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Weed of the Week: Dalmatian Toadflax: Pretty wildflower or problematic invasive?

Weed of the Week: Dalmatian Toadflax: Pretty wildflower or problematic invasive?

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You have likely seen this bright, showy plant lighting up dry, open, possibly disturbed hills and roadsides around Ravalli County during the hot, dry summer months.

It’s yellow flower is pretty and bright looking and might draw you in to check it out more up close. However, after close examination I’m sure you will quickly realize that this is no pretty wildflower.

But what is it? It is Dalmatian toadflax (Linaria dalmatica) a highly invasive plant that is listed as priority 2B by the State of Montana, which means it is abundant and widespread across many counties and must be controlled. Dalmatian toadflax is a short-lived perennial forb that belongs to the genus Linaria of the figwort family, Scrophulariaceae.

This highly competitive plant was introduced to North America from Eurasia as an ornamental, used for fabric dye, and even folk remedies during the 1800s and by 1874 had been introduced to much of the western United States. It has continued to spread and has been a major problem for private landowners, ranchers and land managers ever since.

Its close relative, yellow toadflax (Linaria vulgaris), has been just as big of a problem as Dalmatian toadflax and now, some believe, the two are cross pollinating, producing a hybrid of the two species and a plant that is even more difficult to control.

Dalmatian toadflax is typically found on well-drained, relatively course textured soils in disturbed and degraded sites such as roadsides, waste areas, gravel pits, clearings, overgrazed or otherwise degraded lands, such as along railroad tracks. It is identified by its numerous, alternating heart shaped leaves, and butter-colored flowers with orange centers and is woody towards the base of the plant. Dalmatian toadflax has a robust rhizomatous root system, as well as abundant seed production and treating it can be a labor-intensive and expensive process.

Hand pulling small individual plants in relatively small infestations can be effective, however, pulling large plants can be more difficult and may result in leaving behind more viable root fragments that will eventually sprout new shoots. Although labor-intensive, hand pulling those small infestations can significantly reduce the seed bank, but only if it is repeated annually for 5-6 years.

Mowing can reduce seed production but can spread seeds and root fragments elsewhere to cause new shoots to form later. Burning is also not a great option as it also destroys competing vegetation and can increase the susceptibility of the site to further toadflax infestations. Selective herbicides, such as phenoxy or sulfonylurea types are effective against Dalmatian toadflax when used with a non-ionic surfactant, which enables the chemical to penetrate and stick to the waxy leaf surface of the plant. Last but certainly not least, is treatment with the use of biological control agents that feed on Dalmatian toadflax.

Here at the Ravalli County Weed District, our Youth Biocontrol Program collects and redistributes the insects that feed on this type of plant. There are a variety of beetle, moth and weevil species that can be used for toadflax’s, but we typically work with the toadflax stem-mining weevil (Mecinus janthiniformis) that typically emerges from the stem of the plant in early spring to feed and mate.

Female weevils orally excavate shallow, divot-like holes in the surface of the stem and can lay up to 45 eggs in the stem where they will spend the winter, emerge as adults in the spring, and repeat the process. Biocontrol can be slow to establish, but once it does, it will help keep your Dalmatian toadflax infestation at bay. 

Contact the Ravalli County Weed District office for more questions at 406-777-5842 or contact Josh (extension 4) to get signed up for Dalmatian toadflax biocontrol insects. We ask for a suggested donation of $40 per release (approximately 250 toadflax stem-weevils) and these donations go right back into the youth program to fund the student’s salaries and the projects they participate in.

The Weed of the Week feature is hosted by the Ravalli County Weed District. 

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