Q: There’s a shady area in my yard with a layer of clay several inches below the surface, where grass grows poorly. Two summers ago I let it go and allowed moss to take over, but the moss is so thin just walking on it damages it. How do I encourage the moss to grow more thickly? — Don Avery, Yorktown, Va.
A: A gorgeous ground cover — as soothing to look at as it is to touch — moss is poorly equipped to compete with larger plants for water. It thrives mainly where other plants grow with difficulty. Your yard sounds nearly ideal, however, with its shade and moisture-holding clay. No wonder the moss moved in so readily. Getting it to grow more thickly will take only minor adjustments: you won’t need to water or fertilize. Each season, the many wild moss species in your part of the country reproduce by throwing out billions of spores. Watering your yard will only encourage the thirstiest native mosses to settle in and spread, elbowing out the more drought-tolerant kinds that will liberate you from hose duty. You can discourage that straggly but invasive grass by making the ground it grows in more acidic. You’ll be helping the moss in more ways than one. If your lawn was ever limed to neutralize the soil, the moss may still be suffering from a soil pH that’s higher (more alkaline) than the 5 to 5.5 level it favors. Have the soil tested. To acidify the soil by one pH point — say, from 6.5 to 5.5 — over 100 square feet of ground, spread 2.5 pounds of sulfur or 6.5 pounds of aluminum sulfate or 12 pounds of ferrous sulfate, all of which are sold at garden centers. (If your soil happened to be sandy, not clayey, you’d need to add only half as much of these minerals to achieve the same results.) Pure sulfur takes about two months to work during warm weather; the compounds work yearround and take effect in about two to three weeks. Weeds and tree seedlings that crop up in the moss should be pulled in early summer. Roundup weed killer will not harm the moss, but use caution around vulnerable shallow tree roots. Promptly clean up fallen leaves before they stifle the moss. Leaf blowers are good for keeping that green carpet clear of litter. A quieter method is to spread sheets of black plastic mesh (a 1/4- to 1-inch grid) over the moss in early fall and then lift it to remove the leaves for composting when the autumn drop is over.