Kneel down beside one of the flagstones and decide how much you want to reduce it by increasing the space.
Put on work gloves to protect your hands and then place a chisel on top of the flagstone at the chosen location firmly with your hand. Make sure to hold only the base of the chisel, not the top.
Hit the top of the chisel with a hammer two to three times until it starts to cut into the flagstone.
Move the chisel to a new location directly beside the first. Hit the top of the chisel again to extend the score line. Continue scoring the flagstone along the entire side that you are reducing.
Place the chisel back at the starting location and hit it again with more force than before. Continue the process until the flagstone section breaks off.
Pick up the excess pieces of flagstone and discard them. Repeat the chiseling process on all remaining flagstones to increase the distance between the plants and stones.