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Weed Control
June 17, 2004

By Richard Zollinger
NDSU extension weed specialist

Common ragweed or biennial wormwood - can you tell the difference?
In both photos, ragweed is on the left.

According to the 2002 dry bean survey, the worst weeds in the Northarvest dry bean region are:

% acres reported
Nightshade 57
Canada thistle 42
Kochia 35
Ragweed 30
Lambsquarters 26
Cocklebur 20
Redroot pigweed 19
Foxtail 15
Biennial wormwood 12
Wild oat 5
Other weeds 5

Nightshade has been the top worst weed for several years. The reason nightshade maintains its position as number one is because it produces several flushes during the season and these flushes result from each rain event. Soil applied Prowl, Treflan, Sonalan, Dual Magnum, Outlook, and Lasso only give partial nightshade control and do not control successive flushes.

Only four postemergence herbicides are registered in dry beans. Basagran will control only hairy nightshade and Reflex will control only eastern black nightshade but neither have residue to control flushes of nightshade. Raptor controls all nightshade species but the drybean and weed foliage intercepts most of the spray so very little gets to the ground to help in controlling nightshade flushes.

The effective strategies for season long nightshade control is use of an effective soil residual herbicide program like Spartan or Pursuit applied preemergence or use the micro-rate concept used in sugarbeet where Basagran, Raptor, or Reflex are applied at reduced rates two to three times in 10 day intervals.

Caution should be taken that no postemergence broadleaf herbicide can be applied once dry bean flowering begins.

NDSU research has shown excellent season-long control from Raptor, Pursuit, and Reflex applied three times in 10 day intervals with each application of Raptor at 1 fl. oz/A, Pursuit at 0.5 fl. oz/A, and Reflex at 3.2 fl. oz/A. Growers must be aware that this use is not explained nor recommended on the label so all liability of use is on the user. Also, nightshade has become resistant to Raptor and Pursuit in the Northarvest region. Many new sites are reported each year. Usually the resistance occurs after the grower has used ALS herbicides of  Raptor or Pursuit 10 to 12 times in their history of herbicide selection. Growers must use Spartan and Reflex as other modes of action and use non-ALS herbicides in the other crops of their rotation.

Canada thistle
Canada thistle has continually increased as a major weed problem in dry beans over the last 10 years probably because of the ample moisture across the region, the perennial nature of the weed where one application of a herbicide will not kill the entire plant and the expense of effective herbicides which limits the tenacity of controlling the patches and limits the sustained strategy in control.

Only Basagran applied at least twice in dry beans will control Canada thistle. Killing the Canada thistle and reducing patch size by using cheaper and more effective herbicides in rotation crops, like small grains, is more effective than trying to control the thistle using Basagran in dry beans. Basagran is a contact type herbicide so it will only control Canada thistle top growth and will not kill underground roots.

Kochia as a weed problem decreased in the wet years, but has resurfaced as a major problem due resistance to ALS herbicides, and drier area where the weed thrives. Spartan applied preemergence, two postemergence applications of Basagran, and Reflex will control kochia. Raptor and Pursuit will only control ALS susceptible kochia. Kochia usually germinates early in the spring with only one or two flushes. However, due to selection pressure and pattern of herbicide use it appears we have selected biotypes that can germinate later in the growing season which adds to seriousness of the weed. The prolonged germination has occurred in other weed species.

Ragweed, cocklebur
Common ragweed and common cocklebur are very competitive with dry beans. Reflex is the only herbicide registered in dry bean that controls ragweed which is probably one of the main reasons that growers list the weed as one of the top worst weeds in dry bean fields. Reflex, Raptor and Pursuit controls common cocklebur.

Lambquarters, redroot pigweed
Lambquarters and redroot pigweed are major weeds in most fields in North Dakota. Most soil and postemergence broadleaf herbicides registered in dry beans, except Basagran, control these weeds. NDSU research has shown greater control of lambs­quarters and redroot pigweed by applying Basagran as split treatments either twice each at 1 pt/A, 3 times each at 0.67 pt/A, or 4 times each at 0.5 pt/A as compared to one application at 2 pt/A.

Biennial wormwood
Biennial wormwood continues to infest dry beans and other row crops. Biennial wormwood plants in North Dakota emerge throughout the year, behave like an annual species, and produce numerous seed (approx. 1 million per plant). B. wormwood seeds are very small, can be dispersed easily by wind, water, and all human-related operations related to farming.  B. wormwood thrives in undisturbed (no- or minimum-till) areas, low areas, and areas where soil may remain wet for extended periods of time. Consequently, with every rain events a new flush of wormwood seedlings may appear.

Biennial wormwood survives most PPI, PRE, and some POST herbicides, is then detected and misidentified as common ragweed when plants are larger. In soybeans, rescue treatments are made with herbicides that control common ragweed, such as Ultra Blazer and FirstRate but have little or no effect on wormwood. B. wormwood plants can grow six feet tall with a woody stem that averages 1 to 2 inches in diameter which impedes grain harvest or can damage harvesting equipment.

Biennial wormwood is difficult to control because of an extended emergence period and tolerance to many PPI, PRE (e.g. Treflan, Sonalan, Prowl, Lasso, Dual products) and POST (e.g. most ALS herbicides) herbicides used in row crops.  Also, biennial wormwood can emerge late after most POST herbicides have been applied and is often confused with common ragweed.  However, biennial wormwood is controlled in grass crops by growth regulator herbicides of 2,4-D, Curtail/M, dicamba, Distinct, Hornet, Stinger and by non-selective herbicides, Liberty in Liberty Link crops and glyphosate in Roundup Ready crops. Other herbicides that control wormwood are soil applied Authority/Spartan, Python, Sencor, and Valor.

Limited research and experience indicates Basagran applied as split applications  - first split when wormwood is 1.5 inches tall and second split when wormwood is 3 inches tall will improve control compared to a single treatment. Wormwood apparently rapidly becomes tolerant to herbicides as plant size increases.

Biennial wormwood as a seedling (left), as an immature plant (center)
and as a mature plant (right). Photos: NDSU Extension Service


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