Article Archives
Fall Prime Time to Whack Weeds

Saturday, September 15, 2007
filed under: Weeds

Set up your sunflower crop next spring by managing weeds this fall, starting with Canada thistle. Preharvest and fall-applied treatments provide the most effective long-term control of this perennial noxious weed.

Shortening day lengths, which signal the approach of winter, trigger movement of food energy to the roots of perennial plants for storage. This is why translocation or systemic herbicides are most effective in the fall in controlling perennial weeds such as Canada thistle, quackgrass, dandelions, and field bindweed.

A fall application of glyphosate does a good job of inhibiting thistle growth the following year, says Kirk Howatt, weed scientist at North Dakota State University. It should work particularly well in tandem with ExpressSun™, the new sunflower hybrids with built-in tolerance to tribenuron methyl, the active ingredient in Express herbicide. Research has demonstrated control of Canada thistle among other weeds in conventional, minimum-till or no-till sunflower production systems with the ExpressSun herbicide technology.

Howatt has noticed a difference in research plots where Canada thistle wasn’t controlled the previous fall. Left unchecked, it can choke out emerging sunflower the following spring.

Howatt says the effectiveness, affordability, and safety of glyphosate makes it the best option to consider for Canada thistle control in front of sunflower. Outside of glyphosate, there is little to consider in the fall for effective Canada thistle control that won’t pose soil residual risk to sunflower the following spring. Dicamba (Banvel) is a possibility, but a risky possibility. The 2007 NDSU Weed Control Guide notes the following about dicamba residual:

“Dicamba at rates greater than 1.5 pt/A may remain as a residue in soil. Most grass and broadleaf crops can be planted 4 months or more after application at 1.5 pt/A. Allow 45 days/pt/A of dicamba, excluding days when ground is frozen to rotate to any crop. NDSU research indicates dicamba at 1 qt/A applied in late September caused visible injury to wheat and barley planted the following spring, but effect on yield was minimal. Dicamba at 1 pt/A applied the previous fall prevented seed production in sunflower. The approximate ranking of crops from most to least tolerant is corn, barley, wheat, oat, potato, buckwheat, soybean, dry edible bean, sunflower, flax, and sugarbeet.”

Dry soils can increase the risk of herbicide carryover; since herbicides are primarily degraded by soil microbes, they can become more persistent under dry soil conditions. Soil pH can affect the stability and persistence of some herbicides as well.

NDSU rates glyphosate as having good to excellent activity on Canada thistle, depending upon the rate used, weed size, and environmental conditions. NDSU rates dicamba as having fair to good activity on Canada thistle.

For comprehensive information, go online to - click on the link ‘2007 ND Weed Control Guide.’ Scroll down to the links with specific information on perennial weeds (including Canada thistle) weed control ratings, and herbicide carryover (see soil pH information, and scroll to the bottom of this link for a chart of crop rotation restrictions in N.D. for a number of herbicides in 17 crops, including sunflower).

Spraying after a frost/freeze

Three frost/freeze threshold temperatures are often referenced:

• 32°F, the temperature at which water freezes which may or may not be a plant freezing event.

• 30°F, which will cause damage to all cold sensitive plants.

• 28°F, well below freezing which will kill cold sensitive plants.

However, a damaging frost or killing freeze in the fall will depend on a number of factors, including the plant (obviously some are more cold tolerant than others), geography, soil moisture, wind speed, cloud cover, and dewpoint. “Sometimes we get frost when the temperatures are above freezing, and we often have a freeze without frost,” comments Michigan State horticulturalist Mark Longstroth on his plant frost/freeze web site, .

“It takes a very cold temperature to kill Canada thistle,” says Howatt. “Depending upon how big the plant is, if a frost damages the top one third of the plant, those lower leaves may still be viable for spraying.” Wait for the upper leaves to wilt after a frost to see if the lower leaves are still viable for spraying, he recommends. This will also help make a spray treatment more effective, so that the wilted upper leaves don’t intercept herbicide contact onto the lower leaves.

