Greensward Lawn, Tree, & Pest
Mulch: Rock, Bark, ... ?
Do NOT use rock mulch for Trees or Shrubs!
If you must use a mulch use something soft and giving like wood chips, shredded bark mulch, compost, fabric, clippings,... If you use mulch to hold in moisture keep it as thin as possible. You really dont need mulch to hold moisture in our heavy clay soil. You want to encourage roots to grow deep and freely, seeking water on their own to maximize root growth.
If you must use mulch around Trees and Shrubs for weed control, erosion control, or organic matter soil conditioning lightly use no more than 3 inches (according to some references) of bark mulch at most in drier seasons, and none to 1 inch in wetter seasons. And do not let mulch touch the trunk, keep it back away 3 or 4 inches. 6 to 12 inches away from trunk is even better.
For our Red River Valley Clay Soil in Fargo area Greensward recommends bare soil, or a light layer of 1 inch or less depth of coarse bark mulch, or 1 inch or less of cured compost, or a mix of compost/bark mulch 1 inch or less. Kept 6 to 12 inches away from trunk. No fabric, or plastic underlayment. This recommendation to improve soil structure, micro-life, and add organic matter to the tree's rooting zone. Ideally the tree should be separated from the lawn, flowers, and shrubs by use of the mulch, or bare soil. Emerging weeds can be controlled with approved herbicides.
Coarse bark mulch is possibly the best, with no fabric or plastic underneath. The coarse bark will allow better soil air exchange, and water movement. Clippings, or real fine shredded mulch may act like a thatch roof inhibiting soil air and water movement.
Imagine living your life with rocks pushing up against you all day, every day, all night, every night.
After awhile the rocks begin to injure the trees vascular system. Cutting off the trees circulation. The rocks push against the trunk as the ground swells, the tree grows, the rocks settle, ... eventually sick and dying tree. Yuck terrible picture.
Rock mulch also collects solar heat and radiates the heat down into the soil where the plant(s) are growing. Some species of trees (ex. birch trees), and plants require cool soils for their preferred survival.
If you got rocks there now move them back away from the trunk as much as possible and keep them away from the trunk, and then keep it as light as possible. The extra weight under the tree is not good either.
Do NOT ever use plastic sheeting under your trees. If it is there now pull it out and get rid of it.