Organic mulch or aggregates: what changes in the soil and in maintenance

Organic mulch, gravel and other aggregates reduce the area of exposed soil, but they do not have the same effect.

Organic matter breaks down, feeds the soil and needs topping up. Aggregates keep a more mineral look, can last longer and add no humus.

The choice should take account of the planting, the climate, leaf fall, banks, maintenance and the design language of the project.

Woodchip, bark and shredded plant material form a layer that reduces evaporation, the impact of rain and the germination of weeds.

As they break down, they feed biological activity and add to the organic matter at the surface.

The speed depends on the material and the climate. Fine particles disappear faster; larger pieces last, but can shift.

The mulch should not touch the trunk or form a mound over the root collar.

Woodchip can make use of pruning waste and has a natural look.

When it is fresh, decomposition happens mainly at the surface. It should not be dug into the soil in large quantities close to the roots, where it can temporarily alter the availability of nitrogen.

On the surface, this risk is much lower and the material works well.

The source should be known, to avoid seeds, contaminants or parts of problem plants.

Bark has a more uniform particle size and a look associated with finished gardens.

It can float or shift in heavy rain. On banks, it needs retaining or a complementary solution.

The initial colour fades with time, and topping up should respect the total depth.

Mature compost can be applied in a thin layer to improve the soil.

It does not offer the same visual life as bark and can encourage germination if left exposed.

It works better as a component of soil management than as the only finish in every situation.

Gravel, pebbles, chippings and crushed stone create a durable surface that suits Mediterranean gardens or a mineral design language.

They reduce evaporation and the impact of rain, but can get hotter in the sun. Reflected heat affects sensitive plants and raises the temperature around the base.

Dark particles and nearby walls intensify the effect.

Aggregates do not improve organic matter and can mix with the soil if the base is not prepared.

A separation layer can reduce mixing between gravel and soil.

Membranes with too tight a weave limit exchange, make planting harder and collect organic matter on the surface. In time, weeds germinate on top of that layer.

The choice should let water and air through and allow for future changes.

Impermeable plastic is not suitable over root zones.

No mulch eliminates weeds completely.

An adequate depth reduces light and germination in the soil. Seeds still arrive on the wind and settle on accumulated matter.

Early weeding is needed in both systems.

Organic mulches need topping up to stay effective. Aggregates need cleaning and redistributing.

Organic mulch tends to moderate temperature better and to keep the surface biologically active.

Aggregates let water through when the base is permeable, but can encourage run-off over compacted soil.

Drip irrigation should stay accessible for inspection. Under gravel, leaks can remain hidden.

The depth should not stop light rain reaching the soil, although this rainfall has a limited effect in any case.

Leaves on organic mulch can blend in and break down. On pale gravel, they become more visible and are hard to remove without taking stones with them.

Near deciduous trees, this work should be allowed for in the plan.

Blowers can shift light aggregates and dry out the surface.

Loose organic mulch can slide. Gravel also moves and collects at the bottom.

Ground-cover planting, biodegradable matting and ground modelling help to stabilise it.

The solution depends on the gradient and the run-off, not just on the material.

Rounded pebbles can be unstable on paths. Angular chippings lock together better, but are still different from a bound surface.

Around swimming pools, the particle size, temperature and ease of cleaning are chosen to suit.

In children’s areas or where there are animals, the material should not create a risk or be easily scattered.

Organic mulch is topped up more often, but it improves the soil. Aggregate calls for an initial investment and continued cleaning.

Neither is fully permanent. Mixing, settling and loss all happen.

The cost should include labour and the relationship with the plants, not just the price per cubic metre.

Trees and shrubs from cool environments often benefit from organic mulch.

Mediterranean plantings can work with aggregates, provided the heat and the permeability are compatible.

It is also possible to combine the two: mulch over the root zones and stone in architectural bands.

The cover should serve the soil and the composition. It is not just a colour applied at the end of the build.

• Sandy, poor soils
• How to plan for maintenance in the design
• How to schedule irrigation

1. Literature on the management of mulch and soil covers.
2. Royal Horticultural Society — information on mulching.
3. Diaplant’s garden construction and maintenance practice.