A mulch is a layer of material (usually organic, but not always) that is used to cover the soil. Mulching is natures way of gardening (it imitates the leaf fall in a deciduous forest) and provides a number of significant benefits. It protects the soil, suppresses weeds, conserves moisture and adds nutrients, all without disturbing the soil. It can be so simple and effective that some gardeners give up on cultivation altogether and become no-dig gardeners.
The benefits of mulch
Most annual weeds are adapted to growing on newly disturbed soil, so if the soil is never left bare they won’t be able to get established (this is an example of going along with natural laws rather than trying to fight them).
If the mulch material is free of weed seeds, it will eventually break down to form a weed free organic layer on top of the soil. Any weed seeds in the soil will slowly get buried deeper and deeper and won’t get the chance to germinate. If you don’t allow any more weeds to set seed and don’t disturb the soil, you can eventually almost eliminate annual weeds entirely.
Mulching can also work with perennial weeds if it is thick enough and you give it enough time (maybe two years or more).
Mulch significantly reduces the evaporation of water from the soil and in dry climates this is often the primary reason to use it. Not only does it shade the surface from the direct heat of the sun, it also increases the humidity of the air under the mulch (that in direct contact with the soil).
Soil temperature stabilization
On a sunny day the surface of bare soil may reach 90°F, which is a problem because when the soil gets above 80°F, organic matter breaks down faster than it is created (this is one reason we always try to avoid bare soil). On a very cold day bare soil will quickly freeze, which makes it impossible to do any gardening. A thick organic mulch insulates the soil and slows down these temperature changes (it can reduce soil temperature by up to 10°F). On a seasonal scale it keeps the soil cooler in summer and warmer in winter. It’s common practice to cover root crops with mulch in winter to prevent the ground freezing.
Organic mulch is so effective at insulating the soil from the suns rays that it can be a problem in spring, as it may prevent the soil from warming up quickly. For this reason mulch should be removed from beds at least 2 weeks before you wish to plant, to allow them to warm up.
Light colored mulches keep the soil cooler than dark colored ones.
Plastic mulches can be very effective for warming the soil, though they aren’t much help in cooling it.
Sunlight damages bare soil by burning up humus, liberating nitrogen and killing soil organisms. A layer of mulch protects the soil surface from direct sunlight and prevents these things from happening. It can also prevent erosion and reduces surface crusting and capping.
Mulching is the easiest and most natural way to add nutrients to the soil and improve it’s structure (it could even be considered a form of composting). Most organic materials break down quickly once they are in contact with the soil, as the humid interface of soil and air is an ideal place for decomposing organisms. A two-inch layer of shredded leaves will have all but disappeared into the soil by the end of one season.
Mulch benefits soil organisms by providing food, blocking out the sun (with its lethal UV rays), stabilizing the soil temperature, giving refuge from predators and providing suitable habitat. It is one of the best ways to increase the number of earthworms in the soil.
Organic mulches can make the garden look more attractive and well cared for. They also make it less muddy in wet weather, which reduces the amount of soil splashing onto leaves (and hence the spread of disease).
A thick mulch (up to 12˝ deep) of straw, tree leaves or other bulky material, may be applied after the temperature drops in late fall. This can protect late crops and tender plants and help them to survive in otherwise inhospitable situations. It keeps the soil warmer, reduces nutrient leaching and helps keep soil organisms (especially earthworms) active. It also keeps the ground from freezing, so you can harvest over-wintering root crops. This mulch should be removed in early spring to allow the soil to warm up.
If given advance warning of freeze, you can protect tender plants by heaping mulch right over the top of them.
Problems with mulch
There are situations where mulch can be a problem.
In cool, humid climates where slugs are a big problem, mulching may not work very well, because it provides a perfect hiding place for these pests.
In some situations mulch has been known to create conditions whereby normally harmless creatures such as earwigs and sowbugs become so numerous they become pests.
It is well known that organic mulches can keep the soil cold in spring. Less well known is that a heavy mulch may actually increase frost damage to plants in cold weather. This occurs because the well-insulated soil releases less heat than bare soil.
