what do you use for mulch ....

I'll trade your yellow sorrel for my poison ivy :-)

I feel your plight! It's the number one weed in my garden. It grows better than the shrubs and plants! It's growing like weeds(LOL) on the north side of my house and it's very dense. I haven't gotten to that side of the house yet to start pulling it out and I am not looking forward to it. I few years ago I put down cardboard over it in between the shrubs and then put mulch down over the cardboard and and worked for a few years but this year it came back with a vengeance.

Thank you all for commiserating! The mulch I've tried is about 3" of shredded hardwood. It seems to come pre-seeded with yellow sorrel. :-/

I've got yellow sorrel too. Sun, shade, dry or damp, it grows just about everywhere. Just when I think I've gotten it I find a new colony hidden behind some other plant.

The only trick I've found that sort of works is to plant densely to minimize exposed soil.

I looked up yellow sorrel. You guys -- all these years I thought it was clover. LOL!

Let me be the guy who chimes in with the politically incorrect answer: it truly is easier to hit with herbicide, because it's so hard to pull. unless you get them right after a rain and use a digging tool, they like to snap at the base and regrow. The purple ones are worse than the green ones in that respect. You can use an appropriate selective like Trimec, or the dreaded glyphosate. Former is more 'satisfying' because it kills it faster, and in the case of these, it does so throughly. (for other things like pokeweed or violets I prefer to not use a selective because it seems like a selective just knocks them back temporarily, sometimes. Of course in a lawn, you have to use a selective!)

I have no idea why this page says it is hard to kill with herbicides: https://hort.extension.wisc.edu/articles/common-yellow-woodsorrel-oxalis-stricta/

I wouldn't put it on that list at all - if the herbicide is listed to kill Oxalis, it is easily killed. Maybe whomever contributed that wasn't using enough surfactant. The leaves are among the most very hydrophobic of any weed so you definitely want to up the surfactant in your mix if going after it. I use a methylated seed oil based one that is approved for use around wetlands because I don't want to affect my ample population of toads and frogs that keep slugs at bay.

I don't find it hard to pull at all. It's just that it seems to be everywhere.

@UpperBayGardener (zone 7) My problem with herbiciding it is that it's in so thickly with perennial beds (not a lawn). I don't know how to treat it without overspray and damage to desirable plants. For what it's worth, I do seem to remember trying 2-4-D on it in a prior home with very poor results.

Banana, I do pick and pull if weeds are right at the base of other perennials.* Sometimes there is no other option. What you can sometimes do is hold a piece of cardboard or flexible plastic between two plants and spray one. As long as there is not too much contact when you remove the barrier, you are ok. The stuff isn't plutonium LOL. At normal correct dilutions, a certain % of the plant's surface must be coated for it to kill. Granted I will admit I've sometimes been too careless, especially with especially susceptible-to-selectives families like Asteraceae and Fabaceae. I seriously maimed one of my cardoons last fall by spraying for Oxalis near it. Just a few droplets of trimec (I think but not 100% sure...could have been triclopyr) hitting the Cardoon almost killed it, but it's back now. I knew I should have used a barrier but was rushing.

* or if they are thing relatively easy to pull, period, like seedlings of Celastrus orbiculatus. That plant is pure evil!