Battling Several of Contra Costa County's
More Noxious Garden Weeds

Advice for the Home Gardener from the Contra Costa Master Gardeners' Help Desk

Client's Problem and Questions:

Client originally called the Help Desk telling us that earlier this year he had installed several cubic yards @ 2-4” of redwood mulch over his yard intending to replant the area next spring. He did not follow the normal “sheet mulching” procedure of putting cardboard down before mulching nor did he apply weed killer although he did remove surface weeds. He is now seeing a great deal of “clover” like weeds growing up through the mulch. The client has sprayed the mulched area's weeds with Ortho Ground Clear™ and Weed-Be-Gone™ but the problem persists. He would like some advice on what to do. From his description we think it is probably some form of Oxalis, and ask him to confirm this by comparing his “clover” to a photo we send him of Oxalis stricta, one of many Oxalis often called “Sour Grass” and that are common garden weeds in our area. Believing it is probably Oxalis from his description, we also send him a link to “Woodsorrel and Bermuda Buttercup Creeping”, UC's Pest Note on these weeds. (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7444.html). He immediately responded back via email with several attached photos of the “clover” (and another weed not previously mentioned... see below).

Advice from the CCMGs' Help Desk

Oxalis pes-carpae ... typical leaves

Thank you so much for sending the excellent photos of your yard and weeds - they were valuable in identifying the clover-like plant as Bermuda Buttercup, Oxalis pes-caprae. My ID is based upon the presence of bulbs as well as the tiny purple spots on the leaves. Unfortunately, this is one of the hardest plants to eradicate. As we discussed previously on the phone, your first order of business is to use at least 2 layers of cardboard to cover the area, being sure to overlap the layers, wet the cardboard, and then cover with mulch (see sheet mulching link above). To quote the UC Davis Pestnote previously sent to you, "Woodsorrel and Bermuda Buttercup":

"Researchers around the world are investigating approaches for controlling Bermuda buttercup. Some suggest covering infestations with stiff cardboard, then covering the cardboard with a thick layer of organic mulch to kill the plants and weaken the bulbs, making them less capable of competing with desirable plants. Keep the mulch on the infestation until the mulch and cardboard have rotted, then plant competitive ornamentals into the soil-mulch mixture."

Oxalis pes-caprae "roots"

As we discussed, in Spring, if plants are still coming through, you should consider solarization, which involves covering the area with clear plastic during the hottest months of summer to actually "cook out" weed seeds and bulbs.  The following link to UC's Pestnote "Soil Solarization for Gardens & Landscape" will give you all the information you need for this process, including recommendations on the thickness of the plastic covering : http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74145.html.

The Contra Costa Master Gardener article, "Oxalis: From Ornament to Nuisance" will give you even further insight on controlling Oxalis per-caprae. http://ccmg.ucanr.edu/files/83817.pdf

Himalayan Blackberry

Finally, we also note that several of your submitted photos show possibly another noxious weed growing in your garden. Although not able to confirm with you at this time, it appears to be Himalayan Blackberry (Rubus armeniacus). If not controlled, this rather invasive wild blackberry will take up where your Oxalis stops in taking over your yard.  The UC Pest Note on Wild Blackberries (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7434.html) provides information on identification and management that should be of use to you. My experience is that it is a very tenacious weed and may require more than a few years to bring it under full control.

I know that the management of these weeds is a big challenge and wish you all the best in dealing with it!

Contra Costa Master Gardeners' Help Desk

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Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/

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