What is it really? Pine needles, hardwood chips, pine bark, leaves, newspaper, gravel?
From Merriam-Webster:
mulch: noun; a protective covering (as of sawdust, compost, or paper) spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, enrich the soil, or keep fruit (as strawberries) clean
So, anything that provides the protective covering mentioned above is a “mulch”. Inorganic and organic materials can provide that covering. Some do different things, and all have benefits and drawbacks. They also differ in price and ease of installation.
First look at organic and inorganic mulches. Organic would describe any material that can degrade into organic (carbon) compounds. These compounds can then be used by organisms in the soil (plants, animals, bacteria). Inorganic would include anything else.
Organic mulches:
- pine needles
- bark/wood chips
- straw
- leaves
- newspaper
Inorganic Mulches:
- gravel/stone
- landscape fabric
- brick chips
- rubber chips
Those organic mulches add nutrients to the soil. Some, like chopped leaves and straw break down quickly; others, like pine needles, more slowly. Generally, smaller pieces of mulch with greater relative surface area will degrade more quickly than other materials. Landscape fabric, despite being inorganic, will break down over time especially when exposed to UV light.
Of course when mulches break down they need to be replenished. That’s where most of the inorganic mulches have an advantage. Stone won’t break down (at least not in a landscape’s short geological time frame), so it won’t need to be replaced. However, such mulches can “sink” into the soil below or appear to disappear as silt and organic matter build up in the material.
Some mulches help with water infiltration, some inhibit it. Layered newspapers will keep water from entering the soil until most of the sheets are fully wet. Once the paper breaks down a bit this won’t be such a problem. Landscape fabric also limits water infiltration unless there’s sufficient water to sit on the fabric and be pushed through the holes as more water falls on the surface (water’s surface tension causes it to bead up and roll off). Large leaves can mat up and force water to find circuitous routes to the soil.
Any material covering the soil will help prevent soil erosion. Our Piedmont clay can erode easily and needs to be covered by some type of mulch. When our clay soils compact (and rain can cause compaction) infiltration rates become the same as concrete or asphalt. Water simply runs off, taking soil particles with it. Soil particles are one of our biggest waterway pollutants. If you see muddy water running into storm drains during or after storms, you are seeing pollution.
All mulches can reduce weed growth. The covered soil keeps common weed seeds in the soil from light which inhibits germination. However, weeds can still grow even with mulch. Perennial weeds (weeds that live more than one season) that spread by underground stems or roots can travel through the soil and pop up in the mulch. These weeds are most easily controlled with Roundup-type (glyphosate) herbicides or consistent removal. Weed seeds can still blow or fall into the mulch and sprout right there. These are easily removed when they are small. Keeping little weeds from becoming mature, seed producing weeds is the best way to become weed-free.
On stone mulches: I will use this occasionally, but only when the look is appropriate for the design. The most common use would be for a parterre type garden. White or tan stones are commonly available and reasonably priced, but keep their reflective qualities in mind as they can be pretty bright in full sun. Mexican beach pebbles are really popular, but they are quite costly here in NC. I once used those for an indoor pool. Here’re some pictures showing stone mulches:
- Parterre garden
I do prefer organic mulches. They look good and I love the fact that they add nutrients to the soil. Pine bark is my favorite. I have used decorative stone at one client’s home for some parterre gardens.
I rarely recommend landscape fabric. When used it is still necessary to cover it with another mulch–because the fabric is unsightly and to minimize its degradation. So you really need to use two mulches. On slopes the top mulch may wash away, revealing the fabric. Like this photo shows it can get pretty ugly:
- Exposed landscape fabric
If you need something to stabilize the soil until plants spread out try an erosion control matting or a jute fabric. Both are organic and will degrade over time. Oh, and don’t try walking on a slope covered with landscape fabric and pine needles–you’ll slip unless you have crampons on your boots. The only time I might use fabric is under stone mulches. But be sure it’s not lumpy and the stone isn’t too thin or you’ll see it.
How much mulch do you need? Well, this homeowner went a bit overboard:
Too much mulch
Run, it's gonna blow!
Most mulches are best kept at 2-4″ deep. It’s important to keep mulch from becoming too thick at the base of most plants. When the mulch is deep the trunk can rot. Or trunks might sprout roots in the mulch making the plant much more vulnerable to freezing or drought. Little chewing rodents can hide in the mulch with easy access to the base of a tasty plant if the mulch is too thick.
Stone mulches which also double as paths should not be more than 3″ deep as walking anything deeper can be really difficult.
One more mulch idea is to use living plants. Groundcovers (including grass) do all that mulches do and they keep getting better as they grow. You may not be able to cover all of your soil with plants so keep those bare spots covered with mulch.
*I’ve had this tune in my head since I decided on this title!
