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Beautiful but ecologically harmful shrubs get a foothold in California forests

Deceivingly beautiful French broom is invading California forestland.
A glorious foothill display of yellow flowers and their spicy-sweet fragrance may delight the senses, but they pose a serious problem for California.

Scotch, Spanish and French broom were introduced from Europe in the mid-1800s as lovely, easy-to-grow garden accents and land stabilizers, but they have become aggressive invaders threatening native plants and increasing fire hazards.

“These brooms crowd out our native flora and form large, dense stands of just broom,” said Scott Oneto, UC Cooperative Extension advisor in the Central Sierra. “It’s also displacing the birds and animals that would live in this environment with native plants.”

The characteristics of invasive brooms cause several problems. The plants grow large and upright, developing thick trunks at the base. After its relatively short life span - typically 7 to 8 years, 15 at the most – they die and become tinder-dry woody skeletons that can burn high and hot.

The brooms are also a member of the legume family. Legumes are unique in the plant world. They have evolved a symbiotic relationship with bacteria that live on their roots. The bacteria are able to pull nitrogen from the atmosphere and deposit it into the soil, where it feeds the plant.

“These soils are naturally very low in nitrogen,” Oneto said. “Our native plants thrive in low-nitrogen soils. Large populations of broom are changing the soil chemistry so even after they are removed, the area is no longer ideal for our native vegetation.”

Large stands of broom are also a significant concern for rangeland managers. Cows don’t like the taste; only goats will eat it. When the plant is grazed off or cut back, it readily re-sprouts from the crown. The plant’s spread is bolstered by its intriguing ability to scatter seeds widely. Brooms grow seed pods that are naturally spring loaded. When the seeds are ready for dispersal, they fling from the pod with the force of a tiny explosion.

In collaboration with the non-profit California Invasive Plant Council, which received a grant from the Sierra Nevada Conservancy, Oneto is working to eradicate broom from a particularly sensitive location. Highway 120, for many the “gateway to Yosemite,” has populations of French and Spanish broom.

“We’ve started by mapping the broom along Highway 120 and plan to eliminate the population to prevent it from moving further into the forest and eventually into Yosemite National Park,” Oneto said.

The grant is providing the funding for environmental and regulatory compliance so that control measures can be implemented.

“Once the regulatory compliance is complete, the project will be shovel-ready and we can begin treatment,” Oneto said.

Scott Oneto (left) and Rebecca Miller-Crips stand near landscaping in Sonora's Courthouse Park where invasive French broom was planted as an ornamental.
Despite French and Spanish broom’s invasive nature, the heat-tolerant greenery and pea-like spring floral display still inspire people to plant them in landscapes and even parks. Brooms are occasionally found in retail nurseries along with a cousin, called “sweet broom,” that is touted as a sterile, non-invasive substitute. However, DNA research conducted by scientists in the Department of Plant Sciences at UC Davis found that sweet broom hybridizes with French broom, potentially contributing to the invasive weed problem.

“If broom is growing wild or as an ornamental on your property, we suggest you remove it and replace it with a non-invasive plant,” said Rebecca Miller-Cripps, natural resources program representative with UCCE Central Sierra. For example, forsythia produces yellow flowers and a shrub of about the same size and shape as brooms, but isn’t invasive, she said.

For more about invasive broom, see the video below:

Posted on Friday, June 14, 2013 at 9:01 AM

Comments:

1.
Great piece! Stop that dang broom!!!!!!!

Posted by Wendy West on June 14, 2013 at 4:25 PM

2.
At my local nursery, they are selling a broom called Genista racemosa and said it is not invasive. Do you agree?

Posted by K.Lauritson on June 22, 2013 at 4:06 PM

3.
“Sweet broom” (Cytisus spachiamus or Genista racemosa) is not currently known to be invasive. However, because we lack information on its potential for invading wildlands, we do not recommend it as a substitute for other brooms. In addition, recent research has shown that Genista racemosa hybridizes with its close cousin French broom (Genista monspessulana) which is highly invasive.

Posted by Scott Oneto on June 24, 2013 at 10:23 AM

4.
I am seeing what I think is Mexican Petunia all over Auburn canyon. Is anyone making a conclusive ID and investigating how invasive this thing is? I see it sold everywhere!

Posted by Rose H. on June 26, 2013 at 10:04 PM

5.
Hi Rose H.  
No I am not aware of Mexican petunia being invasive here in California. It has become extremely invasive in Florida so I would be very interested to know more about what you have seen. Could you send me an email with the exact location:  
sroneto@ucanr.edu

Posted by Scott Oneto on June 27, 2013 at 4:06 PM

6.
I came from New Zealand to northern California and was dismayed to see how widespread the brooms are here. I've even seen gorse (Ulex europaeus) in what I think must be late pre-invasive stage up near Crescent City. That's enough to give a wandering Kiwi the fits!  
 
Another significant factor in the long-term invasive nature of these legumes is that seed remain viable a long time - two or three decades for some - so eliminating a seed bank can be a very expensive and protracted exercise.

Posted by David Price on July 1, 2013 at 1:23 PM

7.
leave the broom alone it looks nice

Posted by broomfan99 on April 23, 2015 at 4:28 AM

8.
just purchased 8 #2 sweet broom. genista racermosa from Costco and then read the article on Green Blog and now have questions. They are in full bloom and look gorgeous and smell good. I will plant 2 in large urns and planned to plant the other six interspaced with french lavendar, lavendula dentata on the slope in front of our home. Both are drought tolerant and better than the ivy which was planted 55 years ago and the purple and yellow work well to gether--and they were cheap at $9 each. Should I return them all or keep them all or just the 2 in urns?

Posted by david di ckerson on March 30, 2017 at 9:48 PM

9.
Hi David  
Thank you for the great question. “Sweet broom” (Cytisus spachiamus or Genista racemosa) is very closely related to the invasive French broom (Genista monspessulana). Although French broom is no longer commercially available for purchase as an ornamental, sweet broom is widely available. In recent years, there have been a lot of questions as to the potential invasiveness of sweet broom and whether sweet broom and other brooms hybridize. Some very recent DNA work done by my colleagues at U.C. Davis, suggest that the majority of invasive French broom in California originated from G. monspessulana but that ornamental sweet broom can contribute to invasive populations, particularly in urban locations. Because of this, I do not recommend planting sweet broom in areas where other very closely related broom species have become invasive.

Posted by Scott Oneto on April 4, 2017 at 10:52 AM

10.
Thank you for the article and the experts' comments on the Brooms. I have 4 sweet broom for 5-6 years, there're gorgeous however I discovered that they were growing so fast, the trunks so thick and the roots are invasive went under the concrete platform and eventual cracked the concrete. I had to to cut them all and plan to remove all together. I will be happy to send some pictures.

Posted by K. Qiu on July 25, 2017 at 10:25 AM

11.
Thank you for the informative article. My husband and I recently purchased a track of land in Carmel Valley that is filled with Genista. We are in the process of removing it and were wondering what we should plant afterward? Will natives regrow in the soil?

Posted by Arleen Tarantino on November 16, 2017 at 3:39 PM

12.
I suggest you get in touch with the UC Master Gardeners in Monterey County. Here's their website: http://mbmg.ucanr.edu/

Reply by Jeannette E. Warnert on November 16, 2017 at 3:42 PM

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