Tackling bullies (and other invasive species)

Usually, it’s easy to spot a weed and understand why they are no friend to our garden, consuming space and resources our flowers and crops need to grow. But often we overlook the other invasive plants that threaten our yards, fields, rangeland, communities and world.

“Some invasive species actually come to us through the horticulture trade – plants like ice plant and pampas grass and Scotch broom,” DiTomaso explains. “They are ornamental in nature, but because they are characteristically adaptable and vigorous with a high reproductive capacity, they can thrive in areas beyond where they were planted and choke out native species.”

Here is a list of some common invasive plants.  

And why is it important to protect native species? Biodiversity, for one thing. Healthy ecosystems protect the planet’s biodiversity, which consists of many millions of distinct biological species produced over 3.5 billion years of evolution. When a species becomes rare or extinct, a condition on the rise, they take with them their unique genetic codes often needed to develop medicines and food. Biodiversity is required for recycling essential elements (such as carbon, oxygen and nitrogen), for mitigating pollution, protecting watersheds, combating soil erosion, maintaining genetic diversity for crop improvement and providing habitat for thousands of important organisms, including insects and animals.  

Habitat destruction by humans – roads, housing, logging, mining, for example – are the leading cause of loss of biodiversity. The second leading cause is invasive species.

Take, for example, yellow starthistle (Centaurea solstitialis), a prickly, pesky plant that has gobbled up millions of acres in the United States since its arrival in California around the time of the Gold Rush. It currently dominates 15,000,000 acres in California alone. This winter annual bears a taproot that can reach two meters deep in the soil, allowing it to thrive during dry, hot summers. It’s versatile in its growth patterns and can adapt to low soil moisture by producing smaller plants with fewer seeds during dry years. Yellow starthistle is toxic to horses, is a pest in both field crops and rangelands and acts as a physical barrier to wild animal movement in wildlands.

 “It’s a bully,” DiTomaso says.

So what can DiTomaso do to fight invasive bullies? It’s not enough to kill it or cut it back.

“We’re working on finding the right perennials for the right environment to prevent reinvasion,” DiTomaso says. “It’s important to find species that can occupy the same niches so they can compete with starthistle. But before you do anything, you have to start with weed control, otherwise you won’t have a fair fight. If you have 10,000 starthistle seeds and we plant 50 seeds – that’s not a fair fight.”

Suppressing starthistle takes systematic and persistent effort over several years. The approach varies with size and density of the infestation, terrain, tools and equipment available and planned use for the site. Success has been made with various approaches, including tilling, mowing, grazing, burning, chemical control, manual control and biological control - using natural enemies like weevils to control the pest. Plant competition can be successful, with a few caveats – make sure it’s a fair fight and choose the right plant for the right use and the right environment. One size does not fit all.

DiTomaso, Research Specialist Guy Kyser, Associate Professor Marie Jasieniuk and others work to manage a long list of invasive plants, from medusahead, downy broome and perennial pepperweed the Sierra Foothills and the Great Basin region of California to Dalmatian toadflax in Hungry Valley to tree-of-heaven in the Putah Creek Preserve to jubata grass along the California coast. They also provide data and education on the pathways and mechanisms of invasive plant spread and the potential for a species to invade a region, such as whether a biofuel crop might become a menace.

“For example, we are trying several herbicides for controlling switchgrass and miscanthus, two leading biofuel candidates, as a mitigation strategy before these plants are introduced for cultivation in California,” Kyser explains.

And their work doesn’t stop at land’s end. Together with affiliates like Plant Physiologist Lars Anderson and Ecologists David Spencer and Brenda Grewell who conduct exotic and invasive weed research with the USDA Agricultural Research Service, they wage underwater fights in Lake Tahoe, the San Francisco Bay and wherever invasive species threaten to take root.

Here’s a video on California’s challenge with invasive species.

You can learn more about these projects and more at the UC Weed Research and Information Center (Weed RIC) website. The Weed RIC collaborates with researches and helps distribute knowledge for the benefit of agriculture and the preservation of natural resources.