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<tc>Mulching in the garden: Basics and how it works</tc>

Mulching is a simple and at the same time extremely effective method to protect the soil, save water and keep plants healthy. The protective layer of organic material ensures that moisture is retained and that a stable soil life can develop.

Mulchen im Garten: Grundlagen und Wirkungsweise

Jun 22, 2024

Benji Freeman

In nature, the soil is never left uncovered – leaves and plant debris form a natural protective layer. This exact principle is replicated in the garden. The article explains the basics of mulching and shows how this method ensures vigorous growth and a lively, fertile soil in the long term.

1. Warum Mulchen so wichtig ist

Die wichtigsten Vorteile des Mulchens

2. Was Mulchen bedeutet
3. Grundlagen der Bodenbiologie unter Mulch

Das C/N‑Verhältnis – ein zentrales Grundprinzip
pH‑Wirkung – wie Mulch den Boden chemisch beeinflusst
Wasserhaushalt – warum Mulch Feuchtigkeit speichert
Temperaturregulierung – die physikalische Grundlage
Unkrautunterdrückung – der biologische Mechanismus

4. Mulchen und No‑Dig – warum der Boden nicht mehr umgegraben wird

5. Geeignete Materialien zum Mulchen
6. Typische Probleme und Lösungen beim mulchen

7. Mulch richtig ausbringen

8. Fazit

Why mulching is so important

In natural habitats, ground cover plays a central role. It protects the soil from drying out, prevents erosion and ensures that microorganisms and earthworms can work undisturbed. Even early civilisations such as the Inca, Maya and Aztecs used mulch to keep soils fertile and increase yields.

In agriculture, this condition is referred to as ‘soil tilth’ – a state in which the soil is loose, nutrient‑rich and well aerated. Mulching is one of the simplest ways to achieve this condition in the garden without having to work the soil continuously.

The most important benefits of mulching

keeps moisture in the soil and reduces watering effort
protects against heat, frost and strong temperature fluctuations
suppresses weeds
promotes soil life and humus formation
prevents erosion and surface sealing
improves soil structure
protects ground‑level fruits from rot

When used correctly, mulch can reduce water evaporation by up to 80% – a crucial advantage in hot or dry regions.

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Fresh wild herbs and grasses before seed formation are ideal as a nutrient‑rich mulch for vegetable beds

What mulching means

Mulching means covering the soil with a light‑impermeable but air‑permeable layer of organic material. Depending on the material and purpose, this layer can be a few centimetres up to 15 cm high. It is important that the mulch layer remains loose so that air and water can continue to reach the soil.

As the material slowly decomposes, humus is formed – one of the most valuable components of the soil. Humus improves structure, stores water and nutrients and promotes soil life.

Foundations of soil biology under mulch

A small, stable ecosystem forms beneath a mulch layer, similar to natural forest soil. The mulch cover keeps moisture in the soil, protects against strong temperature fluctuations and prevents rain from sealing or compacting the surface. This keeps the soil loose, well aerated and easy for roots to penetrate. At the same time, the organic material slowly decomposes and is transformed into valuable humus – a natural cycle that makes the soil more fertile in the long term.

Activity of microorganisms

Microorganisms are the first to respond to mulch. They use the consistent moisture and stable temperature to break down the organic material. Bacteria decompose the easily degradable components, while fungi take over the coarser structures. In doing so, they form fine threads, the so‑called mycelium – an underground network that runs through the soil like a web, binds soil particles and unlocks nutrients that plants can later absorb.
Through this activity, a living soil develops that constantly renews itself and reliably supplies plants with nutrients.

Earthworms pull mulch into deeper layers and transform it into valuable worm humus – a key driver of soil fertility

Role of earthworms

Earthworms benefit particularly from a mulch layer because it provides them with protection and plenty of food. They pull mulch particles downwards, feed on them and convert them into nutrient‑rich worm humus – one of the most valuable natural fertilisers in the garden.
Their tunnels loosen the soil, create pathways for air and water and prevent compaction. This results in a naturally aerated soil that remains loose without digging and offers ideal conditions for roots.

Stabilisation of soil structure

A stable crumb structure develops through the combined activity of microorganisms, fungi and earthworms. Fungal mycelium binds fine soil particles into small crumbs, while earthworms mix these crumbs with mineral components.
This results in a soil that:

Can absorb water well without becoming sealed
Allows air to reach deeper layers
Remains loose and easy for roots to grow through
Distributes nutrients evenly

This stable structure makes the soil more resistant to compaction, heavy rainfall and drought. Mulch supports this process continuously because fresh material is constantly added, keeping soil life active.

Further foundations of soil biology under mulch:

The C/N ratio – a central basic principle

For mulch to work effectively, it is important to understand how quickly a material decomposes and what nutrient effect it has during this process. This is precisely what the C/N ratio describes: it shows how much carbon (C) and how much nitrogen (N) a material contains. Both are fundamental building blocks for microorganisms that decompose organic matter.

Carbon is the “fuel” for microorganisms, nitrogen their “protein”.

