Published 04.02.2025
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Canada thistle
Organic guidance
By Lars Egelund Olsen
This website has been automatically translated for your convenience. Reasonable efforts have been made to provide an accurate translation, however, errors or inaccuracies may occur, and some content (such as images, videos, PDF-files, etc.) has not been translated.
Problems with Canada thistle can be solved with knowledge of the biology, prevention and the right control strategy. Canada thistle is a problem in organic fields, but is most prevalent on clay soils and thrives on compact soil.
Prevention against Canada thistle
Control of Canada thistle
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Mapping and spreading
An effective treatment is best carried out by mapping the presence of Canada thisle in the field. Therefore, it is a good idea to record where thistles have been observed in the field. Mapping thistle colonies can help target mechanical control and if the thistles are detected early, they can be cut off at the right time.
A good idea is to make a field map where root weeds are plotted. This makes it easier to target your efforts and prevent the weeds from spreading to the entire field. When treating colonies, it is important to avoid pulling the thistle roots away from the colony and spreading them throughout the field.
FarmTracking can be used to register root weed hotspots at field level, making it easy to keep track of root weeds. Then the crop consultant can also keep an eye on MarkOnline in the office and the right control strategy can be applied.
Strategies for control
Interrupt growth immediately after harvest
Interrupt the growth of Canada thistle immediately after harvest with a full cutting treatment. A cut-through treatment can also be carried out in the autumn to help interrupt the thistle growth and weaken the plant.
Double ploughing
Double ploughing is an effective mechanical control of thistle. The advantage of this method is that the soil treatment will be completed immediately after harvest and a good seedbed is ensured prior to sowing the cover crop. An early established cover crop will reduce leaching and be competitive against weeds.
Double ploughing strategy
Immediately after harvest, ploughing or full cutting is carried out with wing shares. This cuts the vertical root connections and interrupts the growth of the green part of the plant.
A competitive after-crop is sown by 15 August at the latest
In the spring, plough again and sow a competitive crop
The population of field bindweed is affected by ploughing twice. The best effect with 89 percent reduction was achieved by the most intensive treatment with harrowing from harvest until spring plus ploughing in late November and again in April.
The second best effect was achieved by ploughing in late September, sowing cover crops and ploughing again in April with an effect of 64%. 88% effect on thistles was achieved by ploughing in August, sowing yellow mustard and ploughing again in late November plus a harrowing with Kvik-killer before sowing in April. See more about the results in National Trials 2009.
Starvation
Starvation of field horsetail is done by chopping/cutting the plant. It is most effective in spring and summer when the plant is growing and making root shoots and using energy from the roots. The plant is most sensitive at the 6-8 leaf stage, when the energy depot of the roots is lowest.
Birdsfoot trefoil grows in colonies and cutting can be done in patches in the colonies in the field or repeatedly in clover grass or lucerne for mowing.
Recent research has shown that the compensation point for Canada thistle occurs before the 6-8 leaf stage and that mechanical control should therefore be applied earlier than previously recommended, to prevent energy storage in the roots that can be used for regeneration.
Strategy for thistle removal:
Spot treatment can be made in colonies of thistles in cereals, pulses and seed grass by stripping, which is repeated at the 8-leaf stage.
New shoots or root shoots are removed in cereals and pulses without cover crops by tilling just after harvest
In fields with seed grass, the colonies should be stripped at the six to eight-leaf stage in autumn.
In cover crop fields, it depends on the type of cover crop.
In clover-grass fields, three to four cleanings/ploughing is an effective way to starve the thistles.
The treatment ends with a good ploughing, where the weakened and decomposed root weeds are brought down to a depth of more than 20 cm. The weakened roots that are removed may have the power to germinate, but will be weaker plants
Subsequent establishment of catch crops in August or a competitive spring crop will enhance the effect of ploughing down.
Repeated ploughing in clover grass or alfalfa
Ploughing in grass leys consistently weakens the thistles. A good strategy against thistles is to have room for a sward crop in the crop rotation. This is a natural part of cattle farms, where the removal of waste grass and thistle shoots after each grazing round can also weaken thistles.
On a crop farm, the following strategy with a sward crop in the crop rotation can be used:
Immediately after harvesting grain, peas, seed grass, or after whole crop, make a full cut
When there are new thistle shoots, do repeated harrowing
If the soil type allows it, do spring ploughing. Otherwise, winter ploughing is done.
In the following year, sow clover grass as green grain or green pea or choose alfalfa for cutting
From mid to late June, green grain is harvested
Around 1 July, harvest green pea
The denser the thistle population, the more cuts you need to take
Two to four cuts of clover grass are taken and there should be three to four weeks between cuts.
It is important that the cuts continue into spring.
Mini summer fallow
Mini summer fallow keeps the soil black during the summer months. This provides effective control of all root weeds because it is done at a time when there is strong growth in all root weeds and because the strategy is both starvation and drying of roots and runners.
Mini summer fallow is a very effective method of thistle control. This option is used if there is a large thistle problem or there is a mixed population of root weeds, such as chickweed, bracken, lamb's-quarters etc. that cannot be effectively controlled by other means.
Mini summer fallowing can be carried out by repeatedly harrowing between 1 June and 1 August with a heavy-duty stubble harrow with wing blades, which provides a full cut through the soil. The advantage of mini summer fallow is that the control is carried out during a period when there is often low humidity.