Some mulches (sawdust, tree leaves, wood chips, seaweed) may contain toxins that inhibit plant growth.
Some mulches (hay, manure) can contain a lot of weed seeds and other problems (though this may not matter very much if your soil is permanently mulched).
The biggest problem with mulch is that it takes a lot to work effectively. To lay 3” of mulch on a 100 square foot bed, requires almost a cubic yard of material .
The application of mulch
Before applying any kind of mulch you should remove any perennial weeds, otherwise they will simply grow through it.
A weed suppressing mulch must be continuous and impenetrable to be effective. The depth depends upon the material used, it might be a 6˝ layer of straw, 3˝ of chopped leaves, two layers of cardboard, or 6 – 8 layers of newspaper.
Whenever you use a mulch to suppress weeds you must pay particular attention to the edges, as weeds will try to creep around the edges. This is especially true with plastic, so bury the edges to prevent it happening.
If the mulch material is very high in carbon, you might want to put down a high nitrogen layer underneath it, to aid in its decomposition. Compost is probably the best thing to use for this, as it provides its own decomposing organisms to get to work on breaking it down. Horse manure is good too.
A good mulch material should be:
• Relatively long lived.
• Free or inexpensive.
• Available in abundance.
• Heavy enough that it doesn’t blow away.
• Free of weeds, weed seeds, pests, disease and chemical pollutants.
• It should add nutrients and organic matter to the soil when it decays.
• Ideally it will also be attractive.
Tree leaves: (2 – 3˝) Deciduous tree leaves, gathered after they drop in fall, are oneof the best mulch materials. They add humus and nutrients, encourage fungi in the soil and are usually free of weed seeds, pests and disease. They can be used whole, but tend to blow away or mat down (mix with straw to prevent matting). They work better and break down faster if shredded (use a shredder or a lawn mower).
Broadleaf evergreens can also be used, but are best shredded to encourage decomposition.
The needles of coniferous evergreens can also be used, but aren’t as good as deciduous leaves because they contain toxic resins and can lower the pH of the soil. They work best on acid loving plants such as blueberries and evergreens, but can also be used with other plants if you add lime. In some areas these are available commercially under the name pine straw.
Straw: (4 – 6˝) This bulky material is clean, attractiveand a rich source of humus. It can be used to enrich the soil, suppress weeds, protect plants from frost and to insulate the soil. If it isn’t organic it may contain pesticide residues, but this isn’t normally a problem.
Hay: This is similar to straw in many ways, but containsmore nutrients. However it also contains a lot of weed seeds, so isn’t a good mulch material for intensive beds.
Compost: (2 – 3˝) Compost is a good mulch materialbecause it contains a lot of nutrients and can suppress some pathogens. It can also suppress weeds quite effectively if at least 3” thick, Most gardeners don’t make enough compost to be able to use it as mulch, though if you buy it you may be able to. If compost is laying on the surface, exposed to the sun, some nutrients may be lost to oxidation. To prevent this you can cover it with a coarser mulch, such as straw or tree leaves.
Manure: (1 – 3˝) This can be a good mulch material,if you have enough of it that you can afford to lose some nutrients (nitrogen is lost when manure is exposed to sunlight). It is rich in nutrients and organic matter. Some manures (horse, goat) aren’t digested very completely and may contain a lot of weed seeds.
Fresh manure may contain so much nitrogen that itcan actually burn plants. It is often applied to the soil in autumn, so some of the nitrogen can leach out over the winter. Aged manure is safer because it has less nitrogen. Composted manure is even better, as ithas also heated up enough to kill weed seeds.
As with compost it may be a good idea to cover manure with a couple of inches of a coarser mulch, such as straw or tree leaves.
Seaweed: (2 – 3˝) A rich source of trace elements,seaweed breaks down very quickly into the soil, but doesn’t add much humus. It was once widely used in coastal areas.
Grass clippings: (2 – 3˝) These are rich in nutrientsand humus, but decompose rapidly so need replacing regularly if they are to keep weeds down. They are best put down in thin layers as they become available. Thick layers tend to mat down.
