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Master Gardeners of the Blue Ridge
By Andrew Bowman

About the Virginia Master Gardeners and Virginia Cooperative Extension: Virginia Master Gardeners are volunteer educators who work within their communities to encourage and promote environmentally sound horticulture practices through sustainable landscape management education and training. As an educational program of Virginia Cooperative Extension, Virginia Master Gardeners bring the resources of Virginias land-grant universities, Virginia Tech and Virginia State University, to the people of the commonwealth.

This week I would like to cover a little information about mulching, especially around trees, since this time of year it is so easy to procure the item for mulching, or at least one of them.

Mulches are materials placed over the soil surface to maintain moisture and improve soil conditions. Mulching is one of the most beneficial things a home owner can do for the health of a tree. Mulch can reduce water loss from the soil, minimize weed competition, and improve soil structure. Properly applied, mulch can give landscapes a handsome, well-groomed appearance. Mulch must be applied properly; if it is too deep or if the wrong material is used, it can actually cause significant harm to trees and other landscape plants.

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 natures 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.

Trees growing in a natural forest environment have their roots anchored in a rich, well-aerated soil full of essential nutrients. The soil is blanketed by leaves and organic materials that replenish nutrients and provide an optimal environment for root growth and mineral uptake. Urban landscapes, however, are typically a much harsher environment with poor soils, little organic matter, and large fluctuations in temperature and moisture. Applying a 2- to 4-inch layer of organic mulch can mimic a more natural environment and improve plant health.

The root system of a tree is not a mirror image of the top. The roots of most trees can extend out a significant distance from the tree trunk. Although the guideline for many maintenance practices is the drip line-the outermost extension of the canopy-the roots can grow many times that distance. In addition, most of the fine, absorbing roots are located within inches of the soil surface. These roots, which are essential for taking up water and minerals, require oxygen to survive. A thin layer of mulch, applied as broadly as practical, can improve the soil structure, oxygen levels, temperature, and moisture availability where these roots grow.

Mulches are available commercially in many forms. The two major types of mulch are inorganic and organic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, geotextile fabrics, and other materials. Inorganic mulches do not decompose and do not need to be replenished often. On the other hand, they do not improve soil structure, add organic materials, or provide nutrients. For these reasons, most horticulturists and arborists prefer organic mulches.

Organic mulches include wood chips, pine needles, hardwood and softwood bark, cocoa hulls, leaves, compost mixes, and a variety of other products usually derived from plants. Organic mulches decompose in the landscape at different rates depending on the material and climate. Those that decompose faster must be replenished more often. Because the decomposition process improves soil quality and fertility, many arborists and other landscape professionals consider that characteristic a positive one, despite the added maintenance.

As beneficial as mulch is, too much can be harmful. The generally recommended mulching depth is 2 to 4 inches. Unfortunately, many landscapes are falling victim to a plague of overmulching. A new term, mulch volcanoes, has emerged to describe mulch that has been piled up around the base of trees. Most organic mulches must be replenished, but the rate of decomposition varies. Some mulches, such as cypress mulch, remain intact for many years. Top dressing with new mulch annually (often for the sake of refreshing the color) creates a buildup to depths that can be unhealthy. Deep mulch can be effective in suppressing weeds and reducing maintenance, but it often causes additional problems.

Please pay heed to the above paragraph about over-mulching. Many times it is better just to fluff it up rather than put down more, especially around some types of trees, like pine or maple, whose roots are close to the top for breathing purposes. It might be better to under-mulch than to kill the tree. Also, be sure to leave the mulch back from the trunk a couple inches, to avoid damage.

Until next time, just let it grow!
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