Selecting the Right Landscape Mulch

Straw mulch and drip irrigation around vegetable plants in garden.

Francesca Yorke / Photolibrary / Getty Images

Mulching is the action of covering the ground around plants with a layer of loose material. It's a key element of landscaping and gardening, and it can serve many functions. Sometimes mulch is used purely for decoration; other times it's used for controlling weeds, moderating temperatures, maintaining moisture levels, and adding nutrients to the soil.

Organic vs. Inorganic Mulch

Most mulching materials can be subdivided into two main groupings:

  • Inorganic mulches are created from manmade material or stone and used for controlling weeds and decorative purposes.
  • Organic mulches are composed of natural materials. In addition to being decorative, these mulches eventually break down and provide nutrients that improve the soil.

Examples of inorganic mulches include:

  • Stone and gravel
  • Black plastic
  • Shredded rubber or synthetic rubber "bark" chips

Organic mulches include a larger group of materials, including:

  • Compost
  • Shredded leaves
  • Shredded wood and bark chips
  • Pine straw
  • Straw
  • Hay

Benefits of Mulch

In addition to aesthetic improvements, many mulches provide a wide range of benefits:

  • Weed suppression: Just about any layer of mulch that is at least 1 inch deep (but aim for 3 inches in most cases) will help suppress weeds. This happens mostly because the mulch helps prevent weed seeds in the soil from obtaining the sunlight they need to germinate. However, some organic mulches may themselves include unwanted seeds that can that germinate and sprout within the mulch. Additionally, compost that has not experienced the proper heat during decomposition may contain viable weed seeds. Hay also sometimes contains the seeds that the plants growing in the field produced before they were harvested as hay, and these seeds tend to sprout in moist conditions.
  • Soil temperature moderation: A layer of mulch can be very helpful to keep soils from getting too hot, which can be important when growing sensitive plants in warm climates. In cold climates, a good layer of mulch may help protect against the daily frost and thaw cycles that can kill perennials. Some mulches, however, particularly exposed plastic or light-colored crushed stone and gravel may actually overheat the soil. Overheating may render soil sterile or kill helpful organisms, so be sure to research the best type of mulch for your climate.
  • Moisture retention: By preventing soil moisture from transpiring into the air, most mulches can help moisture-loving plants. Plants preferring dry soils, however, may be hindered by the moist nature of mulch-protected soil.
  • Soil improvement: Organic mulches, especially compost, add important nutrients to the soil as they decompose. Compost is especially good at this since it's already partially decomposed when applied. Other organic materials, such as shredded wood and bark chips, will also break down, but it takes much longer, and the nutritional value is less immediate. While the nutritional value of compost has long been known, plant science has also recognized that it improves the texture of soil simply by its presence. A thorough layer of compost over a planting area encourages worms and other helpful organisms to loosen and aerate the soil.

Mulching Effects on Soil pH

The composition of your soil pH has a hefty impact on plant health. Since landscape mulch could influence that composition, it's understandable that there's concern over how mulch impacts soil pH.

For example, spreading fresh pine needles and oak leaves may lead to a more acidic soil. However, while oak leaves may be acidic when fresh, experts such as David J. Williams at the University of Illinois at Urbana-Champaign say that "the net result is an alkaline reaction" as the leaves decompose. Furthermore, it's generally thought that decomposing pine needles lower soil pH to only a negligible degree, if at all. And whatever possibility exists for acidification pertains only to un-composted pine needles as the act of composting neutralizes them.

Alternately, some believe landscape mulch has little to no impact on soil pH. Although, if you wished to be on the safe side, it doesn't hurt to test your soil's pH levels and avoid using the same mulch type year after year.

Preliminary Notes About Mulching

There are many considerations surrounding mulch selection, some of which are quantifiable, while others are a matter of preferences. When choosing a landscape mulch, find the best fit for your planting area's specific needs.

Some important attributes to consider:

  • Insulation value is judged by the degree to which the mulch can keep the soil beneath it cool and moist. A successful summer insulator will both reduce the need for watering and protect roots against extreme heat. Clematis vine is an example of a plant that benefits greatly from having its roots kept cool in summer.
  • Required spring removal is required in some cases as heavy organic mulches can sometimes smother emerging spring plants. This is obviously less of a factor for plants that remain alive and above-ground throughout the winter. But even exposed plants 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 (or landscape fabric), removal is irrelevant since holes are poked through the material to provide air and sun access for the plants.
  • Nourishment and aeration is a key mulch element as decomposition will be hastened as you work the mulch into the soil. However, don't be fooled into thinking that compost and mulch are synonymous; they are not. 

Mulch Types

It's a good idea to compare the various types of mulching materials to determine their advantages and disadvantages. The following varieties are the most common options to consider:

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: Fair
  • Allows water and oxygen to move freely into the soil: Excellent
  • Ease of application and maintenance: Good

Wood Chips, Sawdust, and Bark Mulches

  • Appearance: Feathery texture available in black, browns, and red
  • Insulating value in summer: Good
  • Insulating value in winter: Good
  • Need to remove in spring: Yes
  • Nourishment and aeration: Fair
  • Lets water and oxygen move freely into the soil: Good
  • Ease of application and maintenance: Good

Black Plastic Mulch

  • Appearance: Poor
  • Insulating value in summer: High moisture retention, but it also heats up the ground beneath considerably
  • Insulating value in winter: Fair
  • Need to remove in spring: No
  • Nourishment and aeration: None
  • Lets water and oxygen move freely into the soil: No
  • Ease of application and maintenance: Excellent

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: No nourishment, fair aeration
  • Lets water and oxygen move freely into the soil: Yes
  • Ease of application and maintenance: Fair. Leaves get caught between the rocks and must be removed. Otherwise, they decompose and become soil in which weeds can sprout.

Partially Composted Leaves

  • Appearance: Fair
  • Insulating value in summer: Excellent
  • Insulating value in winter: Excellent
  • Need to remove in spring: Yes
  • Nourishment and aeration: Excellent
  • Lets water and oxygen move freely into the soil: Fair (unless leaves are very finely shredded, in which case they will rate better in this category)
  • Ease of application and maintenance: Fair

Straw and Hay Mulches

  • Appearance: Straw brightens your area nicely; hay is less attractive, but it 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: 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 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.
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  4. Winter Protection and Other November Gardening Tips. University of Vermont Extension