In order to understand the importance of mulch to a tree, think back to the last time you walked through a forest. You may have noticed that the forest floor was covered, not by grass, but by organic matter. Twigs, leaves, dead flowers, rotten wood, and other debris cover the forest floor. This material can be thought of as a mulch. It shades and cools the soil, adds organic matter and nutrients to the soil, reduces compaction, and helps keep grass and other plants from growing under and competing with the trees. Shade from surrounding trees also keeps soil and roots cool and moist in the forest. Trees that are native to heavily forested areas, therefore, are well adapted to having allot of organic matter covering their root systems. Trees roots are very shallow, within 6 to 12 inches of the soil surface, and this organic matter or mulch helps them survive. Roots do best under moist, cool conditions and need plenty of oxygen in the soil. These conditions are ensured by a good mulch layer.

Proper Mulching Techniques

 DO NOT-Mulch higher than 2-4 inches. If using finely textured or double shredded mulch use 1-2 inches because these materials allow less oxygen to the root zone

DO NOT-Mulch against the trunk or stems of the plants. Keep all mulch 3-4 inches away from the trunk, allowing the root flare zone to show above the ground level.

DO-Mulch to the trees drip line if possible. Remember that the "drip line" moves as the tree grows.

Below are overall views of best and worst mulches. Family Tree provides this information as a guideline in your decision making process. Click on the tab for more information.

  • Best Mulches
  • Worst Mulches

Bark chunks or shredded bark that is at least 3/8 inch in size. Pine bark will last longer than hardwood bark

Bark mulch can add color and interest to your landscape while retaining moisture around your. Bark mulch helps protect plant roots from extreme temperatures, cooling them in the summer and insulating them in the winter. It creates an ecosystem around your plants by encouraging the beneficial bacteria that break it down into an organic humus. Bark mulch can last for as long as 6 years, needing only a small addition each year to maintain the 4-inch cover and the fresh color.

Pine Straw Mulch

Pine straw consists of the dead needles shed by pine trees each year. The needles range in color from brown to reddish-brown, providing a garden material that's both useful and attractive. Pine straw breaks down slowly, adding organic matter that helps improve the soil quality in the garden bed.

One Year Old Pine Chips


Leaves that were shredded or composted for at least three months


Fresh Grass Clippings

Grass clippings can compress into a mat of slimy stinky matter. Plant roots need air and water, and a mat of grass clippings depletes the soil of oxygen and moisture. In addition, freshly cut grass is hot. Have you seen steam rising from a mound of cut grass or hay? The heat of decomposition can do root damage to plants.

Fresh Wood Chips

This material contains bark and pieces of wood of various sizes and makes an attractive mulch. A 2- to 3- inch layer of wood chips provides good weed control. Small wood chips decompose very rapidly using nitrogen from the soil, which needs to be replaced by nitrogen fertilizer. Wood chips may attract termites and other insects.


Any fresh organic that smells bad


Peat Moss or Saw Dust

A 2- to 3-inch layer of peat moss gives fair to good weed control. However, peat tends to form a crust if used in layers thick enough to hold down weeds. It is very difficult to wet and tends to blow away if applied dry. Peat also is a relatively expensive mulching material.

A 2-inch layer of sawdust provides good weed control. If applied around growing plants, add 1/2 pound of actual nitrogen per 10 cubic feet of sawdust to prevent nutrient deficiencies. Fresh sawdust contains a great deal of carbon and very little nitrogen, and its breakdown requires that microorganisms take nitrogen from the soil. There is a problem with crusting of fresh sawdust, with resulting impermeability of rainfall. Sawdust is best used for garden paths and around permanent plantings. Readily available from sawmills, it tends to be inexpensive.

Pebbles, Rocks, or Cobblestones

Decorative rocks that come in different shapes, colors and sizes are used as protective mulch. Though expensive and labor intensive, they need no replacements. They add color and a finished neat appearance to a landscape. Microorganisms do not degrade the hard rocks. They have no nutritive value. They do not hold water but retain soil moisture.

Black Plastic

Black plastic is often used for mulching. Do not use plastic sheets, as they will trap heat, which will harm the root system of a plant. Make slits in the plastic or use shredded plastic. 

Ground Up Rubber Tires

Have been tested as soil amendments & surface mulches, & discovered to kill ornamental plants due to its zinc content; to be a fire hazard dangerous to place near buildings; & to smell incredibly bad.
Often a single application is sufficient to kill plants. Nevertheless, more & more companies are obtaining crumb rubber waste product & packaging it in nicely designed bags that recommend it for gardens. Still other companies slip it into commercial composts as a bulking agent. The excuse for this is that rubber does have a slight (very slight) nitrogen value. But the toxicity from zinc, cadmium, & other heavy metals, more than makes up for any faint chance of a benefit.

