After the fire, watch for unwanted weeds

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From record-breaking drought to record-breaking fire! What does nature have in store for us next? Not only is Larimer County experiencing severe drought, but as a result we are in the midst of a devastating fire season. The effects of below-average precipitation and above-average temperatures left forests and grasslands extremely dry, providing ready fuel for any ignition source. Dry, dormant grass, ready to burn, combined with dry forests, pockets of beetle-killed trees and years of accumulated fuels created a “perfect” storm.

Ellen Nelson
Larimer County Weed District

What will our local landscape look like in the coming years as major portions of northern Larimer County’s forests and grasslands recover from the effects of wildfire? What might we expect from invasive weeds that are always poised to take advantage of changing environmental conditions? Against a backdrop of life lost, homes destroyed and lives threatened, concerns about invasive weeds may seem trivial. The often promoted analogy equating the spread of noxious weeds to a slow-motion wildfire feels trite and insensitive. As stewards of land, whether a few acres in a rural subdivision or thousands of acres of National Forest, the decisions we make, the actions we choose to take or not take can affect our landscapes and our lives for generations. Increased population densities in remote and forested locations make the devastating impacts of wildfires more immediate.

As you assess land-management options after the fire, the decisions you make now may have ramifications into the future. Many of the noxious weeds found in northern Larimer County will be happy with the opportunities created by large areas of disturbance resulting from wildfire and the suppression of wildfire. One such weed is Dalmatian toadflax. Dalmatian toadflax looks like a large, yellow snapdragon, with bright yellow flowers arranged in spikes on the stems. It is a native of the Mediterranean area and was introduced to this country as an ornamental garden flower. It is a hardy plant that readily “escaped” from ornamental plantings and invaded thousands of acres in the arid west. Dalmatian toadflax prefers sunny areas with well-drained often coarse-textured soils. You can observe it on steep, rocky slopes in the foothills areas.

Dalmatian toadflax is a perennial that grows 3- to 4-feet tall. It has waxy, bluish-green, heart-shaped leaves that clasp the stem. Flowering occurs from early summer to early fall. 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 can persist in the seedbank for up to 10 years. Seed dispersal begins shortly after flowering and continues into winter. Dispersal is by wind, water, wildlife, vehicles and equipment, forage, and livestock. Because it grows on steep, rocky slopes and because it is a very waxy, tough plant, it is difficult to control.

Mowing and grazing of Dalmatian toadflax is generally not effective. There are reports that Dalmatian toadflax contains an alkaloid that can be toxic to cattle. For established infestations on steep, inaccessible slopes in the foothills, one practical approach is bio-control using the stem-boring beetle, Mecinus janthinus. There have been several releases of this weevil 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.

There is little information about toadflax control with fire. Fire destroys seedlings and established plant top growth, but new shoots will rapidly emerge from lateral root buds with any soil moisture. These new shoots proliferate in the burned nutrient rich environment, especially in the absence of competition. There is evidence that fire may actually lead to increased size and density of a Dalmatian toadflax infestation. Best results controlling Dalmatian toadflax with herbicides is in the fall by treating the newly emerged rosettes with Telar or Plateau, herbicides that are effective against this plant. A methylated seed oil additive helps the herbicide stick to and penetrate the waxy leaves.

Extreme natural events such as the High Park fire remind us of how little control we sometimes have over nature. If we attempt to understand how different plants in our landscapes function we may be able to make better decisions and take actions to set up conditions to move our landscapes in the direction we prefer.

For more information on herbicide treatments for dalmatian toadflax and other invasive weeds, contact the Larimer County Weed District. The Larimer County Weed District provides advice to landowners on managing noxious weeds. 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