Winter is good time to control dalmatian toadflax

By Casey Cisneros
Larimer County Department of Natural Resources

Dalmatian toadflax (Linaria dalmatica) is a foreign, invasive plant that is encroaching on pastures and rangeland in the Rocky Mountains. Extending from Canada into New Mexico, this escaped ornamental has become a weed species of concern within the region.

Throughout the summer months, dalmatian toadflax produces a flashy, yellow and orange, snapdragon flower. In combination with a perennial lifecycle and a strong tolerance to drought, dalmatian toadflax was used as an ornamental plant in flower gardens. When it became evident that dalmatian toadflax was invasive and a threat to natural areas, the plant was declared a noxious weed and is now on the prohibited sales list for Colorado plant nurseries.

Dalmatian toadflax has two vectors by which it reproduces. After flowering, seed pods are formed and transported by wind and animals. Disturbed soil sites are ideal locations for these seeds to germinate. Dalmatian toadflax can also propagate through its underground roots by creating offshoots.

In Larimer County, dalmatian toadflax can be found in the transitional shrub lands and foothills up to an elevation of 10,000 feet. On both public and private lands in Larimer County, dalmatian toadflax threatens plant communities and reduces forage by displacing other forbs and grasses.

Methods to control dalmatian toadflax are currently somewhat limited. Hand pulling, digging and grazing are ineffective because of the extensive underground root system. The Larimer County Weed District is recommending a selective herbicide to control this hearty noxious weed. The herbicide Telar, applied at 1.5 to 2 ounces product per acre in combination with a methylated seed oil or silicone-based surfactant has shown the greatest results when applied in the fall.

The timing of application is very important. Late summer can be ineffective when the plants tend to dry down. After the first freeze of the season the roots will begin to send up crowns to the surface. After the regrowth emerges is the optimal time to apply the herbicide. Private landowners can always call the Larimer County Weed District at 498-5768 with questions concerning noxious weeds.

Research is currently underway in Larimer County to assess the impacts that mecinus janthinus, an insect bio-control agent, has on dalmatian toadflax in this region. In other areas of the United States and Canada the root-boring weevil has shown the capability to reduce the number of dalmatian toadflax to a tolerable level. Weed Scientists from Colorado State University have released thousands of these insects in backcountry areas of Larimer County parks and open lands. CSU is currently collecting data at Horsetooth Reservoir and Red Mountain Ranch. If the insect adapts well to the climate in Colorado, there will soon be additional management tools for controlling dalmatian toadflax.