The only way to work at this problem is one sapling at a time. The tiniest ones--less than a quarter-inch in diameter and one foot tall--may be pulled out with a pair of pliers. On the rest, cut them off at ground level with a pair of hand pruners and then dab the cut stub with undiluted brush killer. Brush killer is pricier than Roundup, but the bottle will last a long time when applied this way. Dab applicators, like one called Weed Wand, are inexpensive. They're basically plastic bottles with sponge-tip applicators. Be careful not to get any brush killer on the plants that you want to save. If you do, rinse it off immediately with water.