Canada thistle infestations have been increasing on western Canadian farms in part due to changes in management practices. More specifically: reduced tillage, extended crop rotation with less summerfallow, and increased acreage of special crops (such as pulses) in which Canada thistle control options are practically non-existent.

Reduced tillage has especially favoured perennials like Canada thistle, because it results in less root disturbance, and the root system is Canada thistle's main means of reproduction and spread.

Canada thistle is an incredibly hardy weed. Some characteristics that contribute to its growing success include:

• Biomass of the creeping root systems of Canada thistle can be as much as 10 times the visible above-ground parts.
  
  
• Rootstocks send up new shoots every six inches, and the roots spread horizontally about 20 feet in one year.
  
  
• The root system contains enough food reserves to generate new shoots for more than one-and-a-half growing seasons without being replenished. Root fragments (e.g. broken off by tillage) have enough food reserves to survive for 100 days under adverse conditions.
  
  
• In one growing season, a single shoot can spread over 20 feet in diameter and produce a total root length of 1,600 feet. In that time, it can also produce 26 daughter shoots, 154 underground shoots and approximately 800 buds.
  
  
• Seed is not an efficient means of spread for Canada thistle, but it does occur. The seed germinates best at high temperatures (25°C to 30°C), and can germinate 10 days after maturity.
• Seedlings are poor competitors initially, and need high light intensity to thrive.
  
  
• Three weeks after successful emergence, Canada thistle seedlings can regenerate top growth from their established taproot.
  
  
• Four to five weeks after successful emergence, horizontal roots can initiate new shoot growth.

CANADA THISTLE - UNFORTUNATELY, ERADICATION IS A LENGTHY PROCESS.
Thistles present a tough management problem. In fact, the ONLY way to eradicate them is to plan a long-term (three- to four-year) management strategy using all the best options at the right times.

Along with early, in-crop, translocatable herbicides, pre-harvest and (when practical) post-harvest treatments, consider using these integrated methods:

Crop selection. Choose a crop that is a strong competitor and allows in-crop thistle treatments. Cereals or canola are the best bets. Well managed forage grasses or alfalfa compete well with thistles, but thistles thrive in poorly managed hay fields or overgrazed pastures.

Seeding. Thistles do not emerge early in spring, so seed early to help the crop seize a competitive advantage by emerging before the thistles. Use high-quality treated seed, and seed shallow to promote fast germination and emergence.

Fertility. Thistles respond very well to nitrogen fertilizer. Apply the rate of fertilizer you normally would as if no thistle were present, but seed place or band it if possible to give your crop the advantage of access to the fertilizer.

When executing your plans, BE PERSISTENT! You can beat thistle by making the right choices.

THISTLE CONTROL: IT'S ALL IN THE TIMING.
Profitable thistle control with herbicides involves taking advantage of thistle biology at two critical times - early in the crop year and pre-harvest.

GET 'EM BY THE ROOTS.
The most widely used herbicide control strategies are either in-crop suppression, followed by pre-harvest treatment with glyphosate (i.e.Vantage* Plus orVantage*); or early in-crop root kill with a clopyralid product (e.g. Eclipse*, Prestige*, Curtail* M, Prevail*, Lontrel* or FlaxMax Ultra') followed by a pre-harvest treatment with glyphosate. Dow AgroSciences offers the products necessary to implement either of these strategies but, for many reasons, the second option is more effective.

In-crop suppression + pre-harvest

• Although the visible parts of thistles are removed by in-crop spray, the 10-times greater sized root system survives to generate new shoots.
  
  
• The grower incurs additional application costs for the crop near harvest - a time when cash flow is typically a concern.
  
  
• The grower must rely on favourable weather, time to spray and/or the availability of custom applicators. Trampling due to application will also create additional crop loss.

In-crop root kill (+ pre-harvest if necessary)

• Products containing clopyralid are unique in their ability to move to the root system. Once there, translocated clopyralid provides significant root kill for the best thistle control of all typical herbicides.
  
  
• Thistles are controlled at the same time as the normal post-emergent spray, which reduces overall application costs. No second pre-harvest pass is required.
  
  
• Thistle competition is eliminated during the critical early growth stages of the crop, which protects crop yield potential.
  
  
• Root kill results in extended control. Research shows that thistle control may extend into subsequent crop years.

Other application timings will not be as successful at controlling thistles.

Pre-seeding burnoff is less practical for thistles than for other broadleaf weeds. It provides only top growth control, and control is further reduced due to incomplete thistle emergence. Late in-crop application (i.e. spot spraying once flower buds are formed) will result in only top growth control, because thistles are translocating nutrients to the flowers, not the roots, at this time. Post-harvest treatment is often inconsistent, since the best part of the thistle for herbicide uptake has been cut off, and thistles must be actively re-growing for herbicidal effect. The variable weather of the post-harvest season can also make this treatment less effective.

THE STRONGEST POSSIBLE CANADA THISTLE ATTACK.
For really aggressive action against thistles, use in-crop clopyralid-based products. If required, follow up with a pre-harvest application of glyphosate (Vantage or Vantage Plus). It's the strongest possible herbicide attack for the toughest thistle problems. Data from AAFC trials at Melfort, Saskatchewan demonstrate that a combination of in-crop clopyralid and pre-/post-harvest glyphosate treatments provide improved control and increased yield.

Sources:
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