Soil, water and drainage: the foundation of a garden

The vegetation depends on what happens in the soil. A correct selection of species does not make up for a compacted layer, watering that does not reach the roots or a planting hole where water sits without oxygen.

Soil, water and drainage should be analysed together. A sandy site can lose water too quickly; a compacted clay can look moist at the surface and stay dry or saturated inside.

The design starts by understanding this relationship and not by applying the same preparation to every area.

Classifying a soil as clay, loam or sand is useful, but not enough.

Structure shows how the particles group together and what pores exist between them. It is in those spaces that water and air circulate and roots grow.

A soil with nutrients can still be unsuitable when it is compacted. The passage of machinery, the storage of materials and work in wet conditions reduce porosity.

Usable depth, stones, fill and abrupt layers also constrain the vegetation.

Water needs to be present and accessible.

In sand, it infiltrates quickly and the reserve is small. In clay, it is held more tightly and may move slowly.

When the soil dries, the tree has to exert greater tension to take water up. When it is saturated, oxygen is lacking and the roots stop working well.

It is possible to have plenty of water in the ground and a crown with symptoms similar to drought.

Drainage is the ability of excess water to find a way out.

It can happen through deep infiltration, lateral movement, surface modelling or built systems.

A layer of gravel with nowhere to discharge is not drainage. Nor is a pipe with no fall or outlet.

The solution has to know where the water comes from and where it can go without creating another problem.

Compaction reduces large pores, hinders infiltration and increases resistance to roots.

It can exist beneath a layer of apparently good topsoil. Digging a hole reveals smooth surfaces, absent roots and transitions between materials.

Decompacting only the opening for the plant can create a loose zone surrounded by dense soil. The work should establish continuity.

A site has microzones. The low point receives run-off; the bank loses water; next to the house there is fill; under old trees there is root competition.

Applying the same mix and the same irrigation programme to the whole space ignores these differences.

The design can make use of the variation and create communities adapted to each zone, instead of trying to make all the soil uniform.

Correction may include decompaction, organic matter, ground modelling, drainage or the controlled replacement of materials.

Organic matter improves activity and retention, but it does not instantly resolve a deep compacted layer.

Very different mixes inside isolated planting holes can concentrate water and keep roots within the same volume.

Preparation should connect the plant to the ground where it will go on growing.

The system is divided by needs, aspect and soil.

Mature trees need deep volumes; newly planted perennials may require more frequent watering. Lawn should not determine the programme for the whole garden.

The output of each emitter is known. Minutes of irrigation, without a flow rate, do not allow the water applied to be calculated.

The programme changes with the season and with establishment.

Falls, paving, channels and permeable areas direct water.

The design avoids sending run-off towards buildings, walls or neighbouring properties. It also reduces velocity on banks to limit erosion.

Roof water can be stored, infiltrated in suitable areas or led to an authorised outlet.

After the works, infiltration, puddles, dryness and the response of the plants are observed.

The theoretical system may need adjusting. A displaced emitter, a blocked outlet or settlement changes how it works.

Records of moisture and consumption help to reduce water use without leaving critical areas unprotected.

Species are selected for the soil that will remain after preparation and for the water the garden can supply.

A drought-tolerant plant still needs establishing. A species of moist soils does not necessarily accept permanent waterlogging.

The right choice reduces corrections, but it is no substitute for building minimum conditions.

Soil, irrigation and drainage are largely hidden after planting. The lack of visibility does not reduce their importance.

A garden can look finished on handover day and show problems months later if this foundation has not been worked on.

The quality of the outdoor space starts before the first plant goes in.

• How to assess the soil
• Garden drainage
• How to schedule irrigation

1. European Soil Data Centre.
2. LUCAS Soil and SoilGrids, as sources for the regional framework.
3. European Arboricultural Standards. European Tree Planting Standard.
4. Diaplant’s soil preparation and irrigation practice.