Depending on how these two substances are distributed within the material, the effect of the mulch changes.

High carbon → slow decomposition → microorganisms need additional nitrogen
High nitrogen → fast decomposition → plants receive additional nutrients

Mulch can nourish the soil, remain neutral, or even bind nutrients – depending on the material.

pH effect – how mulch influences soil chemistry

Mulch affects the soil not only biologically but also chemically. Depending on the material, it can slightly lower, raise or leave the pH value unchanged. The pH value determines how well plants can absorb nutrients. Even small changes can influence growth.

Acidic materials

Bark mulch and some wood chips have a slightly acidic effect. They lower the soil pH a little. This is ideal for plants such as blueberries or lingonberries, which prefer acidic soils. For vegetable beds, these materials are less suitable because many vegetable crops prefer only slightly acidic to neutral soils.

Alkaline materials

Wool acts slightly alkaline and can raise the soil pH a little. This is particularly beneficial for vegetable crops such as cabbage, leek or celery, which prefer slightly alkaline soils.

At the same time, wool stores moisture and releases it slowly.

The pH value influences how well plants absorb nutrients. Mulch materials can slightly lower, raise or keep it neutral.

Neutral materials

Wild herbs, leaves or semi‑mature plant residues have a largely neutral effect. They hardly alter the soil pH and are therefore universally usable. These materials are suitable for almost all beds and are particularly recommended for beginners.

Water balance – why mulch retains moisture

Mulch helps the soil retain moisture for longer. This is not only because it covers the surface, but also due to several physical effects that together ensure plants need watering less frequently while still being supplied evenly.

Interruption of capillary action

In the soil, water normally rises through fine pores and evaporates at the surface. This process is called capillary action. Mulch forms a protective layer over the soil and interrupts this upward movement of water. As a result, more moisture remains in the soil and is available to the plants for a longer period.

This results in:

Plants have access to moisture for longer, especially on hot days.
The soil dries out more slowly and stays evenly moist.

This significantly reduces water loss and keeps the soil consistently supplied without the need for constant watering. Especially important in hot or dry regions.

Protection of soil pores

Sun and wind dry out the soil surface quickly and can damage the fine pores. Mulch protects these pores, allowing the soil to dry out more slowly and maintain its structure.

This has two advantages:

Water can infiltrate more effectively instead of running off on the surface.
The soil stays evenly moist without becoming muddy or compacted.

This reduces both waterlogging and drought stress.

Protection from direct sunlight

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Without mulch, the soil surface dries out quickly and compacts, which weakens soil life.

Mulch acts like a natural sunshade. It prevents the sun from heating the soil excessively and drawing out moisture.

A cooler soil means:

less evaporation
more stable conditions for roots
reduced watering effort

This effect is especially crucial in regions with intense sunlight, as plants can dry out quickly there.

Temperature regulation – the physical principle

Mulch affects not only water and soil structure but also the temperature within the soil. It acts as an insulating layer that protects the soil from heat and cold and softens strong fluctuations. This is important because roots function best within a stable temperature range.

Cooling in summer

Without mulch, the soil surface can heat up significantly. The heat penetrates the upper soil layers and puts the roots under stress. A layer of mulch covers the soil and prevents the sun from hitting it directly. As a result, the soil stays noticeably cooler.

This results in two clear advantages:

Roots remain within a comfortable temperature range and can work steadily.
Less water evaporates because the soil does not overheat.

This ensures that plants remain well supplied even on hot days and do not dry out as quickly.

Heat retention in winter

In the cold season, unprotected soil cools down quickly, and frost can penetrate deeply. This puts stress on the roots and slows down soil life. Mulch acts like a protective blanket in winter. It keeps warmth in the soil for longer and slows the penetration of cold.

For the soil, this means:

Roots are better protected from frost.
Microorganisms can continue working even at low temperatures.

This preserves soil fertility, and the soil becomes active again more quickly in spring.

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Straw protects the soil from frost in winter and keeps warmth in the ground for longer — ideal for active soil life during the cold season.

Buffering extreme temperatures

Strong temperature shifts – for example sudden heat after cool days or the other way around – can stress the soil and disrupt plant growth.
Mulch evens out these fluctuations because it acts as a buffer between weather and soil. The soil warms up and cools down more slowly.

This has two important effects:

Heat and cold affect the roots less abruptly.
Conditions in the soil remain more consistent.

This allows plants to grow more steadily and makes them less susceptible to stress caused by extreme weather conditions.

Weed suppression – the biological mechanism

Mulch naturally suppresses weeds by altering the conditions weeds need to germinate and grow. It is important to understand that weeds mainly thrive where light, space and exposed soil are available. Mulch removes exactly these requirements.

Light deprivation

Weed seeds need light to germinate. Mulch blocks this light and prevents many seeds from becoming active in the first place. Seedlings that still manage to emerge receive too little energy and usually die off early — resulting in significantly fewer weeds.

