Broom Busting: yanking, sawing and nibbling...

Stewardship Mountaineers have been busy busting broom above Buckeye Canyon. Over 1,500 Genista monspessulana shrubs and seedlings, a Mediterranean plant commonly known as French Broom, have felt the tug of our weed wrenches and the teeth of our saws over the past weeks.

French Broom © 2008 Neal Kramer

French Broom © 2008 Neal Kramer

These troublesome invaders have been holding our focus as we await the showers and storms that moisten mountain soils and mark the beginning of our native planting season.

The rich native grasslands south of Buckeye Canyon have undergone extensive invasions by woody plants, including many native species like Toyon (Heteromeles arbutifolia), Coyote Brush (Baccharis pilularis), and Islay (Prunus ilicifolia)—apart from French Broom.

At the moment, however, French Broom has been our main target, and we’ve been ripping them off and cutting them out from patches of autumn’s golden prairies, thick with brittle bunchgrass tufts and lined with the dry outstretched seedheads of Common Yarrow (Achillea millefolium) and Douglas Iris (Iris douglasiana). French Broom shrubs that are too big to pull are cut and their bark peeled.

French Broom peeled stump

French Broom peeled stump

Growing on the slopes just north of where we’ve been working are Silver Lupines (Lupinus albifrons) and California Violets (Viola pedunculata)—host plants of the endangered Mission Blue and Callippe Silverspot butterflies. Both plants would be devastated by broom. First-time teenage volunteers and seasoned volunteer veterans have both joined in the critical effort to challenge the grassy appetite of these relentless prairie consumers.

We’ve been satisfied to learn that the invasive woody shrubs are themselves food—and they don’t just feed our hunger for a hardy day of broom busting.

The University of California’s Statewide Integrated Pest Management Program found a broom gall mite in El Dorado County. To read more see Scotch Broom Gall Mite: A New Partner in Broom Management for California. Below is an excerpt from the article regarding the disruptive effects of the mite on broom shrubs.

The mites cause galls, small abnormal growths on the plant's buds, to form during feeding, greatly reducing Scotch broom's ability to grow and reproduce. This mite is considered to be an ideal biological control agent due to its specialized feeding habits and the debilitating damage it can cause to invasive weeds. In some areas, the gall mite has already killed large stands of broom.

Genista Broom Moth caterpillar defoliating French Broom   photo: Sutro Stewards

Genista Broom Moth caterpillar defoliating French Broom
photo: Sutro Stewards

However, the broom gall mite targets Scotch Broom (Cystisus scoparius), apparently not the French Broom we've been attacking ourselves.

Genista Broom Moth caterpillar defoliating French Broom
photo: Sutro StewardsYet we did notice a local creature that munches on French Broom. The Genista Broom Moth (Uresiphita reversalis) caterpillar was found scrunching its way along broom branches.

It is native to the United States and prefers plants like broom that form part of the pea family, Fabaceae. We are happy to have other species participating in the fight against broom on San Bruno Mountain.

Still, there is something worrisome about the Genista Broom Moth—our native lupines also form part of the pea family and thus the caterpillars may also target them!

The Sutro Stewards, for example, found the caterpillars eating their nursery stock of yellow Bush Lupines (Lupinus arboreus). See Genista Broom Moth - Friend or Foe? to read more about this surprising find.

Could the Genista Broom Moth defoliate the host plants of the Mission Blues, like the Silver Lupine (Lupinus albifrons)? It seems possible.

We’ll be keeping an eye on the lupines for signs of the Genista Broom Moth, especially in areas where French Broom grows near lupines—and now we have one more clear reason why we shouldn’t let French Broom encroach upon precious lupines!

    — Ariel Cherbowsky, Stewardship Coordinator for SBMW

Ariel recently joined Mountain Watch as our Stewardship Coordinator. He has thrown himself into this important work with great enthusiasm. If you're interested in joining Ariel in restoration and stewardhsip projects on the Mountain, contact him at