Curbing Weeds on the Sierra National Forest
D’Artanyan Ratley, Pacific Southwest Region
October 5th, 2023
This map shows overall weed control work on the Sierra National Forest, as part of the trial project with Corteva Agriscience. (USDA Forest Service image)
If you thought weeds in your yard or garden were bad, imagine them on a forest scale. National forests in California are battling the threat of invasive weed species throughout their ecosystems. Non-native weeds can quickly push out native species and disrupt the natural balance. These invasive species affect everything from soil composition to the frequency of wildfire.
To help tackle this growing issue, the USDA Forest Service is working with Corteva Agriscience on a trial collaboration. Partners since 2021, this new trial began in May 2023 with Corteva contributing herbicides, professional pest control advisors, and a professional application company.
They will treat invasive weeds at two high priority sites in the Sierra National Forest — Batterson Work Center and the Bass Lake Ranger District office compound in North Fork. These two sites have become severely infested with invasive weeds, and unless promptly controlled, seeds will continue to move to uninfected lands across the forest.
Spraying for invasive weeds has centered on the 122-acre Batterson Work Center — with its two fire engines, a 20-person hot-shot crew, a fire prevention patroller, a silviculture office, and two helicopter landing pads. If left unchecked, invasive weed seeds could be carried to remote areas of the Sierra National Forest from heavy-use areas like these. (USDA Forest Service photo by Joanna Clines)
“Of the approximately 7,000 plant species growing wild in California, about 1,000 species are not native. A few hundred of these are so damaging that they are actively controlled by Forest Service invasive weed managers,” said Joanna Clines, forest botanist for the Sierra National Forest.
“These especially bad actors — like yellow star thistle, spotted knapweed, rush skeleton weed — are the ones we spend federal dollars on. They're the ones that can take over a watershed really fast, so nothing else can grow there, reducing the ability of the land to support wildlife and livestock (in rangelands).”
Corteva and the Sierra National Forest share the common goals of using the right chemicals, application rates, and timing to effectively control these invasive weeds. To track these results over several years, photos will show changes in the plant species present.
It’s well established in the Forest Service that removing invasive weeds improves watershed integrity and promotes healthy native vegetation. Reducing acres of damaging non-native plants like yellow star thistle, klamathweed, and Italian thistle contributes to more and better water on the Sierra National Forest. This water then flows to agricultural lands and communities downstream, restores healthy native vegetation relied on by wildlife and livestock, and contributes to reforestation success in burned areas.
But how did these invasive weeds even get here? These unwanted plants were introduced from other continents or other parts of the country accidentally (think via soil on wheels or seeds on fur) or on purpose (horticultural plants like Scotch broom or Himalayan blackberry). They can grow fast and displace native plant species, impacting the food chain for animals that depend on the native plants.
“What most of the invasive weeds have in common is that they arrived in North America, especially California, without the predators and pathogens and competitors they had in, say, Europe,” said Clines. “A lot of things that are really bad pests here in California are from the Mediterranean region of Europe, where the climates are similar.
Italian thistle and klamathweed are unwelcome visitors at North Fork on the Sierra National Forest. Through the partnership with Corteva, these weeds are being treated and areas photographed to see progress on restoring this area to its native species. (USDA Forest Service photo by Joanna Clines)
“In their native lands, most of our worst weeds are kept in check, but when they arrive here with none of the organisms that kept their numbers down, they can wreak havoc.”
Generally speaking, invasive weed species are more aggressive than native plants. Without their natural predators and competitors, they can outcompete native species for resources such as water, sunlight and nutrients.
For example, Italian thistle (Carduus pycnocephalus) and klamathweed (Hypericum perforatum) are the primary target weeds at North Fork on the Sierra National Forest. Since 2021, Corteva has treated both species in heavy traffic areas with a high risk of moving weed seeds on vehicles and equipment. These unwanted seeds just love to hitch rides. So Corteva began by treating for these weeds around fleet parking lots, roads department storage, and travel ways where weeds were growing directly next to tires in 2021.
Clines has seen the full cycle of damage from unchecked weeds on a whole ecosystem.
“How invasive species work is you start with a diverse watershed — with a river and the diverse plant, animal, and fungi species that evolved in that location. And the native plants provide wildlife habitat and services to people and pollinators.
Then, all of a sudden — over 10 or 20 years or more, sometimes less — you end up with just one species dominating the vegetation. Yellow star thistle is one example. It's a tall, spiny, really unpleasant weed that has a super deep root system. And even though it's an annual species, it will take over a watershed until little else is growing there.”