Thick layers

A sufficiently thick mulch layer forms a natural barrier. Weeds can hardly penetrate this layer, especially when fine materials such as grass clippings or wild herbs are used, which cover the surface densely. This reliably prevents weeds from growing through.

Even coverage

Mulch is only reliably effective when the layer is continuous. Gaps allow light and exposed soil, and weeds immediately take advantage of these spots. An evenly spread mulch cover prevents such gaps and greatly reduces maintenance because hardly any weeds can break through.

Mulching and no‑dig – why the soil is no longer dug over

benjis-seeds-hand-humus-soil-mulch-image-blog

Under mulch, a loose, humus‑rich soil develops that stores water, provides nutrients, and allows roots to grow easily.

Mulching and no‑dig are based on the principle of not disturbing the soil, but allowing it to improve through natural processes. Instead of loosening or turning the soil with tools, soil organisms, humus formation and the mulch layer take over these tasks. Undisturbed soil functions more efficiently, more steadily, and is therefore more fertile than soil that is regularly dug over.

Natural mixing

Without mulch, the soil often needs to be dug over to let air in and mix the layers. Under a mulch layer, earthworms and other soil organisms take over this work naturally. They pull organic material downwards, loosen the soil with their tunnels and ensure an even mixing.
As a result, the soil remains loose and well‑aerated without any digging.

Preservation of soil pores

When soil is dug over, the fine soil pores that are essential for water and air circulation are destroyed. Disturbed soil silts up more quickly, dries out unevenly and provides less stable conditions for roots.
Under mulch, these pores remain intact because the soil is not mechanically broken apart. Water can infiltrate more effectively, air reaches deeper layers, and roots grow evenly and without resistance.

Layered humus formation

When soil is dug over, humus layers are constantly mixed, disrupting their natural structure. Under mulch, however, humus develops layer by layer, just like in forest soil.
The organic material decomposes slowly and forms stable humus layers that improve the soil in the long term. As a result, the soil becomes more fertile, stores more water and provides plants with a consistently good foundation — all without digging.

Suitable materials for mulching

Mulch materials differ in how quickly they decompose, how many nutrients they provide, and how strongly they protect the soil. Each material has its own effect. Some improve the soil quickly, others offer long‑term protection or retain moisture particularly well.

Here is a compact overview of the most important materials:

Straw – loose, ideal for vegetable beds, decomposes slowly
Hay – nutrient rich, good for heavy‑feeding crops
Grass clippings – very high in nitrogen, apply only thinly
Wild herbs – versatile, nutrient rich, quick soil improvement
Bark mulch – coarse, long‑lasting, slightly acidic
Wood chips – structurally stable, ideal for fruit trees
Sheep wool – retains moisture, warms, slightly alkaline

The choice of material depends on whether the soil should be improved quickly, protected in the long term, or primarily kept moist.

Further information on each mulching material is provided in the article Mulching in the Garden: Mulch Materials and Application .

Typical problems and solutions when mulching

When mulching, mistakes can occur if materials are used incorrectly or conditions are ignored. It is important to understand that each material behaves differently and the soil responds in its own way. The following points help to avoid typical problems.

Wet conditions and weak plants

A too thick mulch layer prevents air exchange, the soil stays wet and plants respond with root rot.

→ Solution: Limit mulch depth to 3–7 cm, depending on the material.

Stem rot

Mulch placed too close to stems or trunks promotes rot and fungal infestation.

→ Solution: Always leave a small gap between mulch and the plant stem.

Dryness after applying mulch

When mulch is applied to dry soil, the dryness underneath persists for longer.

→ Solution: Lightly water the soil before mulching.

High weed pressure after applying mulch

Weeds beneath mulch continue to grow and spread.

→ Solution: Clear the area completely before applying mulch.

Delayed growth

Mulching on cold soil slows warming and delays plant growth.

→ Solution: Mulch only once the soil is warm enough.

Moisture problems, nutrient deficiency or overfertilisation

Unsuitable mulch types cause moisture issues, nutrient deficiency or overfertilisation.

→ Solution: Choose the material according to soil type and crop.

Correct mulching application

When mulch is to work effectively, it must lie on the soil as a loose, continuous layer. Without mulch, the soil surface is directly exposed to sun, wind and rain. With mulch, a breathable carpet is formed that protects the soil while supporting soil life.

The materials are gently worked into one another to form a closed but not compacted layer. A light watering after application helps settle the mulch and make it more stable without soaking it. This keeps the layer in place and allows it to act evenly.

Fine materials such as grass clippings or wild herbs should be applied in a thin layer so they do not become too wet and can dry properly. Coarse materials such as straw or wood chips can be applied much more thickly because they contain more air and decompose more slowly.
Over time the mulch breaks down and should be replenished regularly so the soil remains protected and continues to improve.

Conclusion

Mulching is a simple and effective method for protecting the soil, saving water and keeping plants healthy. Those who understand the basics and choose the right materials create stable conditions for vigorous growth and sustainable soil development. With little effort, a lively, fertile soil emerges that reliably supplies plants and makes the garden more resilient in the long term.

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