Dalmatian toadflax is listed as a noxious weed in most of the western United States, but control of this species has not been extensively studied in California. Studies in other states show effective control of Dalmatian toadflax with picloram, but this herbicide is not registered for use in California. In addition, reports vary as to the optimal timing for herbicide applications. In this study we evaluated several herbicides with combined foliar and soil-residual activity at two times of application: postsenescence (fall) and rosette (winter to early spring). We applied two series of treatments (2008 and 2009 to 2010) on adjacent sites in high desert scrub of southern California. In the year of treatment and the following year, we evaluated Dalmatian toadflax cover and presence/absence of associated dominant species (≥ 5% cover). Although time of application, treatment, and timing by treatment interaction all produced significant differences in Dalmatian toadflax cover in the 2008 trial, only the high rate of aminocyclopyrachlor (280 g ae ha⁻¹) applied to dormant plants in fall consistently reduced cover through the second year. No treatments at the rosette stage consistently produced 2 yr of control. In 2009 to 2010, treatments were more effective, probably owing to higher precipitation in spring. In both dormant and rosette applications made in 2009 to 2010, aminocyclopyrachlor (140 and 280 g ae ha⁻¹) and aminocyclopyrachlor chlorsulfuron (140 g ae ha⁻¹ 53 g ai ha⁻¹) gave second year control; chlorsulfuron at the dormant stage (105 and 158 g ai ha⁻¹) and aminopyralid at the rosette stage (245 g ae ha⁻¹) also gave 2 yr of control. The treatments had only minor effects on grass species. The response of broadleaf species varied among treatments, with aminocyclopyrachlor at the high rate increasing Eriogonum spp., but greatly reducing Asteraceae species. These results provide options for the management of Dalmatian toadflax in California and other western states.

Nomenclature: Aminocyclopyrachlor, aminopyralid, chlorsulfuron, picloram, Dalmatian toadflax, Linaria dalmatica (L.) Mill. subsp. dalmatica LINDA, Eriogonum spp

Management Implications: Dalmatian toadflax is usually controlled with picloram in most areas of the western United States. However, results can vary owing to application timing or to environmental factors. Furthermore, picloram is not registered for use in California, and thus is not an option for controlling this plant in California's high deserts. In this study we evaluated several herbicides with combined foliar and soil-residual activity at two times of application: postsenescence (fall) and rosette (winter to early spring). The trial was conducted twice. The primary chemicals tested included aminocyclopyrachlor, aminopyralid, chlorsulfuron, dicamba, imazapic, imazapyr, metsulfuron, picloram, and 2,4-D, but only aminocyclopyrachlor, aminopyralid, and chlorsulfuron were effective. Aminocyclopyrachlor (280 g ae ha⁻¹), applied to dormant plants in fall, gave the most consistent long-term control, providing > 90% reduction in Dalmatian toadflax cover in both trials in the second season after treatment. This rate also increased native species in the genus Eriogonum, but reduced the presence of members of the Asteraceae. Applications at the rosette stage were less consistent between trials, probably because of differences in spring rainfall between the two trial years. In the first trial, with very low spring rainfall, none of the rosette-stage treatments produced 2 yr of control. However, in the second trial, with considerably more spring rainfall, both dormant and rosette applications of aminocyclopyrachlor (2 and 4 oz ae ac⁻¹) and aminocyclopyrachlor chlorsulfuron (2 oz ae ac⁻¹ 0.75 oz ai