Oak trees generally do not respond well to transplanting. Because of this, most oak trees are started by placing an acorn in the ground. However, not all acorns are planted intentionally by humans. Some acorns fall and are buried. Others are planted by wildlife hiding them for the winter. Oaks sprout quickly, so a landscaper may have to contend with new seedlings growing in their zoysia when they did not plant them there. Fortunately, oak trees are easy to remove once they have sprouted.
Rake the soil of your lawn to remove acorns before they can sprout.
Hand-pull oak seedlings the moment you find them. Oak seedlings will look like tiny trees or sticks emerging upright from the ground with small oak leaves attached. The leaves have serrated edges. To pull the seedlings, wrap both hands around the seedling and yank upward to uproot the plant.
Cut down stands of oak seedlings at ground level with branch loppers. Soak a sponge applicator with a systemic herbicide containing triclopyr. Swab the applicator across the cut surface of the seedling stumps. Be careful not to drip the systemic herbicide onto your zoysia grass. Triclopyr will kill zoysia as easily as it will kill oak seedlings.
Spray a broadleaf herbicide over your zoysia lawn. Broadleaf herbicides will kill oak seedlings, but will not harm zoysia grass. Some broadleaf herbicides are sold with a spray applicator. For those without a built-in spray applicator, purchase a pump applicator. Pour the herbicide into the plastic holding chamber. Prime the applicator by grasping the handle and pumping it up and down to build pressure within the chamber.
Grip the holding chamber in one hand and the spray applicator wand in the other. Depress the trigger on the wand applicator to release a fine mist of herbicide. Walk over your lawn in an even pattern, and apply the sprayer by waving the wand in a broad fan shape in front of you as you walk.
Mow your lawn weekly to cut down oak seedlings. Oak seedlings will die after repeated mowing.