Dalmatian toadflax is an aggressive, noxious weed

Dalmatian toadflax, a large yellow snapdragon-like plant, was originally introduced as an ornamental landscape plant. A hardy, Xeriscape plant it promptly escaped from gardens and invaded thousands of acres in the arid West. It has bright yellow flowers arranged in spikes on the stems. Dalmatian toadflax prefers sunny areas with well-drained, coarse-textured or even rocky soils. You can observe it on steep, rocky slopes throughout the foothills.

By Ellen Nelson
Larimer County Weed District

Dalmatian toadflax is a perennial that can grow up to 3-feet tall. It has waxy, bluish-green, heart-shaped leaves that clasp the stem. Flowering occurs from early summer to early fall. Seed pods contain 140 to 250 small dark brown to black seeds. Dalmatian toadflax can quickly colonize an area because it spreads by sprouts from the lateral roots and by seed. Over its lifetime, a single plant can produce up to 500,000 seeds that persist in the seedbank for up to 10 years. Seed dispersal occurs after flowering and continues through winter. Dispersal is by wind, water, wildlife, forage, livestock, vehicles and equipment. The roots of a Dalmatian toadflax plant extend 10 feet and can send up shoots for a new plant every few inches.

Because it grows on steep, rocky, slopes and is a waxy, tough plant, it can be difficult to control. Mowing and grazing can be effective because they prevent the plant from flowering and producing seeds. Herbicide is used on Dalmatian toadflax, and is most effective when applied in the fall, on new rosettes. The tough waxy cuticle on this plant can make herbicide penetration difficult, and may necessitate higher application rates.

For established infestations on steep, inaccessible slopes, a practical approach is bio-control using a stem-boring beetle, Mecinus janthiniformusus, or a moth, Calophasia lunula, whose larvae feed on toadflax foliage. There have been several releases of Mecinus in Larimer County in an attempt to suppress the population of Dalmatian toadflax and slow its spread in steep inaccessible terrain where other means of control are not feasible. Bio-control methods are not intended to totally eliminate the target weed, but rather to slow its spread and to help it learn to behave like any other plant in the landscape. Bio-control insects can be ordered online from the Colorado Department of Agriculture’s Palisade Insectary.

Healthy grasses in properly managed range and pasture are the best defense against the spread of Dalmatian toadflax. Overutilization of forage by grazing animals create opportunities for Dalmatian toadflax infestations. There is evidence that fire can also facilitate increased size and density of a Dalmatian toadflax infestation. Fire destroys seedlings and burns plants, but new shoots and seedlings rapidly emerge with any soil moisture. These new plants proliferate in the nutrient-rich soil, especially without competition from native grasses.

For more information on controlling Dalmatian toadflax or other noxious weeds, contact the Larimer County Weed District. The weed district provides free site visits, plant identification, advice on controlling noxious weeds, and land and pasture management guidelines. Contact the Larimer County Weed District at 970-498-5768 or http://www.larimer.org/weeds