What do blackberry, English ivy, giant reed, stinkwort, and tree of heaven have in common? They’re all highly invasive plants that grow along creeks in Santa Clara County and much of California. Invasive plants are more than weeds — They are plants that originated outside California and have an outsized impact on local ecosystems. Some, like giant reed, are extremely thirsty and suck up to three times as much water out of creeks as native riparian plants. Others increase fire and flood risk, crowd out native vegetation and reduce habitat for wildlife. Even goats and bugs won’t eat some of these plants (looking at you, stinkwort!).
Santa Clara County is full of these botanical villains, and riparian areas along creeks and rivers are easily invaded. There’s plenty of water to help seeds germinate, and seeds and weeds are constantly flowing in from upstream populations. The good news is that Valley Water is actively working to remove harmful and threatening weeds from creeks through our Invasive Plant Management Program (IPMP). The IPMP began in 2014 and enables us to remove, treat, and monitor invasive plants on creeks all over the county, from San Francisquito Creek in Palo Alto all the way down to Llagas Creek in Gilroy.
We’re up to 35 sites so far, covering some 40 different plant species and counting. The species on our removal list come in all shapes and sizes, from grasses to large trees. They are chosen based on statewide ratings by the California Invasive Plant Council as well as our own observations of local invasiveness. As for how project sites are selected, we consider if the site is habitat for wildlife, the density of invasive plants, will native plants in the area be able to repopulate the cleared site, how close are sensitive species downstream of a site, and basic accessibility for our crews and equipment. Some projects are relatively small while others cover a couple miles along a creek.
Once project boundaries have been selected, the site is thoroughly mapped using handheld GPS units to get the total area covered by invasives as well as where infestations are located. Armed with these maps, vegetation maintenance crews go to the site and get to work. Different plants require different tools, timing, and strategies. We try to use the lowest impact method that will achieve the best results. Sometimes that involves mowing yellow starthistle multiple times in a season to reduce seed production, or using a weed wrench on French broom seedlings, which helps pull the whole plant out of the ground. Other times it means applying herbicide on a newly cut invasive tree to ensure it doesn’t resprout. After an initial sweep through a project site, we return to the same site several times a year for five years, re-mapping at the end of each year to gauge our progress. Destroying invasive plants is difficult in a creek system where water can easily move the seeds around. Our end goal is to reduce the area of invasive plants to less than 10% of the total site.
Six years into this program, we’re wrapping up some of the first sites we started. While not every site has been a complete success, invasive plant populations have dropped at all of them, and we’re seeing native plants expand into areas where invasives used to flourish. In some sites where it’s possible, we are planting native plants including willows, coyotebrush, buckeyes and mugwort. As we apply what we’ve learned to improve the IPMP, we’re also looking to do more, including early detection for new noxious species and experiments with planting native seeds in treated areas. As stewards of Santa Clara County’s creeks and riparian areas, we hope our work leaves our creeks more diverse, resilient and healthier.