Yellow Sorrel/ Oxalis is a weed we are seeing a lot of right now. You have likely seen it in your own lawn or a neighbors lawn...a sea of tiny yellow flowers with foliage that looks a LOT like clover.
It's important to deal with this weed quickly for a few reasons:
First, it will spread very, very quickly. It will start as a small patch here and there...and soon your lawn will be covered in it.
Secondly, this is both a spring AND fall weed. If you don't deal with it now, it will actually disappear on its own the closer we get to summer...then it will come back with a vengeance this fall. That is why it needs to be killed as soon as you start seeing it.
This is one of those weeds that has to be treated since hand pulling is not very effective. It has a very complex root system, and usually returns within a few days of hand pulling it.
Your basic three-way broadleaf post emergent herbicides should do the trick!