According to NDSU Extension weed specialist Richard Zollinger, most all perennial weeds respond the same to fall-applied herbicides – if the leaf tissue that intercepts the spray is green and has good integrity and not damaged by freeze, then the herbicide will be absorbed. The other major issue that affects activity of the herbicide is the temperature and weather following application. If warm temperatures follow for a few days after application, then it is more likely that the herbicide (glyphosate) will be translocated throughout the plant, resulting in better kill of underground roots. If cold daytime temperatures follow application, then control will be diminished.

See NDSU Circular W-799 on thistle control for more comprehensive management information, ‘Perennial and Biennial Thistle Control.’

Other weed control considerations

Monitor fall growth in harvested wheat stubble for problematic weeds such as marshelder and kochia, especially if those fields will be rotated to sunflower next spring. It’s possible for both weeds to grow enough yet this fall to set seed – if this appears likely, consider a glyphosate burndown treatment.

Managing weeds as well as volunteer crops in the fall also helps conserve soil moisture, adds Alan Helm, Colorado State Extension weed scientist. Field bindweed is a perennial that can be a problem in High Plains sunflower. Here again, treatments are limited if rotating to sunflower. To knock back field bindweed, Helm suggests a glyphosate + Aim (carfentrazone) application at planting or shortly after in the spring, before sunflower has emerged.

NDSU points out in its field bindweed circular (online at that a tank mix of dicamba at 0.5 lb/ac (1 pt/ac Banvel/Clarity) plus glyphosate at 1.5 lbs/ac (2 qt/ac of a 3 pound acid equivalent per gallon formulation) gives better and more consistent control of field bindweed than glyphosate applied alone. This treatment has less potential for soil carryover of dicamba than using higher dicamba rates of 1 to 2 lb/ac. Again, mind the residual risk of fall-applied dicamba preceding sunflower – lower rates and later plantings may allow sunflower growers to get away with including dicamba in a fall perennial weed control treatment.

Phil Stahlman, Kansas State Extension weed specialist, reminds growers to be careful about using atrazine in their crop rotation, especially if it includes sunflower. A 12 month interval is generally recommended before planting sunflower after atrazine is applied. So if you applied atrazine on corn last spring, planting sunflower on that ground next spring should be fine. However, Stahlman points out that some growers in the High Plains will apply a glyphosate + atrazine mix in the fall for burndown and residual weed control. “They’ll do this with the intention of planting corn or sorghum the following spring,” says Stahlman, “but if they change their minds and go to sunflower, then they’re going to be in trouble.” – Tracy Sayler

Don’t Blame Canada for Canada Thistle, One Amazing Weed

Saskatchewan Agriculture points out in its online bulletin,, that the suspected origin of Canada thistle is the eastern Mediterranean region of Europe, and it was likely one of the first weeds imported to North America by early settlers. It gets the name Canada thistle since the early residents of New England in the U.S. blamed its arrival and spread on French traders from Canada, but historians now believe it arrived in both places at about the same time from Europe.

The thick, fleshy roots of Canada thistle make this weed difficult to control once established. The plant reproduces very successfully through vegetative “cloning” of itself from the root. Above ground parts produced by Canada thistle die back completely each fall, and new shoots are produced from root buds in the spring – old root tissue is continually being replaced by new, resulting in a particular root living only two years.

Within 19 days of emergence (two-leaf stage), new seedlings can regenerate after top-growth removal. During its establishment phase, Canada thistle initially produces a taproot that penetrates to depths in the soil with consistent moisture. Root depths of six to 10 ft are not uncommon and roots may go as deep as 18 ft.

Lateral roots are not only important for expanding the colony but also contribute to the spread of the weed vegetatively with the assistance of humans. New plants can be produced from root pieces as small as 1/8” to ¼” thick and 3/8” long. Tillage and seeding implements contribute greatly to the vegetative spread of this and other perennial weeds.

return to top of page

   More about Sunflower ►