Mapping the Leading Edge of Invasive Thoroughwort on Mount Tamalpais

December 2021.

Keeping invasive plants out helps native plants and wildlife thrive — but first you have to find them. The One Tam Conservation Management Team set out to do that for one species that land managers want to keep off upper Mount Tamalpais: thoroughwort (Ageratina adenophora).

The problem plant

• Where it grows: places with enough moisture.
• How it spreads: masses of seed carried by wind, water, moving soil and animals that brush past.
• What it does: forms thickets that crowd out native plants.
• On the mountain: concentrated in the drainages of the southwestern slopes, watered by fog drip; still uncommon on the north side. The question was how far up those slopes it had reached — where its leading edge was.

The search

• Plan: the team used the Calflora database to see where thoroughwort was already known, and built a survey around those spots — especially the drainages below Ridgecrest Boulevard and near Bootjack Campground.
• Fieldwork: over five days in October and November, they hiked the length of the Ridgecrest grassland, dropping into rarely visited drainages with hidden seeps and cascades.
• Reading the ground: they soon learned to tell which drainages were likely to hold thoroughwort from the plant community and soil moisture. Chain fern or elk clover hinted it might be near; the upper reaches, dry most of the year, were less likely.

Results

• The team mapped the leading edge and pulled small plants along the way.
• They learned where to look in future surveys.
• The data will guide treatment meant to keep thoroughwort out of more drainages and stop it spreading north and east of Ridgecrest Boulevard.

More: One Tam invasive plant monitoring and management · Pacific Coast Science and Learning Center: invasive plants

Sources

Based on "Team Leads Successful Search for Invasive Thoroughwort on Mount Tamalpais," Bay Area Nature & Science Blog, National Park Service; a work of the United States government in the public domain. The original's photographs, credited to an individual with the One Tam partnership, are left out.