Sawdust: (1 – 3˝) Sawdust is highly resistant to decay because it is very high in carbon, so is long lasting when used as mulch. If incorporated into the soil it may cause a nitrogen deficiency, but this doesn’t happen very much if it simply remains on the surface. Fresh sawdust may also contain toxic terpenes and tannins, which can inhibit plant growth. When it does finally break down it is an excellent source of humus. If you really want to use a highly carbonaceous mulch such as this, you might want to put down a layer of high nitrogen material first.
Shredded yard waste
Wood chips: (2 – 3˝)
These woody materials are very durable and effective weed suppressing mulch materials and work great for perennial plantings. They are not so good in the annual garden as they don’t break down and release their nutrients for a long time. When fresh they may contain toxins that actually inhibit plant growth. They are best suited for mulching paths and around permanent perennial plantings, such as trees. They don’t rob the soil of nitrogen if they are left on the surface, as they would if they were incorporated.
I live in a heavily wooded area and tree trimmers are always looking for places to dump their wood chips. Just ask them and they will dump a truckload on your driveway (the trick is to ask them before anyone else does – and have an easy place for them to dump).
Bracken fern: (3˝) If you can get it in quantity it can be used to enrich the soil, suppress weeds and protect plants from frost.
Miscellaneous plant materials: Cottonseed meal, cocoa hulls, spent hops, tea leaves and coffee grinds all make fine mulch, but are rarely available in sufficient quantity.
Newspaper: (6 – 12 overlapping sheets) This is technically organic in it is made from trees, but it is so thin it doesn’t add much organic matter to the soil when it decomposes.
Newspaper is most often used as a weed suppressing mulch, either alone or in combination with other mulches (it is rather ugly to use by itself). It is most often used as a second weed suppressing layer underneath other mulches. You lay down a continuous layer of overlapping newspaper and cover it with a layer of leaves, hay or other organic mulch. It is so light it may blow away if not weighted or pinned down.
Pages with a lot of colored ink may be safe, or they may contain petrochemicals and other toxins. I’m sure you could use them without problems, but I feel better just recycling them.
Cardboard: (1 overlapping layer of corrugated) Another material that is technically organic, but that doesn’t add much organic matter to the soil.
Cardboard is very effective as a weed suppressing mulch and can even be used to eliminate tough woody weeds like blackberries. It isn’t quite as ugly as newspaper, but still looks better when covered with a layer of more attractive material.
Cardboard is most often used as a second layer, underneath other mulches, to make them more weed proof.
Other mulch materials
These materials don’t add nutrients to the soil, but are useful in some circumstances.
Carpet: May be organic or inorganic, depending on the fibers and backing materials (it’s best to avoid synthetics unless you want to end up with a layer of matted synthetic fibers in your garden). Carpet is good for suppressing weeds and keeping the garden clean and is great for paths. Lay it upside down for the most natural appearance, unless you really want to bring the indoors outside.
Plastic sheet: I prefer to avoid plastic whereverpossible and wouldn’t use new plastic merely to keep weeds down. I might use recycled plastic (such as that from greenhouses or cloches, if it isn’t too far gone) to clear land of weeds, to warm the soil, or to mulch around newly planted trees.
Plastic is so light it may need weighing down to prevent it blowing away. You can use soil or rocks, or pin it down with pieces of wire (old coat hangers can be used).
The color of the plastic has a significant effect on its properties as mulch. Black plastic increases the soil temperature by as much as 10°F and also kills weeds. Clear plastic acts like a miniature greenhouse and can increase soil temperatures even higher (as much as 15°F), but may allow weeds to grow underneath it (if it doesn’t cook them).
A big problem with plastic is that it degrades in sunlight. It is also ugly. Large areas of plastic might have an adverse effect on soil air exchange. It certainly does affect water absorption – watch that runoff
Mylar film: These have many of the same properties as plastic, but are highly reflective which gives them some special uses. They help to keep the soil cool (they shouldn’t be applied until the soil is warm) and reflect extra light back on to the surface of the leaves, increasing photosynthesis. In this way they have been known to increase yields by 100% or more. The extra light may actually disorient insect pests such as aphids, preventing their preying upon the plants (it may also disorient the gardener as well, as it is pretty bright, as well as very ugly).