French broom-A tenacious invader, by Mary Millman and Tamia Marg

IF YOU CONSIDER THE SUBSTANTIAL ADVANCES made by French broom in Claremont Canyon over the past decade, John Burroughs’ invective against the weeds of autumn might come to mind: “They are all outlaws; every man’s hand is against them; yet how surely they hold their own!”

Possessed of extraordinarily aggressive traits, French broom (Genista monspessulana) long ago escaped urban gardens and found free range in the wildlands of the west coast. Thriving on recently disturbed soil, broom seeds easily take root. Once even minimally established, broom begins an insidious process of capturing nitrogen from the air and setting it in the soil, making the soil too rich for most native species—but very inviting to other invasives. Cutting the plant off at the root only intensifies the root structure; pulling up or weed-wrenching disturbs the soil so that nearby seeds tend to sprout. Seeds can survive more than 40 years and are not destroyed by fire, enhancing broom’s aggressive potential. This plant’s arsenal is impressive and requires a response that is both deliberate and long term. Casual or occasional trimming or extracting does nothing to arrest the advance of broom.

The “Vegetation Management Almanac in the East Bay Hills,” published in the 1990s by our local Hills Emergency Forum, illustrated 38 native plants, weeds, and invasive exotics. French and Scotch broom were included but not prioritized. Since then, what little attention has been paid to the broom invasion has
aged to halt the advance of broom and to suppress the vitality of the seed bed, even as broom is expanding in directly adjacent public areas .

The basic strategy for success proceeds from a clear understanding of both native flora and broom. The first step is to survey the land and create an inventory of flora and its location so that, for example, outliers might be identified, threatened sensitive habitat might be located, areas of mixed vegetation might be distinguished, and places where broom seed might spread downhill could be flagged. In a relatively small area like Claremont Canyon, such survey work could be done by direct observation or more sophisticated data-based methods.

With survey information clearly in hand, a strategy can be constructed using triage as the operative principle—identify what needs to be saved first and leave for last that which is least likely to succeed. Concentrate on removing broom first from the most pristine areas—those outliers that could become a serious broom colony that will threaten native flora. Working outward from these areas, the last places to be treated would be colonies in areas likely to be invaded by other weeds when the broom is eliminated. The exact removal techniques will differ depending on the location of the broom colony, the extent of financial and public support for broom removal, the advantages of the season, and the depth of commitment to the goal of elimination, which is to say the amount of follow-up. In any case, complete control of broom in Claremont Canyon is definitely possible.

Perhaps the greatest barrier to the development of a canyon-wide survey and strategy is the multiplicity of jurisdictions among which the canyon is divided: EBMUD on the ridges, UC Berkeley in the upper canyon, the East Bay Regional Park District along the urban/wildland interface in the mid and lower canyon and in Gwin (a north-to-south side canyon to the main canyon), the City of Oakland with the westernmost riparian segment of the canyon in Garber Park, and finally a few undeveloped acres that are still in private hands. While it is unrealistic to expect that these diverse jurisdictions, with land-management agendas that often conflict, could conduct the necessary survey and agree on strategic priorities (let alone concur in the selection of techniques), a volunteer citizen’s organization like our Conservancy, which speaks for the entire canyon, might be an appropriate mechanism for achieving these fundamental tasks. In the 1970s, Claremont Canyon was “saved” from development and preserved as open space by the close cooperation of many citizens with the Park District and UC. It was not on anyone’s screen at the time that the native wildland/open space character of the canyon might be threatened in the future by invasive weeds. Yet broom is a major threat to the canyon today and the jurisdictional circumstances suggest that citizens once again need to come forward to “save” Claremont Canyon.

In the meantime, small, individual efforts can make a difference if they are carried out so not to disturb the soil or advance the spread of seeds. Everyone who walks in the canyon can learn what broom looks like even when it does not have its characteristic yellow flower. In the winter and spring, when canyon soil is saturated, small broom plants should be pulled up carefully and left to disintegrate. At other times, especially in late summer, the best thing to do is to remove the seed pods from the plant and carry them out of the canyon for burning or other disposal. With a good strategy, close cooperation, and persistence, all canyon stakeholders, starting with the public, can defeat this invader.