A neighbor recently asked me to evaluate his pine trees. Two Austrian Pine trees flank his driveway and one was suffering significant needle loss as compared to the other. Upon investigation I discovered that the failing tree had eight inches of mulch at its base completely covering the root flare of the trunk where as the other tree’s trunk was left exposed down to the soil surface. Improper application of mulch is all too uncommon.

While the application of mulch has many great benefits you must be sure that the application of mulch be done properly. The International Society of Arboriculture (ISA) has a great article on the proper mulching techniques found here. I’ll highlight a few sections of the article. Here is a link to a PDF of the ISA article.

Benefits of Proper Mulching

  • Helps maintain soil moisture. Evaporation is reduced, and the need for watering can be minimized.
  • Helps control weeds. A 2- to 4-inch layer of mulch will reduce the germination and growth of weeds.
  • Mulch serves as nature’s insulating blanket. Mulch keeps soils warmer in the winter and cooler in the summer.
  • Many types of mulch can improve soil aeration, structure (aggregation of soil particles), and drainage over time.
  • Some mulches can improve soil fertility.
  • A layer of mulch can inhibit certain plant diseases.
  • Mulching around trees helps facilitate maintenance and can reduce the likelihood of damage from “weed whackers” or the dreaded “lawn mower blight.”
  • Mulch can give planting beds a uniform, well-cared-for look.

As with most things in life moderation is the key. Too much mulch can cause many problems as my neighbor can attest.

More from the ISA:

Problems Associated with Improper Mulching

  • Deep mulch can lead to excess moisture in the root zone, which can stress the plant and cause root rot.
  • Piling mulch against the trunk or stems of plants can stress stem tissues and may lead to insect and disease problems.
  • Some mulches, especially those containing cut grass, can affect soil pH. Continued use of certain mulches over long periods can lead to micronutrient deficiencies or toxicities.
  • Mulch piled high against the trunks of young trees may create habitats for rodents that chew the bark and can girdle the trees.
  • Thick blankets of fine mulch can become matted and may prevent the penetration of water and air. In addition, a thick layer of fine mulch can become like potting soil and may support weed growth.
  • Anaerobic “sour” mulch may give off pungent odors, and the alcohols and organic acids that build up may be toxic to young plants.

Check out this video from Davey Trees, one episode in their series “Talking Trees,” on how to properly mulch your trees. Davey Expert and ISA Certified Arborist Shawn Fitzgerald explains proper application and a few mistakes to avoid.