Landscape Mulch 101

Soil pH and Selecting the Proper Mulch

Person moving mulch from wheelbarrow with pitch fork
Francesca Yorke/Photolibrary/Getty Images

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....

After the initial considerations discussed on Page 1, it's time to compare the various types of landscaping mulch. I'll begin with pine straw mulch, a product sold in bales. The needles are harvested from plantations -- a huge industry! Most of it comes from the southern U.S., where its use is also commonly used in landscaping.

Pine Straw Mulch

  • Appearance: Pine straw mulch provides the reddish-brown color (although less vivid) that redwood bark mulch offers.
  • Insulating value in summer: Good
  • Insulating value in winter: Good
  • Need to remove in spring: Yes
  • Nourishment and aeration afforded to underlying soil by decomposition: Fair
  • Lets water and oxygen move freely into the soil: Excellent
  • Ease of application and maintenance: Good

Wood Chips, Sawdust and Bark Mulches

  • Appearance: Good
  • Insulating value in summer: Good
  • Insulating value in winter: Good
  • Need to remove in spring: Yes
  • Nourishment and aeration afforded to underlying soil by decomposition: Fair
  • Lets water and oxygen move freely into soil: Good
  • Ease of application and maintenance: Good

Black Plastic Mulch

  • Appearance: Poor, unless you are striving for that "hi-tech" look.
  • Insulating value in summer: A mixed bag here. Black plastic mulch keeps the moisture already in the soil from escaping, but also heats up the ground considerably.
  • Insulating value in winter: Fair
  • Need to remove in spring: No
  • Nourishment and aeration afforded to underlying soil by decomposition: None
  • Lets water and oxygen move freely into soil: No
  • Ease of application and maintenance: Excellent

On Page 3 we'll look at further options for landscaping mulch....

Like the plastic mulch we looked at on Page 2, stone mulch is an inorganic option, even though it is natural. Due to its ruggedness, stone (for example, the crushed stone shown in the picture) is particularly effective for landscaping with dogs.

 

Stone Mulch

 

  • Appearance: Good, if used around trees, cacti and succulents; but stone mulch may not be aesthetically appropriate for vegetable or flower gardens.
  • Insulating value in summer: Fair. Stone mulch tends to heat up, but it also retains much of that heat within itself.
  • Insulating value in winter: Fair. Again, while stone mulch easily grows cold, at least it keeps much of that cold off your soil.
  • Need to remove in spring: Yes
  • Nourishment and aeration afforded to underlying soil by decomposition: None
  • Lets water and oxygen move freely into soil: Yes
  • Ease of application and maintenance: Good

 

Mulching With Partially Composted Leaves

 

  • Appearance: Fair
  • Insulating value in summer: Excellent
  • Insulating value in winter: Excellent
  • Need to remove in spring: Yes
  • Nourishment and aeration afforded to underlying soil by decomposition: Excellent
  • Lets water and oxygen move freely into soil: Fair (unless leaves are very finely shredded).
  • Ease of application and maintenance: Fair

 

Straw and Hay Mulches

 

  • Appearance: Straw brightens your area nicely; hay is less attractive, but provides a softer look (and feel!)
  • Insulating value in summer: Excellent
  • Insulating value in winter: Excellent
  • Need to remove in spring: Yes
  • Nourishment and aeration afforded to underlying soil by decomposition: Excellent
  • Lets water and oxygen move freely into soil: Excellent
  • Ease of application and maintenance: Fair
  • Note: Straw is preferred over hay, because the latter tends to be riddled with weed seeds.
  • Note: Straw is one of the best mulch choices for winter protection, due to its insulation potential. Being hollow, each strand of straw provides dead air space -- perhaps the #1 characteristic of an effective insulator.