The Story of Mulch

Leaf mulch being added to a flower bed. Photo: fresh-basil.com

Mulching, the art of adding a layer of material to the surface of soil to reduce weed growth, conserve moisture, improve fertility, etc., is a fairly modern practice. You can’t trace it back thousands of years like growing crops, plowing and manuring.

The term “mulch” originally simply referred to the action of spreading manure over fields (the word mulch is believed to have derived from the Middle English “molsh” meaning strawy dung) and that is an age-old technique. It’s the idea that this technique could do more than just enrich the soil that is fairly new. 

Towards the end of the 18th century, Parisian market gardeners learning to grow strawberries (Fragaria × ananassa), a “new fruit on the block” at the time, discovered that covering the soil between their plants with straw kept weeds down (and strawberries, being low-growing plants, are especially susceptible to weeds) and also solved another problem: it reduced reduce fruit rot, a common problem when strawberries touched the soil directly. Also, they realized that a covering of straw eventually reduced populations of slugs, snails and insect pests. They called this use “paillis”, derived from the word “paille” (straw), and it’s still the term used in French for mulch. You could say paillis means “strawing”. 

Since its first use about 250 years ago, the use of mulch and the comprehension of its full value has grown enormously and has spread over much of the world, especially towards the end of the 20th century, although straw is no longer the main mulching material. (In most regions, wood-based products are the most common mulch.)

Landscaping Mulch or Horticultural Mulch?

A lot of information sources divide mulch into two categories: organic mulch, made of decomposable materials, and inert mulches, ones that don’t decompose. I understand that, but I think it is more practical to divide mulches differently, into landscaping mulches, used mainly to beautify gardens, although with some beneficial results, and horticultural mulches, mostly used to help gardeners to grow better plants, although most also have an ornamental aspect.

Landscaping Mulch

In this group, you find both inert mulches, such as crushed stone, gravel, pebbles, pumice, crushed brick, tumbled glass and rubber mulch, ones that will never decompose (at least, not during the lifetime of the homeowner), and conifer mulches that only decompose slowly, such as cedar, hemlock or cypress mulch, pine bark, pine “nuggets” and wood chips. There are even mineralized wood mulches, treated to make them flame resistant and to prevent decomposition and that thus become as inert as any stone mulch.

These mulches are used in places where you are looking for an ornamental effect, but where you don’t intend to undertake any serious and repeated gardening actions (i.e. sowing seeds, planting annuals and vegetables, etc.). They’re often placed in areas where you want no plants at all, such as around the foundation of buildings, surrounding swimming pools, in parking lots and in playgrounds. Usually, in such cases, a covering of landscape fabric is first laid down to reduce weed invasion, and then the fabric is covered with about 2 inches (5 cm) centimeters of landscaping mulch. They’re also used, ideally without landscape fabric, at the base of trees and shrubs that you plant without ever intending to do any kind of planting at their base.

Landscaping mulches aren’t just ornamental: they do benefit plants (when there are any around!) by keeping down weeds and reducing watering needs, but they don’t really enrich the soil to any notable degree, at least, not in the immediate. And applying landscaping mulch around young trees has the advantage that you never need to approach the trunk with a lawn mower or string trimmer and that helps prevent injuries to the base of the tree.

Interestingly, in arid climates, “lithic mulch” (stone mulch) is being used in plantings, even of vegetables, because it reduces water evaporation, decreases rain runoff and reduces temperature variation. In a more humid climate, on the other hand, stone mulch has an almost opposite effect, drying and warming the soil and is rarely used in growing vegetables.

It’s generally not a good idea to put landscaping mulches in a flower bed, vegetable garden or any garden which you regularly add or move plants. You won’t want to put these long-lasting mulches in direct contact with the soil under such circumstances, because they end up mixing in with it. Since they either don’t decompose (stones and artificial products) or do so extremely slowly (conifer mulches), they come to hinder rooting and healthy plant growth and then have to be removed manually, one piece at a time.

In the long run, landscaping mulches eventually fill in with wind-blown dust, which creates a layer of soil on top of the landscape fabric, and then weeds begin to invade. This can take years, but seems to happen in all climates. When this happens, you have little choice but to remove them, wash them off to remove the accumulated soil and replace the landscape fabric before putting the mulch back into place.

It’s important to always keep bark mulch and cedar mulch somewhat damp during periods of drought, because they can become inflammable when dry.

