More and more practiced, the technique of mulching offers many advantages in a garden. Why mulch, with what materials, when, for what purpose? Here are a few tips on this practical and soil-friendly gardening method!
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What is the mulching technique?
Mulching is a technique that consists in covering the soil with a layer of organic or mineral materials or specific films. The coverage can be partial, just at the foot of certain plants, or more extensive depending on the desired effect.
Is it true that mulching causes nitrogen starvation?
Many plants need mineralized nitrogen to grow. A deficit in this element causes crop problems. Plants turn yellow, stop growing, and become stunted.
Organic mulch (except compost) can cause a deficiency in immediately assimilable nitrogen under certain conditions. For example, if it is spread at the end of winter or at the very beginning of spring when the soil is cold, humid, compact and when biological activity is less. The decomposing organisms (aerobic and anaerobic bacteria and fungi), faced with this freshly arrived manna on the soil, will have to absorb a high proportion of carbonaceous elements to ensure the process of transformation into humus, they therefore need to draw a lot of nitrogen from the surface layer of the soil, which is then no longer available for the newly planted plants. This is why it is advisable to apply mulch in the fall or mid-spring, but never in late winter or early spring at the foot of recently planted plants. If you really must mulch at this time, then spread a good layer of well-decomposed compost under your mulch to avoid nitrogen starvation.
Where to find your mulch?
If you have a garden, you can recycle many waste and plant residues into mulch. Compost is one example!
If you have a shredder, you can make RCW from branches and leaves, or you can use grass clippings from your lawn or even weeds cut with a brushcutter if they have not gone to seed to make a mulch.
You can find straw in bales from some farmers, place an ad on general websites or in free newspapers, or visit agricultural cooperatives and some garden centers. The same goes for hemp and flax straw and coconut or buckwheat husks, which are abundantly available on the Internet.
To get mineral mulch, simply think of building material retailers. Some garden centers also offer large pebbles, pozzolan and even slate!
Why mulch the soil?
There are many advantages to mulching. Depending on the nature of the material chosen, it will have various functions. Generally speaking, it protects the soil from leaching and erosion, preserves and stimulates the biological activity of the soil, prevents the growth of weeds, contributes to the development of small fauna and limits evaporation.
When to use a mulch ?
As a soil cover to protect it from bad weather, mulch should be placed in the fall.
Organic mulches in particular can cause nitrogen starvation when decomposed if placed in late winter or early spring.
Mulching in late spring helps prevent water evaporation and keeps the soil cool in the summer, which results in significant water savings.
Mulch films made of natural materials can be used year-round, as can synthetic sails.
Beware of black polyethylene film which, in summer, in the hottest regions, can cause the soil to become too hot. However, it can be used in this season as part of solarization, which helps kill pathogens in the soil thanks to the heat.
What is the best mulch?
It is difficult to answer this question directly, knowing that each mulch has a specific application.
Mineral mulches (gravel, pebbles, pozzolan, slate…), in addition to being very ornamental and durable, offer good growing conditions to plants adapted to dry environments that need well-drained soil and heat. This type of mulch releases heat long after the sun has gone down, encouraging the growth of cold-weather plants such as cacti, succulents and Mediterranean plants (gazanias, helichrysums, agapanthus, phlomis, grevilleas…). It is perfectly suited to alpine and rock garden plants and works wonders in Zen gardens.
Mulch films or veils are generally used for their technical qualities. The black polyethylene film is appreciated for warming the soil in spring and protecting crops from soil splashes. It is widely used in strawberry cultivation despite its ecological impact.
Synthetic mulch cloths made of polypropylene have the advantage of being quickly unrolled and of preventing the growth of weeds, they are not biodegradable in the short term. Not very aesthetic, they can be used under a mineral mulch to protect it from the growth of weeds or on a specific area to facilitate the weeding work. Cardboard can replace this type of fabric, it has the advantage of being biodegradable and can be added to the compost after use.
Cloths made of recyclable and biodegradable materials (hemp, sisal, coir…) are ideal for mulching the base of a new hedge or for flower pots because they are easy to cut. They have a lifespan of about two years.
Organic mulches are by far the most effective, because in addition to the advantages of other mulches, they decompose gradually, boost soil life, loosen the soil and fertilize it naturally. There are many types of mulch and they are chosen on a case by case basis.
What is the right thickness of mulch?
It all depends on the material used and the goal.
Of course, mulch cloths are sufficient in a single layer, no need to accumulate them or to cross them.
Straw and dead leaves, which are very permeable and airy materials, should be spread in a thick layer (15 to 20 cm), especially if the goal is to limit weed growth.
A 3 cm layer of compost will be more than enough if it is covered with another plant material (straw, hay, grass clippings…).
As for mineral mulch, 3 to 5 cm is enough, but weeds will inevitably creep in if you have not taken care to insert a geotextile fabric between the soil and the mulch.
organic mulching solutions?
- Fresh manure stimulates the biological activity of the soil, but it should not be spread directly at the foot of the plants, as this could burn the roots and aerial parts. It is preferable to spread it on bare soil in the fall or early winter, for example on a vegetable plot. It will then have time to decompose and provide nutrients to the soil, which will be loosened and ready for spring planting. Sheep and goat manure are the richest. Horse manure can be a vector of pathologies for plants, but it has the advantage of warming the soil much more than cow manure.
- RCW (Ramial Fragmented Wood) is made from the shredding of small branches of plants. Very effective and protective, it loosens and fertilizes the soil over the long term and stimulates biological life by imitating the natural process that occurs in the forest. It is interesting at the foot of shrubs and trees, but also in beds at the foot of well-established perennials.
- Well decomposed compost is ideal in early spring because it protects plants from nitrogen starvation. It can also be spread at the foot of trees and shrubs after pruning operations. Airy, fertile, it has only advantages! It enriches poor soils and loosens compact and clayey soils. It can be covered with straw, grass clippings, dead leaves or pine bark to prevent the growth of weeds.
- Grass clippings should be spread at the foot of your plantation only after drying to avoid turning into a slimy mass that could rot the most fragile plants. Once dry, these residues constitute a very good mulch.
- Straw, not to be confused with hay, is yellow and rigid. It is light and airy, and favors water penetration and good soil ventilation. Straw is composed of many carbonaceous elements, it takes longer than hay to decompose and repels slugs and snails because of its pungency, which is not appreciated by gastropods.
- Once cut into small pieces, fern fronds have the same air and water permeability qualities as straw, with the added benefit of antifungal and insecticidal properties. This mulch naturally poisons slugs.
- Green manures, once mowed, can be left as a cover on a fallow plot. They will limit climatic aggressions, preserve the life of the soil and will decompose little by little by releasing nutritive elements. They are particularly interesting in clay soils because their roots help to loosen and permeate the soil before being mowed.
- The dead leaves in mulching reproduce the phenomenon naturally present in the forest. It protects the soil and turns into humus. The very tough leaves will first be shredded to promote their decomposition.
- Pine bark is an aerated, permeable and durable mulch because it takes time to decompose. They are ideally placed at the foot of acidophilic plants and rose bushes. They repel slugs and snails that have difficulty moving around.
- Plant chaff (hemp, flax, miscanthus…) is very light and permeable to water and air. They are also good barriers against gastropods. The only problem is that they can be moved by the wind. Preferably in quiet areas at the foot of dense planted beds.
