Selecting plants by place: space, aspect, soil and water

A plant can be suited to the climate of a region and fail at a specific point in the garden. The proximity of a wall, reflected heat, a compacted layer or a lack of watering change the conditions over a very short distance.

Selecting by place means observing space, light, soil, water and microclimate before looking for species. Appearance still counts, but only once you have understood what can grow there without depending on permanent corrections.

The future size should be compatible with façades, paths, views and other plants. The size at planting is only the beginning.

A broad-crowned tree next to a house may need frequent pruning and lose structure. A vigorous shrub placed in a narrow bed will be cut before it reaches its natural form.

Height is considered too. Columnar forms take up little width, but can exceed the intended scale. Multi-stems create volume from the base and need clearance beside paths.

The choice improves when the design reserves the volume first and then looks for the plant.

Roots need continuous, porous soil with water and oxygen. The planting hole is not the final volume.

Foundations, paving, service chambers and compacted layers reduce the effective space. A large tree planted over a limited underground area may hold on for some time, but it is unlikely to develop the expected crown.

In pots and planters, the limitation is accepted and needs compatible watering, drainage and maintenance. The size should be chosen for that system.

The labels ‘sun’ and ‘partial shade’ are too general for some situations.

Morning sun is different from a west-facing aspect next to a stone façade. Light or dark paving stores and reflects heat. Wind increases water loss, especially in thin leaves and newly transplanted crowns.

A plant that tolerates sun may need cool, moist roots. Another accepts shade, but flowers less or has a more open crown.

It is useful to observe the site at several times of day and to consider the difference between summer and winter.

Texture, structure, depth and drainage determine development.

Clay soils can be fertile, but they become difficult when compacted or saturated. Sandy soils drain fast and store less water and fewer nutrients. Construction fill can mix materials that behave very differently.

A laboratory analysis adds information on pH, organic matter and nutrients, but it is no substitute for opening the profile and observing layers and compaction.

The species should be chosen for the condition that will remain after preparation, not for an ideal soil that the project cannot create.

Excess water reduces oxygen and can produce symptoms similar to drought. Before choosing plants for a wet area, you need to know whether the condition is seasonal, permanent or the result of a drainage failure.

Some species tolerate moist soils, but not standing water. Others adapt to temporary waterlogging.

The problem should not be solved just by looking for a ‘resistant’ plant. Ground modelling, drainage and irrigation still need correcting when the water is badly distributed.

Drought resistance normally refers to established plants. During establishment, the roots occupy a small volume and need watering.

Before choosing, check whether there is an automatic system, the possibility of watering by hand, storage and the quality of the water.

A demanding species can work in a deep, moist soil without excessive consumption. The same plant on an exposed, sandy bank becomes dependent on frequent watering.

The water available should be considered across the whole year, including restrictions and periods of absence.

Coastal sites, ridges, corridors between buildings and open areas have a particular exposure.

Wind increases transpiration, deforms crowns and can damage leaves. Salt carried on the air or applied to roads limits many species.

Protection can be built in stages, starting with more tolerant species to create shelter. Planting a sensitive species directly in an extreme site is rarely solved by staking.

Minimum temperatures remain important, but heatwaves and warm nights are gaining weight in selection.

A cold-hardy species can suffer in a dry summer. Another that tolerates heat may not withstand a late frost.

Altitude, orientation, valley and proximity to the sea create differences within the same region. Observing existing plants helps to understand the microclimate, provided the age and the maintenance they receive are taken into account.

Through-routes, animals and children compact the soil and damage fragile plants. Species beside paths should tolerate contact or be protected by where they are placed.

Fruit drop, thorns and toxicity can also be relevant in schools, hotels and family gardens.

There is no such thing as a suitable plant without reference to who uses the space.

Once the place has been characterised, a shortlist of compatible species is drawn up. Within that list, cultivar, form, size and ornamental effect are chosen.

This order reduces substitutions and maintenance. It also makes it possible to use less obvious plants that respond better to the conditions.

The design does not need to make the whole site uniform. Dry, moist, shady and exposed areas can receive different communities, turning limitations into diversity.

• How to assess the soil before designing or planting
• Selecting plants by function
• How to choose the right tree for each place

1. European Arboricultural Standards. European Tree Planting Standard.
2. European Soil Data Centre — information on soils and physical properties.
3. Royal Horticultural Society — growing requirements and aspect.
4. BGCI — resources for selecting trees for urban and climatic conditions.