Horticultural Mulch

These mulches are made of readily decomposable materials, so if they ever end up mixed into the soil, that doesn’t bother the healthy ecosystem the gardener is trying to create. Some of these mulches are sold commercially, but others are homemade.

Straw, the original mulch, is, of course, a horticultural mulch, but so is coconut/coir mulch, cocoa shell mulch, forest mulch (made from partially decomposed wood residues), pine straw, lawn clippings*, peat moss, sawdust, shredded paper, chopped fall leaves and ramial chipped wood (BRF). The latter is from the French bois raméal fragmenté and consists of hardwood branches shredded with their leaves, giving an exceptionally rich mulch. Finally, any compost, be it homemade or purchased, can also be used as mulch.

*It’s best to dry grass clippings out before using them as mulch; otherwise they tend to clump up and form an almost waterproof crust over the soil.

Although most horticultural mulches also have an ornamental effect, their primary role is to make gardening easier by:

• Preventing weed germination.
• Reducing evaporation and thus keeping the soil more evenly moist.
• Moderating soil temperatures and avoiding thermal shock.
• Preventing erosion.
• Reducing insect, disease, slug and snail infestations.
• Keeping soil from compacting.
• Enriching the soil through their own decomposition.
• Eliminating the need for cultivating and hoeing, thus reducing the gardener’s workload.
• Increasing the presence in the soil of beneficial microorganisms and also of other useful animals, such as earthworms and ground beetles.

Before installing horticultural mulch, always thoroughly weed the site, because while these mulches help prevent weeds from germinating, they don’t keep those already present from growing back. In a short time, any weeds left in place will grow up through the mulch layer and take over space the gardener intended for more desirable plants. To eliminate invasive plants before applying a mulch, you can cover the ground with 7 to 10 sheets of newspaper, cardboard or kraft paper. Another possibility is to place a black tarp or piece of old carpet over the weedy section for a year before starting: that will eliminate pretty much any weed.

For other ideas on weed control before applying mulch, read How to Control Invasive Plants.

After weeding, plant the garden, then generously cover the soil with horticulture mulch to a depth of 3 to 4 inches (7 to 10 cm). If you need to plant or sow in the future, just move the mulch to one side temporarily, plant, then put it back into place. (In the case of seeds, wait until any seedlings are taller than the mulch before putting it back in place). Again, horticulture mulch decomposes rapidly, so if some of it mixes into the soil, no harm is done. 

You can apply horticultural mulch in any season. Many gardeners, for example, apply their mulch in the fall, when autumn leaves—an excellent mulch!—start to accumulate on the ground and are therefore readily available. Run them under the lawn mower to shred them (this will give a finer mulch less likely to blow away in the wind and ensures a more even appearance), giving the home gardener not only free mulch, but a very rich one as well. And do note that yes, you can use leaves with minor leaf diseases in leaf mulch. Just ignore anyone who tells you otherwise!

Horticultural mulches are short-lived: most last no more than or two or three years; even one year in the case of many kinds of shredded leaves. But that’s not a bad thing, because if they disappear, it means they have decomposed, thus enriching the soil in minerals and largely eliminating the need to apply fertilizer. So, you just have to be ready to renew them quite frequently (annually in the case of shredded leaves). No need to remove the old mulch, of course: just cover it with fresh mulch. Simple!

Two Oddball Mulches

I’ve kept two “mulches”—black plastic mulch and living mulch—for the last, because I don’t consider them to be true mulches. 

Black plastic mulch is a sheet of thin black plastic, often biodegradable, that is applied to the soil of a vegetable bed early in the season to exclude weeds and to warm the soil. When used for a summer crop, black plastic mulch is generally used for heat-loving vegetables, such as peppers, melons, eggplants and tomatoes, because it usually keeps conditions quite toasty, not something all vegetables can stand. Of course, its ornamental value is pretty much zero. I see it as a weed barrier and a soil warmer, but not really a mulch.

Finally, there is “living mulch”, a term sometimes used to describe groundcover plants. And it’s true that once they are well installed, groundcovers do just about everything that a real horticultural mulch can do … and what’s more, renew themselves without requiring any effort on your part. I have nothing against living mulches and use them widely in my own garden. It’s just that I personally don’t seem them as mulches, but rather as plants. To me, they’re groundcovers, period.

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So, there you go, at short portrait of mulch. If you’ve never used mulch before, beware: it can radically simplify the way you garden!