When grass grows between flagstones, it creates the illusion that the stones are floating across the lawn. The stones also keep visitors from treading on your lush grass. For grass to thrive between flagstones, place the stones at least 5 inches apart. Since most flagstones are located in shaded walkways, consider planting a shade-tolerant variety of grass such as fescue, rye or bluegrass.
Clean up any debris or weeds growing in the planting area.
Loosen the soil around the flagstones to a depth of 3 inches with a hand cultivator. Break up any clumps of dirt and remove any roots or rocks. Smooth the area with the back of the cultivator or your hand.
Spread enough quality potting soil over the area to reach halfway up the side of the flagstone. This will prevent any dirt or debris from washing off of the flagstones and covering the grass.
Sprinkle grass seed over the planting area by hand. All commercial brands of grass seed include the application rate on the packaging. Spread the grass seed according to the manufacturer's recommended rate.
Spread a starter fertilizer over the planting area at the rate recommended by the manufacturer.
Press the seed gently into the soil with your hand so it makes good contact with the soil.
Keep the grass moist by watering it three times daily for a week. Each subsequent week, cut out one of the scheduled waterings until you are watering the grass once daily. After this point, water the grass with the same frequency with which you water the rest of your lawn.