regrowth following harvest, combined with the defoliation when cutting hay ‘slows down’ and will decrease your thistle population. Paul Mugge, from Paullina, Iowa, uses this strategy. Each year Paul has seven to eight percent of his crop acres seeded to alfalfa for hay. This seeding is moved around the farm and placed each cycle where the Canada thistle population is the worst. Paul maintains his hay stand for three years and cuts the hay (and Canada thistle) four times during the growing season. Paul estimates a 90 percent reduction in the Canada thistle population under this system. Hay is sold off the farm, as Paul doesn’t raise ruminants in his system.
4. BIOLOGICAL CONTROL AGENTS
Insect biological control agents must be specific to the target species to avoid off-target impacts.
Researchers have introduced and evaluated six different insect species for biological control of Canada thistle. A stem-mining weevil, Hadroplontus litura, whose larvae feed on thistle stems had a small impact in North Dakota, though plants easily recovered. No other insects have been found effective for control.
Disease-causing fungi and bacteria can be more specific than insects. At least three have been evaluated for Canada thistle control. Both Puccinia puncitiformis (a rust) and Psuedomonas syringae pv. tagetis (a bacteria that causes leaf spot and chlorosis) were found to infect Canada thistle. The bacteria, Psuedomonas, showed promising control in one year of a two-year experiment in Minnesota, but no effect the second year. None of these has been commercially successful.
CONVENTIONAL MANAGEMENT METHODS
Conventional growers have the option to employ a longer crop rotation or seed a hay crop as part of their management strategy. Historically, however, they primarily have relied on systemic herbicides to control Canada thistle. Herbicide options vary in their efficacy on this persistent perennial. Preemergence and incorporated herbicides are ineffective. Products rated ‘Fair to Good’ for post-emergence control include Basagran, dicamba, Beacon, 2,4-D, Liberty and Roundup. Effective control may require multiple applications in a growing season. Check with your supplier and read your product label before treatment.
Diligent observation for and removal of Canada thistle new growth is important to prevent reestablishment of the root system. Preventing seed production and spread is also important in the control of Canada thistle. If a new patch of thistle is observed due to seed germination (or from a newly introduced segment of root) control it immediately to prevent establishment of a mature root system.