polymeric sand?

Polymeric sand is used so grass will not grow between joints. You can try, but will have some issues down the road. I used a sod cutter and removed the sod. I screeded (leveled) and compacted the soil. Instead of sand, I used soil with starter fertilizer to fill joints between rock. Compact soil and make sure it is lower than the top of rock. Apply grass seed and cover with straw and water.

The “stones” as formed by the Country Stone Walkmaker Concrete Mold are all connected by a thin layer of concrete at the bottom, so grass will not be able to readily grow through the concrete. The use of polymeric sand will further inhibit the growth of the grass. You may consider running the edge of a trowel around each “stone” while the concrete is still wet to facilitate grass being able to grow through, or you may consider actually sprinkling dirt and grass seed in the joints. The first of these will result in the “stones” no longer being joined, which may provide less stability.