Someone told me this yellow flower is from South Africa but I don't remember the name

#1 is Malva neglecta or something vey similar.

thank you. I googled and #2 (the yellow flower) seems to be Oxalis pes-caprae. That led me to a list of CA garden weeds which mentioned Malva neglecta and Malva parviflora. #1 in my garden has leaves up to 6" wide.

You're right about the Oxalis pes-caprae id. aka Bermuda Buttercup.

Thank you, this is #3, it's prettier but coming up everywhere

#4 this doesn't look good.

#3 looks like Acanthus mollis.

#4 looks like Cyperus alternifolius.

#4 looks like nutsedge to me. The first one is quite prolific...I spray it with Roundup on a regular basis but haven't eradicated it yet.

Sorry, Cyperus involucratus, formerly mistaken for C. alternifolius.

Yes, #1 is mallow (Malva). It is cultivated in China and they have selected for much bigger leaves and some very interesting looking leaves. You can also find different species of Malva that put on quite a flower show. It is related to hollyhocks.

The malva gets to 15 feet in my neck of the woods.

Wood Sorrel is the common name for all Oxalis species as a group, and not a single species entity with 2 different binomial names. Oxalis pes-carprae is not the same species as Oxalis stricta. The scientific Latin names never change due to location, just the common names.

This is the original poster. I lived in this house for 6 years and moved out a couple months ago.

The Oxalis was very stubborn. As an experiment I plucked hundreds every winter/spring, even though I knew it left little bulbs and would come back in the same spot. After a few years it exhausted itself and was almost all gone. My backyard lawn was very small so this was doable. But's it's so pretty I left some in the borders and it didn't spread much to the grass. Eventually different weeds found the lawn, maybe some hitchiked on my dog.

The Malva was not a big problem in the back yard, worse in front but easy to pull out after a rain.

I loved the Acanthus, and transplanted some plants to cover up ugly pots, spigots, etc. It was nice and green throughout the winter and came back nicely every year.

I didn't like the Cyperus. I think it was planted deliberately, there was quite a bit of it in another part of the yard. It's exotic looking, but the tips of the leaves seemed to turn brown quickly, it was hard to keep up with the deadeading/keep it looking fresh. My dog liked to hide behind it, so I kept it (she also liked the Acanthus).

Thanks for all the responses.

Glad to know about the oxalis elimination - thanks for that update!

I currently have a pink flowered oxalis that's taking over my veggie garden and I despair of ever being able to remove all the tiny bulblets.

It was more of an experiment, my lawn was very small, maybe 15 by 10 feet, and the buttercup was only in about half of the area. I pulled up hundreds of plants every year, some large, some tiny. But eventually it was almost all gone and I saw very few plants in the lawn for the last 2-3 years.

Eventually new weeds colonized the yard--a spreading clover with tiny yellow flowers, maybe Oxalis corniculata? and sticky weed (Galium). These are both very common at my mother in law's house and probably my dog or me introduced them.

Tilling ground containing Oxalis will not kill it. It will merely spread the bulbils far and wide.

Yes, I have. I’ve also seen acres of fields around the Mediterranean covered in Oxalis pes-caprae after tilling. They thrive in disturbed ground. That’s why they're an invasive species of cultivation around the world, including California. I fail to understand how you can ’grind’ the tiny succulent bulbils (they’re not bulbs, different structure) into ‘dust’. The plant forms many bulbils along the rhizomes which regrow if broken off. Only by persistent and repeated pulling or cultivation, over and over again, can you begin to get control.

FWIW, the oxalis in my yard produces a main thick root and dozens of very tiny bulbils attached to that, so when you dig it, it's difficult not to scatter them. They are the size of a grain of millet.

Here's a pic - the tiny brown things that look like seeds are the bulbils:

I spent a long time trying to carefully dig some out of my veggie garden and in a few weeks a big new patch had sprouted up 😣