There are almost never seedlings growing from acorns. If there were, you would be able to pull them up easily. What you see are sprouts from the roots of the existing tree; therefore, you do not want to spray a herbicide on them in an attempt to eliminate the sprouts for you will hurt the “mother” tree along with the sprouts.
Only a small percentage of oaks send up suckers from the roots. It is a genetic trait, like freckles, except I like freckles. But like freckles and sunshine, some trees have the ability to sucker, but do not unless stimulated to do so. Oaks having a slight tendency to sprout suckers will often do so when roots hit a barrier, such as trees confined to a parking lot planter, or between a sidewalk and driveway. Also, when roots are disturbed and damaged by rototilling, they are more likely to sprout suckers. But some trees never will make suckers. When choosing an oak in a garden center, if there are sprouts coming up at the inside edges of the container, I would avoid that tree.
You may choose to mow them along with the grass, if grass still exists. Or if the grass has thinned too much, you might plant Asiatic Jasmine groundcover, and use hedge trimmers to trim the jasmine and oak sprouts to a uniform height. You can cover the area of sprouts with a heavy gauge woven geotextile, and then either mulch or spread large gravel or decomposed granite over the top of the geotextile. My favorite solution, when appropriate, is to cover the ground with geotextile and then build a wood deck.
Or if you prefer a thick green lawn, you may remove the oak tree, and all of the tree roots with a backhoe. If you just cut down the tree, grind down the stump and all the large roots you can see, there will still be thousands of oak sprouts emerging from the remaining roots in your new lawn or bed area for a few years afterwards. The area will need to be continually sprayed with an herbicide.
James 
Rosanna says
What a lively discussion about those pesky live oak sprouts! So, I’m not alone in the battle…. Years ago we put down landscaping cloth under a row of live oaks and covered it with a thick layer of rocks. Our hope was that enough water would get through the cloth so that the trees got sufficient water, but the suckers would die out. In the spring I would blow the leaves away and had no baby oak trees to deal with. Eventually we had to remove the rocks and cloth. What did we discover under the cloth- a four inch thick layer of sturdy live oak roots/runners/whatever they are! It’s taken about three years now, but I’m finally getting close to bare earth when I weed wack the forest of baby trees. I keep a good supply of nylon weed wacker line and just knuckle down to the job periodically.
Jane says
After 20 yrs. of dealing with these live oak suckers, here are my theories on these pesky painful suckers, which are impossible to get rid of under live oaks – at least in TX!:
1) pull by hand to keep suckers to a minimum!
2) plant zoysia (spelling?) grass, as it is the only grass that you are going to be able to grow under your live oaks over time due to the shade & lack of sun, so give up the thought of a lush green sucker-less lawn under your live oaks now!
3) Haven’t ever tried chemicals, but I actually don’t think ANY chemical could kill our live oaks – at least in TX – & why would you want to kill these magnificent evergreen beauties? – yes, even with the suckers all around them (which can get pretty heavy during certain times of the year – but just stay the course at that steady hand pulling)!
4) resign yourself to the fact that you will not get to skip barefoot through the grass under your live oaks as a) you’ll likely barely have any grass to speak of due to the lack of sun under your live oaks, & b) you’ll likely end up in the ER by being stabbed in the feet by those suckers!
5) I think James is 100% right about these suckers becoming more “plentiful” under large oaks in a smaller yard when he said the live oak’s ever-growing roots running into a structure like a sidewalk, house or wall will just redirect those tree roots upward out of the ground as they have to grow somewhere & will also produce even more suckers to contend with! These tree roots & suckers are HUGELY powerful, and we have already had to completely replace our brick and concrete front walkway once due to live oak root walkway “upheaval”! You can’t keep a live oak root – or sucker! – down!
6) Mowing and hand-pulling the suckers are the only things that will allow you to grow any grass whatsoever under your live oaks!
7) Wear heavy duty rubber gardening gloves when you pull the suckers & put your back into getting those sucker roots! It’s great exercise, although you might have to pace yourself (or develop carpel tunnel)!
8) I do think every time you pull 1 sucker, you create 3, but that’s life – & the beauty of the huge live oaks above the mowed down, hand-pulled suckers is worth it!
9) Pull suckers when you are mad at your husband or boss! Yes, put those airpods in, play your favorite podcast & get after it then! You’ll get twice as many pulled, eliminating suckers while also eliminating taking your anger out on those 2 – or saying something you may just regret later!
10) Stand back & look at your work after a good sucker pulling session! You will have 1 of 2 reactions!: You’ll either think “it’s rewarding” or think “oh my Lordy, I haven’t even made a dent!” (depending on the time of year!)
11) In TX, even “Snowmageddan” in the winter of 2020, with ice on the ground for several days, & no power, killing most of our other landscaping & plants, didn’t kill a one of those live oak suckers, so forget your rocks & gravel, herbicides, poison & decks…..you’re not killing the suckers! All you can do is wk toward eliminating them for a time by hand-pulling & mowing!
12) Learn your lesson for your next house….Never plant little live oaks in a small yard – you’ll just end up with large live oaks and suckers in a small yard & a big annual tree trimming arborist bill! You can’t kill a live oak, too, no matter what your arborist tells you! Have them really trim – AND THIN – those live oaks so at least a little sun can break through & you just might have some grass!
13) Build a large stone border under the edge/perimeter of the live oaks outer branch tips & fill that stone border in (after your thin grass sod removal) with solid mondo grass sprigs! Did that 8 years ago & haven’t seen a sucker since (the mondo is either hiding the suckers or killing them out)!
14) Finally, hang a wooden/rope swing under your live oaks & swing to your heart’s content after a good aggressive couple of hrs. “sucker pulling” (but be sure to wear some athletic shoes to be able to get to the swing w/o your feet getting “stabbed” by those suckers)!