An infilled artificial turf surface [18] includes a particulate infill [24] with at least a top layer [28] that comprises a mixture of Black walnut shell particles [30] and English walnut shell particles [32], the walnut shell particles [30, 32] having been treated so as to eliminate or substantially remove tree nut allergens that are known to activate allergies in some humans. Preferably, treatment occurs via heat treatment in a rotary furnace, which also rounds and smoothes the particles [30, 32]. Particularly if used in the top layer [28] of a particulate infill [24] of an artificial turf surface [18], the shape and size and proportion of the Black walnut shell particles [30] and the English walnut shell particles [32] provide stability for the resulting turf surface [18], while also being able to absorb water applied thereto, thereby to hold moisture and to provide evaporative cooling of the artificial turf surface [18] for up to about five hours.