Yellow toadflax (Linaria vulgaris P. Mill.) infestations in North Dakota increased 300-fold from 1997 to 2011, when the plant was added to the state noxious weed list. Long-term control of other invasive species had included biological control agents, but no effective agents for yellow toadflax had been identified, so a control program using herbicides was needed. The objective was to shift from short-term control with picloram applied in the fall at maximum allowed rates to long-term management with minimal nontarget species impact with an adaptive management approach. Yellow toadflax control was increased from an average of 64% with picloram at 1,120 g ha⁻¹ alone 12 mo after treatment (MAT) to over 90% when applied with diflufenzopyr while the picloram rate was reduced 50%. Yellow toadflax control with aminocyclopyrachlor applied at 140 g ha⁻¹ ranged from 91 to 49% 12 MAT when applied in June or September, respectively. In contrast, yellow toadflax control with picloram plus dicamba plus diflufenzopyr averaged > 90% regardless of application date during the growing season. Land managers now have at least two options for long-term yellow toadflax control with a wide window of application timing. The goal of replacing a single high-use–rate herbicide treatment was met but both picloram and aminocyclopyrachlor can injure many desirable forbs. However, application timing can now be adjusted to have the least impact on nontarget species. The adaptive development program led to a 58% reduction in yellow toadflax infestations in North Dakota by 2014.

Nomenclature: Aminocyclopyrachlor; dicamba; diflufenzopyr; picloram; yellow toadflax, Linaria vulgaris P. Mill.

Management Implications: Yellow toadflax (Linaria vulgaris P. Mill.) rapidly increased in North Dakota from approximately 20 ha in 1997 to over 6,000 ha by 2011. At the time, the only herbicide labeled for control of this weed was picloram at 1.1 kg ha⁻¹, which only provided short-term reduction and had limited application potential because it is a restricted-use pesticide. Biological control of yellow toadflax was considered an ideal solution but no successful agent was available. To develop a stop-gap management plan to reduce yellow toadflax spread, a series of experiments were conducted with a goal to discover a more effective herbicide control program with minimal impact on nontarget species. The addition of diflufenzopyr to picloram increased yellow toadflax control from < 50% 2 yr after treatment to > 90% and allowed for reduced application rates. Diflufenzopyr is only available in combination with dicamba and in a three-way mixture with picloram provided long-term control when applied throughout the growing season from June through September. Aminocyclopyrachlor applied at various rates provided an average of 85% yellow toadflax control when applied in June but control gradually declined to 50% or less when the treatments were applied in September. Land managers now have at least two options for yellow toadflax control. Picloram plus dicamba plus diflufenzopyr can be applied throughout the growing season to control yellow toadflax, which would allow for well-timed application to co-occurring invasive species such as Canada thistle (Cirsium arvense L.) or leafy spurge (Euphorbia esula L.) because this herbicide combination is effective on all three species. Aminocyclopyrachlor is also effective on these species, but the optimum application window would be early in the season (June) for optimum yellow toadflax control. Both picloram and aminocyclopyrachlor can injure desirable forbs. To alleviate a portion of forb damage, the application of picloram plus dicamba plus diflufenzopyr could be timed for when a desired forb species would be least affected, such as after seed-set. With several available options, managers