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Landscape Mulch 101

Soil pH and Selecting the Proper Mulch


When determining which landscape mulch is best suited for your landscaping needs, there are many factors to consider. The question can best be tackled by presenting the various types separately, but judging each based on the same criteria.

Before beginning, let's look at a preliminary question that many landscaping enthusiasts have: how does landscape mulch affect soil pH? More specifically, does any mulch lower soil pH, as many have long suspected?

The composition of your soil pH has a hefty impact on plant health. Since landscape mulch could influence that composition as it decomposes, it's understandable that people (myself included) have often expressed concern over how mulch selection impacts soil pH. Does the use of pine needles lead to an acidic soil, for example? What about oak leaves?

A view I'm coming across more and more (although there is plenty of room for disagreement, especially as new research comes in) in my reading is that landscape mulch has a minimal impact on soil pH unless the same type is applied year in and year out. Also, while oak leaves may be acidic when fresh, experts such as David J. Williams now say that "the net result is an alkaline reaction," as they decompose. Furthermore, based on my reading, it is now generally thought that pine needles lower soil pH to only a negligible degree, if at all. And whatever possibility exists for acidification pertains only to uncomposted pine needles. Composting neutralizes them.

Nonetheless, it couldn't hurt to rotate between materials traditionally deemed "sour" and those thought of as yielding "sweet" soil; and just to be on the safe side, have your soil tested occasionally.

With the question of the potential impact of landscape mulch on soil pH out of the way, let's reflect on some other issues surrounding mulch selection -- some of which are quantifiable, others of which boil down to personal landscaping preferences. We will have to prioritize in order to make a decision, since a mulch that scores high in one category might perform miserably in another. Two obvious uses of mulch to which the reader will find little or no reference in this article are weed suppression and erosion control. They have been omitted for a simple reason: any landscape mulch employed properly will cut down on weeds and erosion. These are the two constants in this discussion.

Preliminary Notes About Mulching

  • "Insulation value in summer" is judged by the degree to which the mulch can keep the soil beneath cool and moist. A successful summer insulator will both reduce the need for watering and protect roots against extreme heat.

  • The consideration of whether or not the mulch needs to be removed in spring is grounded in the fact that heavy organic mulches can smother emerging spring plants. This is obviously less of a factor, however, for plants that remain alive above-ground, throughout the winter. But even the latter can profit from having the soil around their roots warmed by the spring sun, a process facilitated by the temporary removal of the mulch. In the case of plastic sheets, this factor is irrelevant, since holes are poked through the material to provide access for the plants.

  • "Nourishment and aeration afforded to underlying soil by decomposition" is one of the criteria used in the following pages to compare the various landscape mulches. However, do not be fooled by the word "nourishment" into thinking that compost and mulch are synonymous. For details on the distinction read, "Fall Cleanup: Raking Leaves for Compost and Mulch."

On Page 2 we'll begin comparing mulching materials in earnest, beginning with a landscape mulch very popular in the southern part of the U.S. We'll also look at the bark and plastic types....

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