Eradicating French Broom

By Elaine De Man

The last few days of rain. . . . .with a few more on the horizon. . . . .make this the perfect time to get out and do some serious damage to one of the most invasive and destructive plants in the Napa Valley, French broom (Genista monspessulana). The ground is soft and the broom’s roots are so shallow, that most of it can be pulled out by hand. And if it can’t do that, there are some tools to help you.

“But it’s so pretty,” you might be thinking, “especially in the spring, with its lovely little yellow flowers dancing daintily in the breeze.”

Don’t be deceived. Those pretty little flowers harbor a nasty secret. During the dry summer months, those mesmerizing little yellow flowers, turn into nasty, dark, hardened, fuzz-covered pods, that open with a “pop” and a mini-explosion that launches the seeds into the air where they can travel for yards before landing. A mature plant, which can be up to 10 feet tall, can produce as many as 10,000 seeds a season! And while about 40% of those seeds will germinate immediately, and 25% the following year, because of an impervious seed coat, the remaining seeds can remain dormant in the soil for up to 50 years!(1. U.C. Agriculture and Natural Resources, Statewide Integrated Pest Management Program,

French broom grows quickly, as much as 3 – 4 feet the first year, and the long, skinny stems grow very close to each other, ultimately forming dense stands that most wildlife finds not only impenetrable, but unpalatable, as well. The caterpillar of the genista broom moth does feed on the leaves of French broom, and can strip the plant bare. But even that doesn’t seem to kill the plant.

In its second or third year, the stems will start to branch near the top and become covered with bright green leaves and pea-shaped flowers that effectively block the sun from reaching the ground, thus preventing the germination and regeneration of native plants. And because it “fixes” atmospheric nitrogen, it can over-fertilize the soil, as well. And in the fall, this thick, dry, woody undergrowth provides fuel for devastating wildfires.

If you cut it down or even burn it, French broom will simply re-sprout, unless you cut it down within its first year, or so. Fire, in fact, can stimulate germination of all those dormant seeds, which means that here in Napa County (and Sonoma County, as well) we could face an onslaught in the next few years. And if you spray it with herbicides, you risk killing the other native plants in the vicinity and introducing toxins into the soil that will eventually find their way into our water supply.

Fortunately, French broom has one weakness that we can exploit to help eradicate it safely: a surprisingly small and shallow root system, which makes it relatively easy to simply rip out of the ground with your bare (gloved) hands, especially if the ground is wet, such as it is right now! And if you come across a plant that requires more than a little manual tug, there are special tools available designed to grab the stem near the base, so all you have to do is exert a little leverage to rip it out, roots and all!2.
Winter is the perfect time of year to take on the broom, whenever and wherever you see it. The idea is to simply pull out as many plants as possible, especially the bigger ones that will soon be flowering. It’s a lovely and productive way to get a little winter-time outdoor exercise between rain showers and explore the ground beneath our feet. You may reap the rewards right away and discover some lovely mushrooms while you work. And in the spring you might see a variety of lovely wildflowers, Fremont’s star lily, Western hounds tongue, and native irises, popping up in places where there was once only broom.

If we all pull together and pay attention, we could eradicate French broom and send it packing. But it’s going to take many, many hands and a concerted effort for a number of years. But what a great and productive way to enjoy the beautiful outdoors during the winter months.

For many years the Weed Wrench was the tool of choice, but the manufacturer has recently gone out of business. Fortunately, the Napa County Agricultural Commissioner’s Office (707-253-4357) has a number of them available for local residents to borrow. Otherwise, you can purchase similar tools directly from the manufacturers: the Pullerbear (; the Uprooter (; and the Extractigator (

How did it get here?
This nasty little plant, along with Scotch, Spanish, and Portuguese broom, was first intro¬duced into North America from Europe in the mid-1800s. And why not? It was so pretty. And it was hardy! So hardy that someone at the Natural Resources Conservation Service of the US Department of Agriculture thought, “Wouldn’t this be a great tool to combat erosion along roadsides and areas that had been heavily mined.” And so, it was planted throughout the state, a solution to one problem that created another. It is now one of the most menacing and destructive invasive plant species in California.

How do you know if your broom is French?
French broom has four to ten clusters of bright yellow flowers at the tips of their branches and dense, trifoliate leaves, or leaves that come in clusters of three. The stem is not round, but instead has 8 to 10 angles. French broom is an evergreen, meaning it keeps its leaves all year long.

One of the reasons broom does so well is because its roots have tiny nodules full of bacteria called “rhizobia” that have the ability to add nutrients to the soil by turning nitrogen (N2) from the air into ammonia (NH3), an essential nutrient that helps the plant grow.

When the plant dies, that nitrogen, “fixed” within the plant, is released into the soil making it available to other nitrogen-needy plants.

The problem is that our native plants and wildflowers have, over eons, adapted very well to local conditions and don’t need or want this extra nutrient boost that gives a competitive edge to non-native weeds that thrive on higher levels of nitrogen.