Strategy to keep the soil black with mini summer fallow:
Sow a spring crop in the spring.
Do a ploughing or trimming in June.
Keep the soil black with repeated harrowing with full cutting in June and July.
Do a good ploughing at the end of July and sow yellow mustard or oil radish as a catch crop.
Plough again in the spring and sow a competitive spring crop.
Figure 1. Effect figures for mini summer fallow for vetch, coltsfoot, field horsetail and pigweed.
Wheel cleaning
Row cleaning cannot stand alone in the fight against thistles. Row cleaning in row crops and cereals sown at increased row spacing can inhibit thistles, but not control or solve the problem. When the plants are small, they will be easier to control with the row cleaner.
The challenge with row cleaning is the thistles in the row. Trials with row cleaning of the stubble have shown that row cleaning can inhibit thistles. With well-executed row cleaning of the stubble and provided that there is row cleaning in the crop, thistles can be inhibited by approximately 45 per cent compared to no row cleaning.
Thistle biology
Canada thistle is a vigorous plant that blooms in June-July with violet flowers. The plant forms a strong root system that can grow to a depth of over two metres.
Sow thistle is colony-forming, which means that buds from the horizontal roots in the soil form new shoots and thereby form a colony that typically spreads by up to 2.5 metres per year depending on competition from the crop. The propagation of thistles can therefore be followed year by year in the field.
Field thistle is a perennial weed. The plant's life cycle extends over several years, during which the development of strong roots, which act as an energy depot for the plant, takes place. New shoot growth can therefore occur by using energy from the large root system, which can reach down to a depth of two metres.
Repeated cutting of the plant at the time when the energy depot in the roots is lowest weakens the plant and the population can be eradicated. At this point, the plant has used most of the stored nutrients from the roots and has not yet replaced them with new energy from photosynthesis. This is called the plant's compensation point.
It is important to interrupt growth and the supply of nutrients to the roots. Deep ploughing is important to reduce new shoots and the number of root shoots decreases the deeper the plough layer is. Small root fragments that are buried deep have a harder time sprouting than fragments that are buried shallow.
Research shows that the root fragments that remain in the plough layer after deep ploughing are less important for regrowth compared to the undisturbed roots that sprout from below the plough layer.
The depth to which the roots that remain intact after tillage end up has been shown to determine the ability of thistles to regrow. Thus, undisturbed roots that are at or below a depth of 20 cm will produce more aboveground biomass than roots that are at a depth of more than 20 cm.
Until now, recommendations for thistle control have been at the 8-10 leaf stage, but recent research has shown that the compensation point for thistle occurs at the 6-8 leaf stage. Cutting thistles at the 8-10 leaf stage has a good effect on thistles, but compared to mechanical control such as row clearing, there may be an advantage to keeping the plants small.
Occurrence and propagation
Thistles are most commonly found on clay soils and thrive on compact soil where there is, for example, structural damage caused by driving on too wet soil.
In annual crops to maturity, such as spring cereals, winter cereals and pulses, thistles are allowed to complete their life cycle in peace and reproduce. In contrast, thistles don't do well in crops that are cut frequently, such as clover grass and lucerne for cutting.
In a dense, tall and fast growing crop such as lucerne, rye or winter oilseed rape, thistles are sensitive to the fierce competition for light.
When tilling where roots are cut, it is important not to drag the roots around the entire field, as the root pieces can shoot again and increase the spread to the rest of the field.
Row crops with row cleaning and repeated hand hoeing are suitable for weakening thistles.
Seed multiplication
Seed dispersal also occurs in thistles and can be relevant in the formation of new colonies. Most seeds are dispersed only 20 metres from the mother plant. The majority of seeds that are often seen flying from the plant with the wind in the summer are barren seeds because the distance between male and female plants is often too far for pollination to take place.
Normally the seeds have a lifespan in the soil of one year, but can survive in the soil for up to five years.
Germination occurs in both May and August, but it's spring-germinating plants that matter because they can send out underground runners in a 1 metre radius from the main shoot already in the first year.
Autumn growth
Thistles, like several other weed species, do not have a definite dormant period, but there is limited shoot growth in autumn.
The limited growth in autumn means that repeated mechanical control, in addition to the important interruption of growth after harvest, will often be wasted effort. In a warm autumn, however, thistle growth can continue and it is therefore important to monitor growth and apply control measures at the right time. Tillage may be relevant during this period if new shoots form.
Yield loss
Thistles can cause significant economic loss, especially in uncompetitive crops. In spring barley, two to five thistle shoots perm2 have shown a yield loss of 20 per cent.
In winter wheat, oats and spring wheat, the yield loss is approximately 10 per cent of the yield for the same population.
The yield loss in pulses will be greater, which is why you should avoid growing pulses in fields with thistle problems.
Crop rotation
It's a good idea to prevent thistle problems by combining several things in crop rotation.
A crop rotation that inhibits thistle can include one or more of the following elements:
- A strategy with a minimum of 20 per cent clover grass with mowing and/or slashing. This can effectively starve the thistles and the greatest effect is achieved if done for more than one year.
- Competitive crops such as oats and winter rye.
- Vigorously growing cover crops.
- Avoid uncompetitive crops for more than two years in a row.
- Repeated row cleaning can reduce thistle propagation, but cannot control the population.
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