Last updated: January 7, 2022
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Team Leads Successful Search for Invasive Thoroughwort on Mount Tamalpais
December 2021 - Keeping invasive plants at bay is a powerful way to help native plants and wildlife flourish. But that’s no easy task when you aren’t quite sure where they are; first, you have to find them. The One Tam Conservation Management Team recently set out to do just that to help land managers keep one particular species of concern off of upper Mount Tamalpais: thoroughwort (Ageratina adenophora).
Thoroughwort is an invasive plant species found in areas with sufficient moisture. It produces abundant seed that is widely dispersed by wind, water, soil movement, and by sticking to animals as they brush against it. Once established, it forms thickets that outcompete native vegetation. Thoroughwort is heavily concentrated in the drainages on the southwestern slopes of Mount Tamalpais where it receives ample moisture from fog drip, yet still uncommon on the north side of the mountain. But how far up the southwestern slopes is it? Where is its leading edge?
One Tam / Michael Sturtevant
The Conservation Management Program developed a strategy to find out. They started by using the Calflora Database to look at where it is already known to grow. From there, they developed a survey plan around those locations. In particular, they focused their attention on the drainages below Ridgecrest Boulevard and near Bootjack Campground. Then, they headed out into the field.
Over five days in October and November, team members hiked across the length of the Ridgecrest grassland, dropping into rarely visited drainages with hidden seeps and cascades, to map the population’s leading edge. Pretty quickly they were able to identify where drainages were likely to have thoroughwort and where they weren’t based on the plant community and soil moisture. If they saw chain fern or elk clover, for example, it was an indicator that thoroughwort may be nearby. Upper reaches of these drainages are often dry most of the year, reducing the likelihood of thoroughwort.
The survey was a success. The team not only mapped the leading edge and removed smaller plants along the way, but gained a better understanding of where to look for thoroughwort in future surveys. The data they collected will inform future treatment which aims to prevent thoroughwort from moving into drainages and spreading north and east of Ridgecrest Boulevard.
For more information
- One Tam Invasive Plant Monitoring & Management webpage
- Pacific Coast Science and Learning Center Invasive Plants webpage
- Contact One Tam Conservation Management Assistant Michael Sturtevant or Conservation Management Specialist Rachel Kesel