Dangers of Over Mulching

  • Excessive Moisture and Root Rot
  • Inner Bark Tissue Death
  • Canker Diseases
  • Excessive Heat (Preventing Hardening)
  • Rodent Chewing and Stem Girdling
  • Nutrient Deficiencies &Toxicities
This is the number one cause of death by over-mulching. Repeated applications of mulch can cause a waterlogged soil and root zone, causing root suffocation. Roots must respire and take in oxygen, unlike leaves that give off oxygen. The problems that are caused from yearly over-mulching are not immediate. The symptoms may take three to five years to express themselves, and sometimes longer, depending on the species and soil type. When oxygen levels drop below 10 percent, root growth declines. Unfortunately, by the time you recognize the symptoms (off-color foliage, abnormally small leaves, poor growth and die-back of older branches), it is generally too late to apply corrective measures. At this point, the plant has gone into an irreversible decline. When roots decline and die, so does the plant.
Above-ground stem tissue of most trees, shrubs and perennials is morphologically different from roots and must be able to freely exchange oxygen and carbon dioxide. Mulch that is piled onto the trunks decreases gas exchange, with inner bark tissue eventually dying. When the inner bark dies, roots no longer receive the energy produced by the leaves through the process of photosynthesis, and the plant dies.
Another mortality factor that is associated with the application of mulch next to stem tissue involves fungal and bacterial "canker" diseases. Most plant diseases require moisture to grow and reproduce. These lethal trunk diseases are no exception and usually gain entry into the stressed, decaying bark tissue. Once established, these cankers will eventually encircle the tree, killing the inner bark, starving the roots and ultimately killing the plant.
Thick mulch layers that are placed against the stem will begin to decay and can produce excessive heat. Similar to composting, where inner mulch layers may reach 120-140 degrees F, the heat may directly kill young trees and shrubs or may prevent the natural autumn "hardening" process that plants must go through to prepare themselves for winter. If trunk flare tissue does not adequately harden before freezing weather arrives, the tissue will die, the roots will starve, and again, the plant will go into decline. The tree needs healthy roots to produce the hormones and chemical growth regulators that allow it to properly prepare for winter weather.
Placing piles of mulch adjacent to tree trunks and other plants can kill plants by providing cover and habitat for chewing rodents such as mice, voles, etc. With lots of cover from predators, the critters will usually live under the warm mulch in the winter and chew on the tender and nutritious inner bark. Often you may not notice this chewing until the following spring or summer when the tree no longer looks good. If the chewing is extensive (more than 50 percent of the circumference) or goes around the whole tree (girdles it completely), there is little you can do to save the tree.

Continuously using the same type of mulch may cause plant death by changing the soil's acidity level, commonly referred to as soil pH. Acid mulches like pine bark may have a pH of 3.5 to 4.5, and when applied continuously, can cause the soil to become too acidic for most plants to grow. Conversely, hardwood bark mulch, although initially acidic, may cause the soil to become too basic or alkaline, causing acid-loving plants to quickly decline. Soil pH's above 6.5 usually create micronutrient deficiencies of iron and manganese for many common landscape plants. You can avoid this by periodically rotating the type of mulch used.

Finally, non-composted, "fresh" or non-aged mulches may cause nitrogen deficiencies in many young trees, shrubs and flowers. Decomposing bacteria and fungi that ultimately break down mulch must have an ample supply of nitrogen to do their job. Most landscaping mulches are comprised of bark or wood that has very little nitrogen available for the decomposing bacteria. Hence, the bacteria in the soil utilize the existing nitrogen to break down the mulch. This process may cause nitrogen deficiencies and yellow leaves as a result of the excessive mulch.

Proper Mulching Benefits

Mulch helps the soil to hold water for the tree's roots

Mulch helps to prevent soil compaction that suffocates tree roots

Mulch helps add organic matter to the soil as it gradually breaks down, thus acting like a slowly released natural fertilizer for the tree

Mulch helps prevent the soil from washing away. Soil erosion is very harmful to the tree's exposed roots. Soil erosion not only stresses the tree but can increase the chance of blow down in a storm or windy day.

Mulch helps to moderate soil temperature. It behaves like an insulating blanket. It helps to keep soil cooler in the summer and warmer in the winter. This is much less stressful for the tree's roots.

Mulch helps to reduce weeds and grasses around trees. Weeds and grasses will compete with the tree's roots for water and nutrients.


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Member of the American Rose Society Research Committee,
Science Editor for the electronic American Rose Journal, and Horticultural Judge







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