Whether you're installing a new path or rehabilitating an old one, you have several choices for filling the cracks between the stones. The biggest decision is whether you want to add living filler, such as plants and moss, or nonliving filler, such as sand or gravel. Perhaps coloured sea glass in alternating colours will complement your decor perfectly, or an exploration into "Gap Gardening" to extend your flowers and herbs into those narrow nooks and crannies. Once you have made your choice, the rest is limited only by your creativity.
Spread coarse sand or stone dust over your path. The colour and material will vary depending on the side of the cracks you are trying to fill. Sand is best suited for small spaces between bricks. Stone dust is preferred for larger spaces such as those between roughly laid fieldstone.
Sweep the path. You should choose a broom with wiry bristles meant for dirt or shop debris. Don't use a finer bristled dust broom. Work in a pattern back and forth over the stones, brushing the filler into the cracks.
Tamp the ground to help settle the filler. If you do not have a professional tamper, you can lay a 60 cm x 60 cm (2 foot x 2 foot) piece of plywood on the ground and jump on it. Tamp each area several times, then move to the next area. If using the jump method, move the plywood every few jumps.
Reapply the filler across the path. There should be space in the cracks where the previous layer settled. Sweep the filler into the cracks as you did before and tamp the ground again. Repeat this process until the cracks are completely filled. You may have to reapply filler again after a few days or after the first rainstorm.
Select low-lying plants to grow in the cracks of your path. The size of the crack you are filling will determine which type of plant you should use. Many herbs such as thyme are ideal for small cracks. The Deltoidea family, including Aubrieta and Dianthus, are appealing for tight areas. Thyme can also be used for larger cracks, although many mosses are preferred for this duty. Talk with a local landscaper to get recommendations for your specific climate and application.
Scrape out the dense filler used when the path was laid. If the area is tight, you may need to use a screwdriver to work out the sandy base material. You should remove several centimetres of the filler if possible.
Replace the filler with soft loamy soil. You may need to use the screwdriver again to help work it into tight spaces. Make sure you do not pack it too tightly. Looser soil will encourage the roots to grow down and will avoid damaging the stones around it.
Spread your seeds evenly across the soil just as you would if you were planting a flower garden. Water the seeds when the soil becomes dry.
To replace a single large cracked tile, chisel around the edge of the tile and remove the broken piece entirely. Scrap off the sandy base underneath and add several centimetres of potting soil. Plant a small shrub in the new soil. Cover the exposed soil with wood chips. This will help retain moisture, protect the roots and add a finished look.
Plan your path project for a dry sunny day. Wet sand will not brush easily into the cracks.
If planting between the cracks, be wary of invasive species. While they may take to their new home quite well, you could soon find yourself with more plant than path.