The natural beauty of a flagstone patio or pathway is enhanced by planting ground cover in the empty space between stones. Not only do the plants help tie the flagstone in to the surrounding landscape, they also make a garden trail or patio feel more inviting. Plants typically used around flagstones are low-growing, hardy and don't require pruning or mowing. They also keep weeds down, which often means less maintenance than required by gravel, sand or mulch used between stones.
Dig out the soil or gravel filler between the flagstones using a hand trowel. Remove the top 1 to 2 inches of soil.
Remove the plants from their pots or plastic planting cells. Turn the pot or cell upside down and gently shake the plant out. You can push against the bottom of thin plastic pots and cells to help get the plant out. Unlike other types of plants, it's alright if some of the roots of ground cover plants are damaged while removing them from the pot. Low-growing ground cover plants that spread in a bushy manner rather than sending out long runners work best between flagstones. Plants commonly used between flagstones are Woolly thyme, Pink Chintz thyme, Roman chamomile, Scotch moss and Blue Star creeper.
Spread the roots by gently grasping them with one hand on each side and slowly pulling them apart. It is OK if a few roots break away at the bottom, but take care to keep the roots near the plant intact.
Dig additional small holes, about 2 inches deep, between the flagstones where you will place individual plants. Spacing varies among plants, but for the most popular, such as Woolly thyme, Pink Chintz thyme and Blue Star creeper, 6 inches between plants is standard. Slower spreading plants like Scotch moss and Roman chamomile may be planted closer, about 4 inches apart. Since ground cover plants are intended to spread and meet up with each other, spacing is not critical, but placing them too close together, within 2 to 3 inches, can cause them to compete with each other for nutrients. Spacing them farther apart, such as 8 to 12 inches, is acceptable, but it will take longer to cover the empty space between the flagstones.
Place the plants in the holes, filling in around the plant with the soil dug out of each hole. The bottom of each plant should be even with or just slightly below the top edge of the flagstone. If the ground is uneven, fill in low areas with dirt or lower high areas by removing additional soil. Flagstones that are uneven should be leveled prior to planting by lifting them up and placing small gravel pieces in low spots.
Water each plant. Insert your finger 2 inches into the soil to make sure the water has penetrated far enough to reach the roots. If the soil is dry at 2 inches deep, water again until the top 2 inches of soil are moist.
Mix together equal parts sand and top soil.
Spread the sand and soil mixture between the flagstones, tamping or pressing it down very lightly, just enough to keep it in place. The mixture needs to remain loose enough to allow good drainage and the roots of the new plantings to spread easily.
Water regularly, keeping the soil constantly moist, until the plants are well established.