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Weed Control in Sunflower Starts Now!

Saturday, September 1, 2001
filed under: Weeds

Weed Control in 2002 Sunflower Starts Now

Fall Key Time for Canada Thistle Control

Now is the time to start thinking about weed control in your 2002 sunflower crop. Think about which fields you want to rotate to sunflower, the weed problems in those fields, and any fall treatments. Fall is a good time to control key broadleaf weed problems such as Canada thistle and field bindweed, but be mindful that some herbicides have rotational restrictions, meaning restrictions to planting sunflower next year in fields where products are applied this fall.

“The weed control programs in soybeans, corn, and wheat have spillover benefits for sunflower, and suppressing perennial weeds in the fall is certainly going to pay off,” says Leon Wrage, South Dakota State University extension weed specialist.

It’s no wonder that Canada thistle has zoomed to the top of the noxious weed “most wanted” list in the Northern Plains, enabled by wet conditions the last few years that has favored plant establishment and thwarted control efforts. Canada thistle is able to produce over 5,000 seeds per plant, which can be carried by wind as far as 200 feet from the “mother” plant. Further, it has an extensive root system that that may penetrate the soil to a depth of 10 feet or more and grow laterally 12 to 15 feet per year. Because Canada thistle has a massive capacity to produce roots and shoots eventually growing up as new plants, trying to kill top growth, as one Canadian weed scientist puts it, is like giving thistle a haircut.

Richard Zollinger, extension weed specialist at North Dakota State University, says a key to successful control of Canada thistle is to understand how it grows. Over 90% of Canada thistle’s root system is below cultivation depth. Thus, the only way to effectively control the root system is to apply herbicides when maximum translocation of the chemical to the underground roots will occur. This happens in late summer or fall after day length is less than 14 hours, says Zollinger.

Thistle plants that emerge after there is less than 14 hours of sunlight will not bolt and produce flowers but stay in the rosette stage. At this stage the plants pump more food reserves down to the root system, and fall-applied herbicides are better absorbed and translocated compared to thistle that is bolting or flowering. Applying mobile or systemic herbicides like 2,4-D, Banvel (dicamba – dma salt), Tordon (picloram), Roundup (glyphosate) Stinger (clopyralid) and Curtail (clopyralid + 2,4-D) at this time in the fall results in greater translocation of herbicide to the roots and subsequently improved root kill, Zollinger says. Again, be mindful of crop rotation restrictions with some herbicides.

Research in Canada showed that thistle treated at the rosette stage had 88% fewer shoots present a year after treatment compared to plants treated in the bud stage with twice as much herbicide. Roundup applied to Canada thistle in the rosette stage at half the rate recommended for the bud stage resulted in 98% control for two years after treatment, Zollinger says.

Fall application of Banvel or Roundup Ultra should be made prior to a killing frost, and when soil moisture is good. Canada thistle should have at least 8 to 12 inches of regrowth. Dicamba at 1 to 2 qt/acre or glyphosate at 1 to 2 qt/acre provides better total stand reduction than 2,4-D. Fall application of 2,4-D at 1 to 2 qt/acre is not effective in reducing Canada thistle stands the following spring. Glyphosate has no soil residual but fall-applied dicamba may carry over to injure crops the following spring. Only corn, sorghum, wheat and soybeans are recommended for planting the spring following a fall application of dicamba.

Tillage in late fall can enhance control of perennial weeds such as quackgrass and newly-established Canada thistle. Late fall tillage, on average, reduces the number of thistle shoots by 20%, according to Canadian research data. However, it should be used cautiously or not at all in fields having light, scattered infestations of Canada thistle, since root segments of 1/2 inch or longer are capable of producing new plants, and can be spread to uninfested parts of the field. About two weeks should elapse between a fall herbicide application and any tillage operation, which may be combined with a fall anhydrous application. The time delay allows for adequate movement of the herbicide in the plant to take place.

Glyphosate is cleared for preharvest applications to certain small grains not being saved for seed. Curtail/Stinger is effective on Canada thistle, but has rotational restrictions. It is also labeled for use in fallow.

Don’t neglect control of thistle and other perennial weeds such as leafy spurge in fencelines, ditches, and pastures, Wrage notes. “Don’t ignore out-of-the-way spots, because in the long-term scheme of things we need to reduce the seed source in those areas,” he says. Apply herbicides in the fall several weeks before a killing frost, to allow herbicide time to translocate. Herbicide applications may not be as effective if perennial weeds are already stressed by drought.

Butterflies But Not Bacteria Bugging Canada Thistle

A yellowish to almost white appearance on the upper parts of Canada thistle plants has been noticed by some this season. This is due to a natural microorganism (Psuedomonas) that may temporarily stunt shoots but won’t kill the plants, according to Zollinger. The University of Minnesota has done considerable work to isolate the agent and find a way to formulate and market the agent, but with limited success thus far.

Better news for biological control of Canada thistle may come this year from the painted lady butterfly (Vanessa cardui). The insect generally is only found in southern states such as Arizona and New Mexico and will build up populations large enough to migrate north only once every eight to 11 years. This happens to be one of those years. Larvae of the butterfly feed on Canada thistle plants and can reduce the Canada thistle population, particularly newly-established plants. Larval feeding may not totally kill well-established thistle plants, however. If thistle plants have incurred damage from painted lady larvae feeding but there is still adequate leaf surface for spraying, fall treatments are still recommended, say area weed control experts. – Tracy Sayler


Field Bindweed More a Problem in High Plains

Field bindweed is a bigger problem in the High Plains than Canada thistle, says Phillip Stahlman, Kansas State University weed scientist, Hays. Paramount (quinclorac) controls field bindweed in fallow, postharvest or preplant prior to seeding wheat, including durum. Bindweed should be actively growing with regrowth at least 4 inches long. Paramount should always be applied with methylated seed oil or crop oil concentrate and usually works best when tank-mixed with 2,4-D, Stahlman says.

Tordon and 2,4-D are also good choices for bindweed control, but Tordon should not be used in fields planned for sunflower next spring, because of its residual effect. “It’s best to have a program approach looking at all the crops in your rotation and keep after it every year,” says Stahlman.

If harvest isn’t too late, and producers are able to take sunflower off in late September, consider coming back in with Banvel to control some perennial broadleaf weeds, says Ron Meyer, Colorado State University extension agronomist, Burlington. If harvest is late, into October or later, than perennial weed control normally isn’t as effective because weeds are normally dormant after having been frosted a number of times. “In planning rotations, if it’s semi-irrigated, come back in with corn after sunflower and you can do a nice job with controlling broadleaf weeds. If it’s dryland, fallow after sunflower, then come back with wheat which allows aggressive action against broadleaf weeds,” he says. “The best seedbed for sunflower, dryland or irrigated, is wheat stubble, no-till or reduced till. That rotation also allows you to address broadleaf weeds in wheat and grassy weeds in sunflower.”

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