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mulch pile

The Scoop on Mulch

With the advent of spring your planting beds are likely looking a little dull and you are probably anxious to spruce things up with a fresh coat of mulch. But before you blanket those beds consider for a moment the various factors that can come into play as a result of this regular garden activity. 

Mulches can be found in a variety of forms from ground buckwheat hulls or shredded bark to pebbles or even ground up old tires (no joke). Mulches in our urban and suburban landscapes are employed primarily as decorative elements and as weed barriers. While these may be useful attributes there are other noteworthy benefits mulches can provide that impact the health (and beauty) of the plants living in these beds.

Water Absorption and Retention

One of the most important benefits mulch can provide is absorption of rainwater. During periods of heavy or prolonged showers water in lawn areas (and paved surfaces) will runoff rather than be absorbed. Mulched surfaces tend to act as sponges while also acting to retard its flow. In this way mulch can help to control erosion while at the same time making water available to plants long after a rain event. In addition mulched areas provide a protective barrier against the drying effects of sun and wind. These combined benefits eliminate the need for irrigation in properly prepared bed areas once plants become established. In the larger scale, mulch helps to reduce flooding by providing a vehicle for water to be transferred into the soil (and water table) rather than directty into streams and rivers.

Further Benefits

In addition to maintaining soil moisture levels, the use of mulch can provide other valuable benefits. Organic mulches such as composted bark mulch, or, better yet, straight compost (comprised of leaves, grass clippings, vegetable kitchen waist, etc.) enriches the soil and, over time, improves soil structure. Mulches also play a valuable role in moderating extremes in temperature of the soil near the surface where roots are concentrated.

Living Layer

There is no better demonstration of an effective mulch than what we find in a nature. The duff layer created by years of leaf litter, decomposed trees and roots along with animal activity provides the ideal growing environment for natve plants (as well as many exotics). A plush, insulating layer of new leaves and fallen branches rests above a moisture rich semi composted layer which intern covers nutrient rich, black compost, alive with beneficial fungi and bacteria. The plants and organisms maintain a symbiotic relationship within this environment; what is good for the plants is good for the organisms and vise versa.


Bloodroot growing in leaf litter

photo, L. W. Brownell color, W. Federbusch

A Note of Caution

The most common mistake made in the application of mulches is using too much. In the interest of that fresh look mulch is applied each spring on top of the previous layer of mulch that has not yet degraded. After several years of this practice the mulch layer gets thicker and thicker to the point where the ability of water and air reaching the soil surface becomes severely restricted. By limiting the availability of water and air the breakdown of the mulch is slowed thus compounding the problem. The breakdown or composting of organic mulches utilizes oxygen and nitrogen, which in turn limit their available to plants. If that clean bright mulch look is a must then the solution may be, first, to remove all the mulch from the previous year (or years) then add a thin (2-3 inch) layer of mulch or compost. Finally, avoid piling up mulch around the trunks of trees and shrubs. The area at base of a tree or shrub where the trunk flares to meet the root system is particularly susceptible to fungus and the development of cankers.



A poor application of mulch

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