There is a moment on a new lawn when the slurry goes down green and the whole yard already looks grown in — and then two weeks of Merrimack Valley rain go ahead and make it true. Spray N Sprout Hydroseeding works that stretch of the calendar, seeding bare ground, filling in thin lawns and bringing new grass across the raw slopes and regraded yards that Haverhill's hilly lots keep producing. Soil here runs sandy down in the river bottoms and full of ledge up on the ridges, so no two seedings are quite the same conversation with the ground. The reward comes slow and then suddenly: nothing, nothing, a haze of green, then a lawn. Watching a bare hillside turn over like that is reason enough to keep doing this.